Identifying spawning sites and fidelity of Ogaa (Walleye, Sander Vitreus): implications for fishery stewardship

Positionality statement

Indigenous people rely on fish to meet spiritual, cultural, ceremonial, and subsistence needs globally. Since Colonization, their ability to maintain relationships with the environment and fish has been restricted or limited by threats to fish stocks and habitats, as well as implementation of regulations and management practices based on Western science and worldviews (1, 2). Access to self-sustaining fisheries often correlates with Indigenous food sovereignty (3) and can positively influence the ability of Indigenous peoples to preserve their culture, language, and communities (4). Among these threats to fisheries are human development and climate change, which degrade or reduce spawning habitats and may impede fish from successfully reproducing. Current regulations and management practices have been insufficient at addressing these concerns, despite the fact that many Indigenous groups have sustainably harvested fish over spawning habitats. For example, for thousands of years, Indigenous tribes in the Laurentian Great Lakes region have harvested Ogaawag (Walleye Sander vitreus; plural) over shallow gravel beds in the early spring when other food is scarce (5), and tribes in the Pacific Northwest have harvested salmon during their migration to spawning areas (6). Preserving spawning habitats will support a critical part of a fish’s life cycle and support the lifeways of Indigenous peoples for future generations. Colonization has resulted in state and federal governments interfering in the ways that Indigenous communities interact with fisheries, but now many groups are asserting their rights and reasserting themselves as co-stewards of the resources by creating their own ethical guidelines and protocols for permitting research in their territories, and research and policy instruments are being designed to protect Indigenous knowledge systems (including scientific, traditional, and ecological knowledges; 7). Many Indigenous groups believe in reciprocity, including that with the natural world (6, 8). To honor this belief, humans should work to conserve the Earth and the beings that inhabit it.

Ojibwe worldview

The Ojibwe creation story in which the loon, muskrat, and turtle worked together to form Turtle Island (North America), teaches respect for all animal beings and how we need them to survive. According to the Ojibwe Creation Story (9), there are Four Orders of Creation, with each order being dependent upon the former. The First Order of Creation is the earth, including soil, rocks, winds, and water; the second is the plants; the third is the animals; and the fourth is human beings, the least of creation. Because we are dependent upon all of the previous orders while none depend on us, we must be humble, never taking without need and always be grateful for the gifts from the other orders of creation. Many natural resources are viewed by the Ojibwe not only as sources of food, medicine, and other things that aid human survival, but as relatives to humans. Ojibwe teachings impart that people must work to protect their relatives, on whom they depend, and this perspective is common among many other Indigenous nations. When the Ojibwe accept gifts (i.e., gather or harvest beings/species), they perform ceremonies and give offerings such as asemaa (tobacco; see 10 for details), one of the four most common nookwezigan, or sacred herbs, as a symbol of recognition of the sacrifice of that being, and follow timing and place cues given to them by nature to guide their harvesting practices. Like many Indigenous nations in North America, the Ojibwe follow the Seventh Generation philosophy, which instructs people to be mindful of future generations, seven generations into the future, when they hunt, fish and gather (11). This philosophy emphasizes all decisions should nurture and sustain the relatives and Turtle Island (i.e. the environment) for generations to come.

Cultural significance of Ogaawag for Ojibwe

Ogaawag are part of the cultural identity of Ojibwe tribes that currently live in the Ceded Territories of the Laurentian Great Lakes region. We wish to view Ogaa (Walleye; singular) management with two-eyed seeing, a practice in which one sees from one eye with Indigenous knowledge and values, and from the other with mainstream, Western knowledge and scientific practices, integrating both viewpoints for the benefit of all (12). Ojibwe tribes rely on giigoonyag (fish; plural) and particularly Ogaawag to meet subsistence, cultural, and spiritual needs (5). Since time immemorial, Ojibwe tribes have harvested Ogaawag via spearfishing and gill netting in early ziigwan (spring), a time when other protein sources were often scarce. Their extended familiarity with their relatives (resources) allows tribal members to use environmental cues such as the singing of agoozimakakii (spring peeper, Pseudacris crucifer), moon cycles, and spawning behavior of other giigoonh species such as Ginoozhe (northern pike, Esox lucius) to mark changes in the seasons and the start of the Ogaawag aamiwag (Walleye spawning; Great Lakes Indian Fish and Wildlife Commission (13). In recognition of this cultural significance, we have incorporated Ojibwemowin vocabulary (a full glossary is located at the end) to help emphasize the importance of Indigenous knowledge systems contributing to a fuller understanding of environmental change. Indigenous knowledge is embedded in their language, and knowledge may be lost when Ojibwe words are translated into another language.

Treaty rights

Seven generations ago, the 1837 Treaty of St. Peters (7 Stat. 536) between the United States government and several Ojibwe tribes, including (but not limited to) the ancestors of the Lac Courte Oreilles, St. Croix, and Mille Lacs bands, ceded much of modern-day Minnesota and Wisconsin to the United States in exchange for the tribes retaining hunting, fishing, and gathering rights (i.e., their lifeways) within this territory. It wasn’t until a 1999 court ruling that these retained rights were upheld in Minnesota by the state and federal governments (14, 15). Since then, Mille Lacs tribal members have harvested Ogaawag via spearfishing and gill netting every year, with harvest being monitored by conservation wardens and biologists (10, 14). This ruling means tribes can practice their ways of life, including harvesting Ogaawag through traditional means, without state interference. It also reaffirmed tribal entities as co-stewards of resources within the Ceded Territory with Minnesota and the federal government. Many other tribal entities in the United States and around the world have made similar agreements, but Indigenous fishing rights have not universally been reaffirmed, which impacts the extent tribal entities are involved with stewardship of natural resources.

