Spawning patterns and reproductive biology of yellowtail scad (Trachurus novaezelandiae) off south-eastern Australia

We characterized the reproductive biology of the ecologically and commercially important small pelagic fish Trachurus novaezelandiae (yellowtail scad) in south-eastern Australian waters using a 25-year monitoring dataset. Sexual maturity occurred at 15.5 cm fork length (FL), with 95% of individuals mature by 23.2 cm FL. There were no significant differences in maturity ogives between sexes. Trachurus novaezelandiae exhibited an extended spawning season from July to March, peaking in October during the austral spring, consistent with sympatric small pelagic species in eastern Australian waters. Sex ratios in the commercial purse-seine fishery through 10 years were significantly skewed toward females (58%) and were consistent through years. Similarly to some other species within the Trachuridae, females predominated in all months except for those immediately following the spawning season, suggesting some differential sex-based availability to the fishery related to spawning. The fishery for T. novaezelandiae is currently classified as sustainable, with relatively low fishing mortality applied to the population. However, commercial harvests of small pelagic teleosts are rapidly increasing in Australian waters, and with developing markets it is likely that T. novaezelandiae will be fished more intensively in the future. The baseline information on reproductive biology in the present study may support sustainable exploitation through improved estimates of spawning stock biomass.

Understanding the biology and life-history characteristics of exploited teleosts are vital to their sustainable management (1). Without knowledge of underlying biological parameters of fish stocks, stock assessments and effective management controls on fishing mortality are challenging (2), with data-poor fisheries often failing to achieve sustainability (3). The world's fisheries resources are vital for food security and nutrition, and despite global fisheries production remaining reasonably stable at around 90 million t p.a., there are ongoing concerns around future sustainability (4).

Many of the largest wild-capture marine fisheries exploit small-bodied, schooling planktivorous teleosts (4). Amongst these, species within the genus Trachurus are highly represented. The genus Trachurus comprises 13 recognized species, each occupying specific geographic regions and contributing uniquely to local fisheries. Seven species of Trachurus with listed landings information by the Food and Agriculture Organization of the United Nations, being Trachurus capensis, Trachurus declivis, Trachurus japonicus, Trachurus mediterraneus, Trachurus murphyi, Trachurus trachurus, and Trachurus trecae peaked at a combined 5 million t in 1995. Landings have declined since that time but in 2022 were close to 2 million t (4). T. murphyi was ranked among the top 20 marine species in global catch volumes between 2010 and 2020, with cumulative landings totalling approximately 5.9 million tons during this period (4).

Australia's largest and most rapidly expanding fisheries are for small pelagic species, including Trachurus spp