Introduction

Many species of giigoonyag, across many families, display spawning site fidelity, which refers to a behavior in which individuals spawn (in Ojibwe aamiwag means; they spawn) at the same site for multiple years (16–20). Previous reproductive success at spawning sites may indicate that these sites offer specific features, such as suitable substrate, water temperature, and food availability (21), that facilitate hatching success and juvenile survivability (17). Spawning site fidelity likely arose from its efficacy in dispersing young, thereby reducing predation risk, establishing territory to reduce competition, or copying social cues from familiar individuals (22). This behavior is well documented in the Salmonidae family (23) and can also be found in lake-dwelling species, including Ogaa and other members of the Percidae family (16). Though the behavior is well-documented in these families, the proportion of giigoonyag within a waterbody exhibiting spawning site fidelity is unclear. Previous studies have found threatened species that exhibit spawning site fidelity have unique considerations for rehabilitation and remediation efforts (18, 24, 25). Members of the Salmonidae family exhibit the most rebound when endaad (habitat/home), particularly habitat that is used for reproduction, growth, rearing, and migration is restored and/or remediated (26, 27). Identifying spawning sites and the extent to which giigoonyag show fidelity will allow stewards/managers to make conservation decisions based on the species life stage.

Spawning site fidelity ties an individual to a place, and the success of that individual and their offspring becomes tied to habitat quality and stability of that place. Human disturbances and climate change (Aanji-bimaadiziimagak o’ow aki; loosely, “a changing world”) have drastically altered habitats, large and small; and animals with strong site fidelity may not survive or reproduce as well as animals that have more flexible behaviors (13, 28). Ogaa spawning typically occurs in shallow water (0–2 m; 29), so shoreline manipulation and erosion may negatively affect the availability of and increase inter-specific competition for adequate spawning habitats (17). Similarly, ecological changes brought about by the introduction of nonnative species may reduce the viability of a spawning site, making fidelity to that site a net negative (30).

Despite abundant and diverse efforts across many systems, Ogaawag and other cool-water species have faced declining populations across the upper Midwest in recent years (31). Their populations have been threatened by warming waters due to climate change, habitat degradation, overfishing by anglers, and changing ecosystem dynamics due to the introduction of nonnative species (32–35). Many Ogaa populations have been particularly threatened by decreasing reproductive success and/or survival of young Ogaawag, which may be linked to decreasing spawning site quality (36). The exact mechanisms by which individual populations are threatened depend on the system, though the physical, chemical, biological, and ecological characteristics of a system may be used to extrapolate likely causes for declining populations elsewhere. In a co-managed fishery, the primary method for managing declining populations in Minnesota and Wisconsin has been to reduce or eliminate harvest (37), but that often fails to address habitat concerns (38).

Spawning site fidelity is an under-studied aspect of Ogaa ecology that may be important to understand if stewards (managers) aim to rehabilitate Ogaa water bodies. Ogaawag are a species known to exhibit spawning site fidelity (16, 17, 39), but to what extent in large inland lakes with altered ecosystem structure and function is unknown. Adult Ogaawag use the disappearance of ice and rapidly warming water temperatures (baakibii’an, the annual timing of the melting of lake ice in spring) as cues to begin spawning over sand and gravel bars in the early ziigwan (spring; 5, 16, 39). Ogaawag typically spawn in a relatively small portion of a lake’s shoreline, preferentially choosing nearshore, shallow sites that are dominated by gravel, cobble, or generally coarse substrates when water temperatures are between 3.3 and 15.5°C (40). They broadcast their eggs onto suitable substrates, and do not build nests or provide parental protection (41). After hatching, Ogaawag fry typically remain on the spawning grounds for one to three weeks before dispersing into the open water (29). This means that spawning sites also provide important nursery habitat, and, if Ogaawag exhibit a high degree of spawning site fidelity, then the loss of this habitat may contribute to the bottleneck that causes low survivability of Ogaawag from hatching to age-2 now and into the future. It is also relatively unknown which habitat characteristics, such as water temperature, substrate type, and depth, might influence their likelihood of returning to a particular site. The exact mechanisms and nature of this site fidelity may vary across individuals, which has rarely been examined in the literature.

Mille Lacs Lake in the 1837 Ceded Territory of modern-day Minnesota boasts a world-renowned Ogaa fishery that has seen many changes over time. The lake’s relative shallowness and resulting lack of a thermocline (a layer of water in which temperature decreases rapidly) allow Ogaawag to forage throughout the lake, thereby maximizing their range (42). Mille Lacs has historically been one of the most heavily fished and productive Ogaa lakes in the state (43). Ogaa abundance in Mille Lacs Lake has been variable but has generally declined since the 1980s (43). In 2014, adult Ogaa abundance reached an all-time low and has plateaued since then (42, 44), primarily due to low recruitment and decreased juvenile survival (age-0 to age-2; 45). Causes for the decline in juvenile survival may be related to habitat degradation, increasing water temperatures due to climate change (35), and a lake-wide decrease in zooplankton due to the introduction of nonnative species such as zebra mussels (Dreissena polymorpha) and spiny water fleas (Bythotrephes longimanus; 33). Though the locations where Ogaawag currently spawn do produce offspring, it is unclear if production of fry would be similar if adult Ogaawag had to use alternative spawning areas or if the ones that they currently use were damaged or degraded. Identifying spawning sites that Ogaa show fidelity to will allow researchers and stewards (managers) to focus efforts on areas that may influence Ogaa recruitment dynamics and ultimately adult abundance.

Given the changes in the ecosystem, ongoing habitat alteration, degradation due to lakeshore development, the effects of climate change, and uncertainty about Ogaa recruitment, it seems prudent to investigate the species’ spawning behavior and ecology. As Kelly Applegate, the Mille Lacs Band of Ojibwe Commissioner of Natural Resources, puts it, “As the very first protectors of water and fish, we care deeply about the habitats which give life to the Ogaawag that sustain us. By learning more about important Ogaawag spawning sites, we can obtain valuable knowledge allowing us to further build on our century’s old reputation for successfully and sustainably managing our fisheries resources”. The objectives of this study were co-produced with the Mille Lacs Band of Ojibwe and align with the interests of the tribe. Specifically, this study seeks to 1) use multiple lines of evidence, including both western science and Indigenous knowledge, to determine the timing and duration of the Ogaa spawning period of multiple years, 2) identify Ogaa spawning sites; 3) characterize the habitat of those sites, including substrate type, bottom hardness, shoreline development, and wind/wave action; and 4) quantify spawning site fidelity of individual adult Ogaawag in Mille Lacs Lake. Previous studies have found that Ogaa management is more effective when management practices are prescribed based on the specific conditions and ecosystem dynamics of a system (38), so this may act as a first step in fully examining the mechanisms that can lead to low annual recruitment of Ogaawag. Findings from this research will identify areas of the lake that may be hotspots for Ogaa spawning activities and inform stewardship and conservation decisions about Mille Lacs’ Ogaa fishery and its adjacent terrestrial habitats.

Methods

Site description

At 51,900 hectares, Mille Lacs Lake in central Minnesota is the 29th largest lake in the United States (Figure 1). It is a mesotrophic (TSI score 43) lake that was formed by the recession of the Superior lobe glacier approximately 12,000 years ago (42). Like most glacial moraine lakes, Mille Lacs is shallow, with an average and maximum depth of 8.7 m and 12.8 m, respectively. The northern portion of the lake is dominated by mud flats and sand bars. The southern portion is diverse, with sand, gravel, boulders, islands, weedy bays, and silt-dominated areas that provide abundant, diverse habitat. The lake is fed by several rivers and creeks from the north, east, and south and is the headwaters for the Rum River, which flows out from the western shore. Due to its shallowness, the lake is not well stratified and sees near-constant mixing during the ice-free periods of the year. Thermoclines rarely appear in the lake, except during brief periods of very warm weather and very little wind (44). Mille Lacs typically ices-up by late November and ices-out in Mid-April. The lake is a popular destination for recreational fishing and is home to Ogaa, Ginoozhe (Northern Pike, Esox lucius), Maashkinoozhe (Muskellunge, Esox masquinongy), Asaawens (Yellow Perch, Perca flavescens), Noosa’owesi (Smallmouth Bass, Micropterus dolomieu), Ashigan (Largemouth Bass, Micropterus salmoides), Agwadaashi (Black Crappie, Pomoxis nigromaculatus), Mizay (Burbot, Lota lota), Odoonibiins (Tullibee, Coregonus artedi), and many other non-game species. This popularity has also led to the accidental introduction of nonnative species, including the Spiny Water Flea (Bythotrephes longimanus), Zebra Mussel (Dreissena polymorpha), Chinese Mystery Snail (Cipangopaludina chinensis), Rusty Crayfish (Faxonius rusticus), and Eurasian Watermilfoil (Myriophyllum spicatum).

Ogaawag collection

Adult Ogaawag were collected throughout the lake via electrofishing at depths of 0–1.5 m in three phases in the summer and fall of 2018. Giigoonyag were released from their collection locations (Figure 1). Sixteen Adult Ogaawag were tagged in July of 2018, with the remaining (n=54) tagged in late September of 2018. Collected Ogaawag were immediately put into a 363-L aerated livewell that constantly circulated fresh lake water. Ogaawag were allowed to recover in the livewell until they regained equilibrium; their length and weight were then recorded.

Surgery

Briefly, transmitters (V13TP-1L acoustic tags; InnovaSea Inc.^©, Halifax, NS) were surgically implanted posterior to the pelvic girdle on the ventral side of each Ogaa along the midline in locations around the lake (47, 48 for surgical procedures). The V13-TP tags transmitted depth (± 1.7 m), temperature (± 0.5°C), time, and date with a nominal delay of 180 seconds and an estimated battery life of 1285 days. Prior to surgery, Ogaawag were anesthetized via electrical nerve stimulation and were provided fresh, oxygenated water throughout the surgical process. Tags were inserted into the coelomic cavity of the Ogaa, and sex identification was attempted by visually inspecting the gonads. Adult Ogaawag received a green Dart FLOY tag (Floy Tag & Mfg., Inc., Seattle, WA) that had contact information and a unique Ogaa identification number printed on it. Based upon recommendations in Shultz et al. (48), target tag burdens (tag to body weight ratio) were 2%. All Ogaawag were released immediately into Mille Lacs Lake or held in a cooler for a few minutes until they recovered (i.e., the Ogaa had regained equilibrium and was ventilating). Incisions were closed with monofilament absorbable sutures. The average surgery time was ~5.5 min. Almost all Ogaawag recovered immediately after tagging, with only 1 Ogaa being held for more than 10 minutes.

Telemetry array

Range testing was completed prior to this study and for further details see 48. Acoustic receivers (n=61, VR2W, InnovaSea Inc., Halifax, NS) were placed in a grid and spaced 3,000 m apart in Mille Lacs Lake from June 13–20, 2018 that covered different habitat types and water depths (Figure 1). Receivers had a mean detection radius (i.e., range at which 50% of detections were recorded) of 800 m, but may range from as little as 300 to as much as 1,200 m. Acoustic receivers were moored by placing them inside a 76.2 mm PVC pipe anchored to a 40.8 kg concrete block that rested on the lakebed, with the hydrophone protruding out of the top. HOBO loggers (HOBO Pendant^® Temperature/Light 64K, Onset Computer Corporation, Bourne, MA) were attached to the receiver moorings to collect water temperature and light penetration data. Additional receivers (n=12) were deployed in the rivers adjacent to Mille Lacs Lake in fall 2019 (for locations, see Supplementary Materials Figure A). Retrieval first occurred in May 2019 and then in June 2020 and 2021.

Data quality control

Detection data were downloaded from receivers, imported into VUE (v 3.4.38; VUE Development Team 2014), and then exported into.csv files for analysis. Dead, lost tags, or false signals were identified using the following criteria: The false_detections() function in the GLATOS package in R-studio (v 4.0.5; 49) was used to identify and remove false detections. The threshold for time between detections was 5,400 seconds, or 30 times the nominal delay on the tags, as per GLATOS network recommendations. Based on observations in captivity it took approximately 18 days for surgical wounds to heal, so Ogaawag that weren’t detected after 18 days were removed and considered surgical mortalities (48). If the Ogaa survived for more than 18 days, the initial 18 days of detections were included. Data from Ogaawag suspected to have died or expelled their tags were removed. Live Ogaawag were typically detected by several receivers, at several depths throughout the year, and have biologically plausible temperatures. Ogaawag that were detected for short periods of time throughout the year were included in the dataset because they may have migrated into one of the rivers/streams or were in a location where they could not be detected by the receivers.

Biotic and abiotic variables

Substrate type was determined using previous surveys conducted by the Mille Lacs Band during initial receiver deployment and existing bottom type maps created by the Minnesota DNR. A substrate type map was generated in ArcGIS Pro (v3.4.0; Esri Inc. 2024) based upon this data (Figure 1). Shoreline development was adapted from a map included in Simon et al.’s Aquatic and Shore Habitat of Mille Lacs Lake, Minnesota 2009–2019 Report (2020) and based on the citizen science Score Your Shore initiative, in which landowners assessed the development level of their land according to guidelines. A bottom hardness map was generated using data collected from a Lowrance HDS 9 with BIOBASE mapping capabilities from three transects within the detection range of each receiver. After downloading, data was then extrapolated using ARCMAP spline tools to match contour lines in Mille Lacs. Finally, wind and wave directions were denoted according to weather reports from the Brainerd Lakes Regional Airport, the site of the nearest weather station to the study area that were accessed by Weather Underground’s Weather History and Data Archive (50). Prevailing wind direction and average speed for each date within all spawning periods are provided in the supplemental materials (Supplementary Materials Table A).

Assigning spawning sites and periods

Using cluster and factoextra packages in R, we grouped the receivers into three categories (“primary”, “secondary”, and “tertiary”) based on the number of unique fish detected, the number of detections, the average residence time at each receiver, and the average depth of detection (all these parameters were centered and scaled). The factoextra package was used to visualize the optimal number of clusters, and the k-means function from the Stats package was used to define the clusters. Once the three groups were identified, we compared the average substrate hardness and the proportion of receivers exposed to either N or W prevailing winds. ANOVA and Tukey post-hoc comparisons were used to identify differences between spawning habitat categories for each of the variables, except for the proportion of receivers exposed to prevailing winds, and over substrate types were compared using Marascuilo Procedure for multiple proportions (51). The full results of these analyses can be found in the supplemental materials (Tables B-G). The predominant substrate type (Rock, Sand, Gravel, Muck [i.e. silt, mud, and/or organic matter substrates]) was identified for each receiver via ponar grabs. Receivers assigned the “primary” class were considered to be the primary spawning hotspots for Ogaawag. In this analysis, “site” refers to a receiver and all its adjacent receivers.

The beginning of the annual spawning period for each year was determined using both Western science and traditional Indigenous knowledge. Spawning periods were determined by finding the median date of the first Ogaa catch in the reported Mille Lacs Band ziigwan (spring) gill net harvest, the date of ice out, and the date of the first spring peeper observations. The first singing of spring peepers has been used by Ojibwe peoples as a signal of the beginning of Ogaa spawning for time immemorial. The spawning period was considered to have lasted the duration of the time that tribal gill net catch comprised mostly Ogaa. The dates that were derived using these methods were compared against water temperature and photoperiod to confirm that they fell within generally accepted parameters. These methods were chosen to remain true to the signals used by Ojibwe peoples for hundreds of years to indicate the beginning of the Ogaa spawning period. Reported tribal harvest at boat landings were used to define the spawning period and ripe fish also provided evidence that the fish were indeed spawning at this time. Ogaa spawning sites have been identified by tribal members who have captured gravid male and female Ogaawag along the shoreline of Mille Lacs Lake (for locations, see Supplementary Materials Figure B). Information about habitat use, number of detections, and number of unique fish one month pre-spawn and one month post-spawn is available in the supplemental materials (Supplementary Materials Figure C; Supplementary Materials Tables H, I).

Data analysis

All data analysis was performed in R-studio (v 4.0.5; 49). Once spawning periods and sites were assigned, we mapped the detections of each Ogaa during the spawning periods for 2019, 2020, and 2021. After the total number of animal ID’s and dates of detection in each site category for each year were calculated, abacus and detection bubble plots (both total and unique detections) were created using the GLATOS package. Ogaawag were categorized and summarized by sex.

Because the pool of data was so small for the 2021 season (most likely due to transmitter battery life), these detections were omitted from analysis but will be mentioned where relevant. Unless noted, metrics only include data from 2019 and 2020. Using the animal ID summaries, the total number of Ogaawag that returned to each receiver’s detection radius in 2020 were determined. Spawning hotspots were identified by quantifying the number of detections, number of unique Ogaawag, and average residence time of Ogaawag detected by each receiver during the spawning periods. These values were mapped onto bubble plots.

Based on detections at receivers across years, Ogaawag were divided into four fidelity categories. These categories were derived from those used by Bade (52) in their analysis of Ogaa spawning behaviors. The fidelity category that each Ogaa was assigned acted as the model for analysis of spawning site fidelity. If it returned to the same or adjacent receiver in 2020 as in 2019 during the spawning period and the Ogaa did not enter another spawning site during that period, it was categorized as “stayed.” This is one of two types of spawning site fidelity that we describe in this study. If the Ogaa returned to its 2019 spawning site but subsequently traveled to another site (farther away than the adjacent receiver), they are categorized as “wandered.” This is the other type of fidelity that we describe here. If the Ogaa did not return to their 2019 spawning site but entered another spawning site, they were categorized as “strayed” (i.e. did not display fidelity). Finally, if an Ogaa was detected on spawning grounds in 2019 but not in 2020, they were categorized as “skipped” (i.e., assumed to have skipped spawning). All detections of the Ogaawag in this category were reviewed to verify that they were alive after the spawning period and justify this classification.

Next, the residence time of each Ogaa was measured as the period between the first and last detection by a receiver recorded for an individual during a given spawning period. Because detection radii may have overlapped, detections by adjacent receivers were included in a single residence period. Detections by non-adjacent receivers marked the end of a residence period. Residence time of an Ogaa at each spawning site was separated by sex (“female” or “unknown”) and total length (“small”, “medium”, “large”, “xlarge”). It is likely that many of the “unknown” Ogaawag were male, since their sex is more difficult to identify. Ogaawag categorized as “small” were less than 47.7 cm long; Ogaawag categorized as “medium” were between 47.8 and 52.3 cm long; Ogaawag categorized as “large” were between 52.4 and 60.5 cm long. Ogaawag categorized as “Xlarge” were over 60.6 cm long.

Shapiro-Wilk tests were used to test for normality. For variables that met the assumptions of normality (number of detections (log transformed), average residence time (log transformed), substrate hardness), ANOVA was used to compare overall differences between sites and TukeyHSD for site-specific differences. For those that did not meet normality assumptions (number of giigoonyag, average mean depth of detection, vegetation biomass, distance to shore, receiver depth), Kruskal-Wallis tests were used to analyze differences in Ogaa length between fidelity groups and sex and pairwise Wilcox test for site-specific differences. Residence time analyses were used to support the assumption that fish were spawning (i.e., long residence time over shallow areas, primary and secondary receivers).

Results

Spring spawning

Based on several metrics (Table 1), spawning periods were 15 April to 15 May in 2019, 11 April to 8 May in 2020, and 2 April to 6 May in 2021. Water temperatures and photoperiod fell within generally accepted parameters for Ogaa spawning. These periods likely encompass staging, spawning, and recovery periods. All these phases are important for identifying habitats used by Ogaawag for reproduction. Suitable spawning habitat occurred around most of the perimeter of the lake. Some areas were more suitable (rock/gravel substrate; hard bottom) than others (sandy substrate; soft bottom) for spawning (Figure 1; for bottom hardness map, see supplemental materials; Supplementary Material Figure D), but most spawning seems to have occurred in smaller, specific areas. Following the spawning period, Ogaawag went to deeper, soft-bottomed areas of the lake for several weeks, after which they were detected in various areas of the lake throughout the year.

Table 1. Reported dates and water temperature ranges that factored into determination of spawning period dates for the study period.

Water temperature (± 0.5°C) was recorded on pendant HOBO loggers attached to each receiver. Tags implanted in Ogaawag transmitted temperature (± 0.5°C) of the fish to the receivers. “Harvest” refers to Mille Lacs Band tribal gill net harvest. “Agoozimakakii” refers to spring peepers (Pseudacris crucifer), whose songs have been recognized by tribal members to signal the onset of spring for hundreds of years.

During the 2019 spawning period, 51 adult Ogaawag (12 females, 39 unknown sex) were detected and 29 Ogaawag (5 females, 24 unknown sex) were detected in 2020 (Table 2). Of the 22 Ogaawag that were not detected in 2020, the last detection for most of the Ogaawag occurred in the summer or early fall of 2019. In 2021, only two Ogaawag were detected; one female and one whose sex was unknown. Detections for seven Ogaawag were removed due to continuously pinging the same receiver for more than one month (six were removed prior to the 2019 spawning period, and one was removed between the 2019 and 2020 spawning periods). These fish are assumed to have died or shed the tag. Additional deaths may have occurred in areas with no receiver coverage and/or been harvested. Two Ogaawag from this study were reported to have been harvested but more may have been harvested without being reported. It is also possible that some of the loss of Ogaawag prior to the 2020 spawning period is due to some of these fish traveling up one of the streams during the spawning period, thus evading detection (river receivers were not deployed until fall 2019). Remaining losses of Ogaawag over time are attributed to transmitter batteries being depleted.

Table 2. Statistics for age, sex, and length of Ogaawag included in this study by year.

Ogaawag were tagged in 2018.

Number of total detections and unique Ogaawag per receiver

Based on the receiver categories derived from the cluster analysis, “primary” receivers detected an average of 7.5 Ogaawag in 2019 and 2020 (Table 3). The “secondary” receivers detected an average of 3.8 Ogaawag in 2019 and 2020. The “tertiary” receivers detected an average of 1.5 Ogaawag in 2019 and 2020. There was statistical significance between the number of individual Ogaawag detected per receiver in each category (χ²(2) =28.57, p=<0.001). Individual Ogaawag were detected by an average of seven receivers. Four Ogaawag were detected by only one receiver during the spawning period were included in this analysis because they were detected by other receivers after spawning. The most mobile Ogaa in this study was detected by 35 receivers. The first week of spawning was the most mobile time of the season among Ogaawag.

Table 3. (a) Receiver numbers, average number of detections during spawning, number of Ogaawag detected, and residence times per receiver by category as assigned by cluster analysis.

(b) Habitat characteristics of receivers by category as assigned by cluster analysis.

In both 2019 and 2020, receivers with the most detections were located in areas with minimal development near Isle (rocky bay in southeast corner of the lake, Father Hennepin State Park, receivers 53 and 54), Malmo (sandy northeast corner of the lake, receivers 2 and 8), and the undeveloped Rainbow Island (# 47; Figures 1, 2). Though shoreline development increases south of Malmo, the rocky substrate found along the eastern shoreline appears to be suitable for spawning (receivers 23 and 31). In 2021, a limited quantity of detections (n=391) came from receiver 16. Receivers with the most unique Ogaa detections were nearly the same for both years (2019 top receivers: 2, 8, 15, 53, 54; 2020 top receivers: 3, 8, 15, 53, 54), and, similar to the receivers with the most overall detections, were in Isle and Malmo Bays. There was statistical significance between the number of total detections per receiver in each category (F (2,58) =25.29, p=<0.001), with “primary” having the most detections and “tertiary” having the fewest. The number of detections, unique fish detected, residence time, and other statistics per receiver are listed in the Supplementary Material Table J.

The river receivers detected only two Ogaawag, both during the 2021 period. All detections were in the Thaines River (receiver R1), which flows into Mille Lacs in Isle Bay. One Ogaa was detected on 3 April, then again on 5 April, and the other was only detected once, on 9 April. It was not detected returning to the lake. This Ogaa was one that “stayed” (i.e., used the same spawning grounds in 2020 as 2019). This detection is possibly an anomaly, since this Ogaa was never detected in the main lake that year. The Ogaa that went upstream and came back down was categorized as “stayed” between 2019 and 2020. It was assumed to have spawned near receivers 53 and 54, also in Isle Bay, and was detected near these receivers before and after it entered the nearby Thaines River. It seems that though the smaller streams are not being used extensively for spawning by Mille Lacs Ogaawag, but the Thaines River might be.

Spawning site habitat

Habitat characteristics varied among the three receiver classifications. The substrate was categorized into four groups based on the dominant type (rock, gravel, sand, and muck). The substrate at the “primary” receivers was predominantly rock with some sand and gravel but no muck (Figure 3). At the “secondary” receivers, the substrate was mostly sand and rock, with some muck. Substrates at the “tertiary” sites were equally distributed among the four groups. Receivers in the “secondary” and “tertiary” categories were statistically more likely to have muck substrates compared to “primary” receivers (Marascuilo procedure: P < 0.05; Supplementary Materials Table E). In addition to differences in substrate composition, the “primary” sites were on average closer to shore (mean = 0.93 km, SE = 0.14) and shallower (mean = 5.73 m, SE = 0.80) than the secondary (mean distance from shore = 2.25 km, SE = 0.23; mean depth = 8.07, SE = 0.39) and tertiary receivers (mean distance from shore = 5.60 km, SE = 0.41; mean depth = 9.95 m, SE = 0.31; Table 3b). Six of the seven “primary” receivers were likely exposed to prevailing winds out of the North and or West. Only 12 of the 31 “secondary” receivers and one of the 24 “tertiary” receivers were exposed to North or West prevailing winds. These differences in exposure to prevailing winds across categories were statistically significant (Marascuilo procedure: P < 0.05; Supplementary Materials Table F).

Residence time

The receivers with the most overall detections and unique Ogaa detections were generally also those with the longest average residence times. The average residence time at the “primary” receivers was almost three-fold greater (11.6 days) compared to “secondary” (4.1 days) and almost six-fold greater than “tertiary” (1.9 days) receivers (Table 3a). Differences in length of residence times between receiver categories were significant (F (2,58) =30.18, p=<0.001). Female Ogaawag had much lower average residence times than those of unknown sex (4.5 and 8.0 days, respectively; Table 4). Small and medium Ogaawag, which were more likely to be of unknown sex, spent longer at each receiver than larger Ogaawag (approx. 7.25 and 6.75 days, respectively; Table 5). Xlarge Ogaawag spent the least amount of time near each receiver (approx. 3.85 days). Ogaawag that “wandered” had much shorter residence times (approx. 5.3 days) than those that “stayed” (approx. 8.7 days). The Ogaa that “skipped” had a very short average residence time (approx. 2.5 days) and the Ogaa that “strayed” had an average residence time of approx. 6.0 days. Though it appears to have spawned near receiver 47 in 2019, the Ogaa that “strayed” remained between three and four meters deep until April 28, 2020 (when most other Ogaawag had already moved to the shallows that year), at which point it traveled inland, near receiver 53, where it remained in about two meters of water until the end of the spawning season.

Table 4. Statistics for total numbers of, sex, size, age, and residence time by fidelity category.

Ogaawag categorized as “stayed” returned to the same site in 2020 that they spawned at in 2019 and did not enter any other spawning grounds that year. Ogaawag categorized as “strayed” did not return to their 2019 spawning site but were detected at other spawning grounds in 2020. Ogaawag categorized as “wandered” returned to their 2019 spawning sites in 2020 but also entered other spawning grounds during that season. Ogaawag that were not detected at any spawning grounds during the 2020 spawning period were categorized as “skipped” (i.e. skipped spawning).

Table 5. Fidelity category and average residence time for each year by Ogaa size category.

Ogaawag categorized as “small” were less than 47.7 cm long; Ogaawag categorized as “medium” were between 47.8 and 52.3 cm long; Ogaawag categorized as “large” were between 52.4 and 60.5 cm long. Ogaawag categorized as “Xlarge” were over 60.6 cm long.

Designating hotspots

A cluster analysis of which receivers had the most overall detections, most detections of unique Ogaawag, and longest residence times revealed six “hot spot” receivers (8, 9, 23, 47, 53, and 54). Two of these receivers were in Malmo Bay (8 and 23), two were in Isle Bay (53 and 54), one was near Rainbow Island (47), and one was near the town of Garrison (9). Five of these receivers were also among those with the most overall detections (8, 23, 47, 53, 54), and three were among those with the most unique fish detected (8, 53, and 54). Receiver 9 was elevated to its status as a “hot spot” by having exceptionally long average residence times among the fish that were assumed to have spawned there. These areas are known as spawning sites by tribal harvesters. The boat launches near Isle (Cedar Creek), Rainbow Island (DNR), and Garrison (North Garrison) all had high catches of ripe Ogaawag during the spawning period (Table 6). Catch near Malmo was low, but that boat ramp is shallow and difficult to launch from, so it is less used than the others.

Table 6. Average Ogaa harvest per year by landing from 2013-2023.

During the spring harvest, sex for the first 100 Ogaa at each landing each day was identified via gamete expression.

Site fidelity

Of the 51 Ogaawag that were detected during the study period, 26 Ogaawag were detected by the same receiver during both years. Most of the remaining Ogaawag (n=23) were only detected during one spawning period. The shortest residence time among returning Ogaawag was about 32 hours, so these Ogaawag were assumed not to be “just passing through.” Seventeen (60.7%) Ogaawag were categorized as “stayed”, meaning that they appeared to spawn only at the same receiver(s) as the previous year. Among these Ogaawag, two (11.8%) were female; the remaining 15 were of unknown sex. Nine Ogaawag (34.6%) were categorized as “wandered”, meaning that they entered both their previous year’s spawning ground and at least one other spawning ground. Among wandering Ogaawag, 2 (22.2%) were female, and the other seven were of unknown sex. Six of the wandering Ogaawag (66.7%) displayed fidelity to multiple sites between years. Together, 26 out of 28 (92.9%) total Ogaawag detected in both 2019 and 2020 demonstrated some degree of spawning site fidelity between those years.

Only two Ogaawag showed no fidelity. One female Ogaa was categorized as “skipped” and one Ogaa of unknown sex was categorized as “strayed.” The majority (77.8% and 63.6%, respectively) of small and medium Ogaawag were categorized as stayed, whereas only 25% and 50% of large and xlarge Ogaawag were categorized as such. There was no statistical difference in total body length between the different fidelity groups (χ²(3) =4.21, p=0.2401) or sex (χ²(1) =0.706, p=0.4008). The largest Ogaa detected during both spawning periods was the aforementioned female that was categorized as “skipped” (66.8 cm). The Ogaa that strayed from its 2019 spawning grounds was also large (60.2 cm), suggesting that it may have also been female. Ogaawag that “wandered” were detected shallowest on average, with Ogaawag that “stayed” also being shallow (1.8 and 2.4 m, respectively; Table 4). The Ogaa that “strayed” was typically detected at moderate depths (approx. 3.7 m), while the Ogaa that “skipped” was comparatively deep (approx. 5.6 m). The Ogaa that appeared to have skipped spawning in 2020 was detected during the 2021 spawning period, where it appeared to have spawned near receiver 16. Notably, this Ogaa appeared to have spawned in this area in 2019, thus exhibiting every-other-year spawning site fidelity.

Fidelity (of both types) most commonly occurred near receivers in the Isle and Malmo areas, with receivers 3, 6, 15, 53, and 54 having the most Ogaawag return from year to year (Figure 4). Receiver 6 stands out as an unexpected fidelity hotspot due to its depth (approximately 9.1 m) and the surrounding sandy substrate. Detections at this receiver peaked approximately four days after the mean date of peak detections for all receivers during the spawning period. This may be an important recovery area rather than a spawning area. Out of the 26 Ogaawag that demonstrated site fidelity in this study, all but one of them are assumed to have spawned near receivers in Malmo Bay, Isle Bay, Rainbow Island, or Garrison in 2020.

Discussion

Fidelity among Ogaawag

This research demonstrates that Ogaawag in Mille Lacs Lake display a high rate of spawning site fidelity (collectively, ~93% of the Ogaawag displayed some degree of fidelity). Fidelity to a spawning site is a highly conserved trait among Ogaa populations, with estimates ranging from 70-95% (17, 52, 53), which is considerably higher than intra-annual fidelity rates among other species such as Coho and Sockeye salmon (57% and 65%, respectively; 54, 55). Interestingly, ~35% of the Ogaawag in this study wandered between spawning sites. Bade (52), upon which we based our fidelity categories, observed wandering behaviors among far fewer Ogaawag than we did here (approx. 3%). Exploring multiple sites may be advantageous for Ogaawag in systems where spawning habitat is not limited and allow fish to populate new sites (17). Using multiple spawning sites has been found to increase mating opportunities among marine fish species such as white trevally (Pseudocaranx dentex; 56), but the behavior has rarely been recorded among freshwater species. Of the fish that wandered, nearly 80% were of unknown sex (likely male Ogaawag). Previous research suggests that male Ogaawag tend to have longer residence times on spawning grounds than females and may spawn multiple times as they try to fertilize as many eggs as possible (57), while females are much more likely to enter the spawning grounds, deposit eggs, and move on (58). Taken together, spawning habitats do not seem to be limiting in Mille Lacs Lake; Ogaawag of unknown sex are more likely to wander than female Ogaawag; and Ogaawag exhibit a high degree of spawning site fidelity, particularly to a small number of sites.

Spawning site fidelity was primarily restricted to a few sites in Mille Lacs Lake. Specifically, 89% of the Ogaawag in this study displayed fidelity to spawning areas near Isle Bay, Malmo Bay, Rainbow Island, or Garrison. Hayden et al. (17) suggested that fidelity to a spawning site was likely influenced by both abiotic and biotic factors. Raabe et al. (59) identified characteristics of quality spawning habitats as shallow, cobble substrates, low embeddedness, well-oxygenated flowing water, and exposure to prevailing winds/wave action, and limited development in the lake and along the shoreline. The sites identified here meet these criteria and account for about 13% (approx. 6,900 of 51,900 hectares) of the surface area of the lake. In sum, habitats with these criteria and a high degree of spawning fidelity should be conserved and potentially enhanced through stewardship/management actions.

Spawning hotspots

Spawning hotspots were more likely to occur along the eastern shore of the lake. During springtime, winds tend to come out of the west or north, meaning that the eastern and southern shores of the lake receive the most wind and waves. Ogaawag tend to spawn in areas with lots of waves and wind because the increased water motion impedes the ability of egg predators to locate egg deposits, increases oxygen flow to eggs, and may increase egg dispersal, though that additional movement may result in some egg mortality (60, 61). Indeed, according to historical weather records from the nearby Brainerd Lakes Regional Airport, west winds were the second most common and strongest wind direction during the spawning periods of 2019, 2020, and 2021 (Weather Underground). North winds were the most prevalent wind direction in all three years as well, which supports the use of Isle Bay by spawning Ogaawag along Mille Lacs’ south shore. These prevailing winds from the west, northwest, and north in the spring may have influenced Ogaawag’s historical spawning-site selection in Mille Lacs.

Spawning hotspots (Isle Bay, Rainbow Island, and Garrison) are often targeted by tribal harvesters due to the shallow gravel areas and abundance of spawning Ogaawag in these sites. Isle Bay and Rainbow Island have long been known by tribal members to be Ogaa spawning areas. A notable newly identified hotspot is the Malmo area, which, though known to have high Ogaa harvest rates during the spawning period, has been largely dismissed as potential spawning habitat due to its sandy substrate. Though the substrate of the northeast corner of Mille Lacs Lake is largely sandy (with some dredging reports finding clay as well), off the shore of Wealthwood, north of receivers 2 and 3, there is a large rock pile that may provide prime spawning habitat for Ogaawag (indicated by long residence times). One potential explanation for its high detection rates is that the sandy substrate in the area may allow for a larger detection radius (48), thus allowing receivers in the area to detect Ogaawag spawning near adjacent receivers. Another is the confluence of two creeks in the area, which, as previously mentioned, provide a flow of oxygenated water. Ogaawag are known to spawn over sand if there is a lack of gravel (16, 39), so, if the nearby spawning grounds were crowded (both adjacent spawning areas had high numbers of Ogaawag detected), the flow from the creeks may have been enough to attract Ogaawag to this area. Multiple factors (flowing water from rivers, wave action from prevailing winds, prime (gravel) to subprime (sand) spawning habitat, and little shoreline development/disturbance) are likely drawing Ogaawag to this spawning site. Collectively, both streams of evidence (tribal members’ knowledge about spawning sites, i.e. Indigenous knowledge) and Ogaawag movements during spawning (Western science) support classifying the Malmo area as a spawning hotspot.

This study identified “hot spot” spawning locations and the fidelity of adult Ogaawag to these locations, but we acknowledge that we tagged a relatively small proportion (<1%) of the adult Ogaawag in Mille Lacs Lake. Our estimates of spawning site fidelity are similar to other studies and, therefore, are likely representative of the entire population. We suspect, based on boat landings used in the spring by tribal fishers (IK), that other major and minor spawning locations exist around the lake. For this reason, further research efforts that identify additional spawning areas and reefs at a finer scale and identify pre- and post-spawning habitats are warranted and would likely result in more areas being designated as reefs.

Skipped spawning

Since the 1990s, female Ogaawag have been postulated to periodically skip spawning (62). Spawning is especially energy-intensive for females, and, as they age and lipid production slows, they may begin to spawn every other year due to decreased hatching success, fry survival, or survival of the female Ogaa herself (57). The detection radii of the receivers collectively cover most of the lake, and throughout the year that this female Ogaa skipped spawning, it remained in the pelagic environment at an average depth of 5.6 m, over muck and silt bottom types. It is unlikely that this Ogaa would have spawned in any of the locations in which it was detected or that it would have managed to evade detection long enough to spawn outside of the detection radii of the receivers. Given the small sample size in this study, this observation is not representative of the true rate of skipped spawning in Ogaawag but provides evidence that the phenomenon occurs. Bade (52), which had a much larger sample size than our study, found that approximately 14% of Ogaawag in Lake Erie skipped spawning, which may be a more representative metric.

Implications for management

Many giigoonh species across several families (salmonids, percids, etc.) have demonstrated these forms of fidelity, yet it is unknown how prevalent it is among all giigoonyag. Studying giigoonh movements during the spawning period has the potential to identify new areas and habitats that giigoonyag are using and re-align stewardship priorities accordingly. In this study, the importance of an area of the lake with extremely sandy substrates for spawning indicates that sandy areas may be more important for spawning than is typically thought, and while Ogaawag may spawn in these areas, their reproductive success is unknown. Additionally, a majority of Ogaa spawning in this study seems to have occurred on the windy/wavy shore of the lake. This may have created habitat that facilitated hatching and juvenile survival, encouraging Ogaawag to return from year to year. Environmental variables, such as wind direction (this study), temperature, and wave action have the potential to play a role in selecting which sites Ogaawag display fidelity to and could be used to predict where Ogaawag may spawn in each waterbody. If these characteristics were to be altered (such as a breakwall interrupting wave action on a reef, development of marinas, addition of docks, and chemical control of vegetation), spawning could be disrupted, and reproduction and/or survival of offspring may decrease (13, 28). Stewardship (management) actions should be taken to prevent further changes in this system and conserve these distinct areas and populations.

The information presented in this study should be of special interest to organizations that make decisions regarding regulations affecting not only aquatic environments, but the land surrounding them as well. The areas that have been identified as primary Ogaawag spawning habitats largely consist of sparse areas of shoreline with little to no anthropogenic development. Because Ogaawag display spawning site fidelity, it is unclear whether they would move to other spawning grounds should these areas be damaged, destroyed, or their characteristics were otherwise altered, which may negatively impact reproduction and recruitment. If these areas were to be developed, it may have temporary or permanent negative effects on Ogaa reproduction and recruitment, which previous studies have shown is essential for achieving and maintaining healthy populations (63). Establishing conservation areas onshore and in the water can help to prevent development from negatively affecting recruitment (64). This may take the form of a network of conservation areas that restrict the amount and nature of development that may occur on or near the shoreline, which can be accomplished through altering local zoning rules or forestry practices (65). In Minnesota, Ogaa spawning habitats may be designated through the Minnesota Department of Natural Resources (DNR) and protected via Aquatic Management Areas (AMAs), which protect lakeshore from development, pollution, and other damage by putting additional restrictions on dredging and prohibiting landowners from putting out docks during the spawning period. These areas may provide benefits not only for Ogaa spawning success, but protect habitat for many species, including Asaawens (Yellow Perch Perca flavescens), which have also experienced declines in Mille Lacs Lake and are an important diet item for Ogaawag across life stages (45, 66). Low survival from hatching to age-2 is considered to be the primary driver of Ogaa decline in Mille Lacs Lake, and conservation areas would provide habitat for spawning and nursery habitat not only for Ogaa but for other species that support their survival. Physical protection of relatively small sites may benefit a very high proportion of the spawning population. Such benefits for Ogaa and other species may also be achieved through cooperation among management agencies (e.g., state, tribal, federal, inter-tribal agencies) by working together to conserve habitat (e.g., including Indigenous knowledge in permitting decisions). Watershed-level management actions that mitigate the effects of erosion, pollution, and climate change would also be appropriate. This information may also be used by state and tribal decisions regarding water-level and watershed-level management.

Conclusions

The Ojibwe worldview is not top-down; it focuses on forming a relationship with the natural world (9). Extended cohabitance and respect for this land have given the Ojibwe and other Indigenous peoples an advanced understanding of our natural resources and our relationships to them that western science is only just coming to understand. This is part of why reaffirmation of rights for Indigenous communities is so important; it allows them to use their cultural and spiritual beliefs to guide management actions that center the interests of the threatened resource/beings (7). Traditional harvest of Ogaawag during spawning has occurred in this region since time immemorial and both beings (Ojibwe people and Ogaawag) have coexisted together during this time. Humans are an inherent part of the environment, and participation in interacting with and caring for the environment is important. Spawning likely occurs in many areas of Mille Lacs Lake, but a few areas appear to be particularly important. Maintaining the quantity and quality of these habitats is one way all humans can be stewards of our environment.

In this study, we identified new areas that were previously not considered to be primary spawning habitat and found evidence that Ogaawag return to the same spawning sites year after year. However, adjusting our relationship (or management actions) with giigoonyag based new information should not be limited to physical protection of spawning sites. To sustain giigoonyag (all fishes) for future generations, we need more research to identify spawning sites and fidelity to these sites because these habitats and behaviors are key to the life cycle of any given being. (Co-)stewards of these beings need more ways to ensure giigoonyag have a place to reproduce and create more giigoonyag for future generations, and management actions should be taken proactively, rather than waiting for problems to occur. Establishing in-lake, habitat-based conservation areas at Ogaa hot spots could be one piece of a larger stewardship plan (e.g., an ecosystem or watershed plan) that works to protect Ogaawag and other giigoonyag species for the future. Returning to the Ojibwe creation story, the Creator formed man from the earth, he entrusted them to care for Mother Earth. The Ojibwe worldview teaches us to respect the natural world because it takes care of us. All things that western society considers to be “resources” are also our relatives, and they make sacrifices in order to support human health and wellbeing. We must offer that same respect to them by being good stewards of the environment that supports all of us. If humans can conserve these habitats and achieve sustainable fisheries, we may fulfill our duty to care for these beings, as they do for us.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frish.2026.1758559/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Periklis Kleitou, Marine and Environmental Research Lab (MER), Cyprus

Reviewed by: Daniel Swadling, NSW Government, Australia

Timothy Grabowski, United States Department of the Interior, United States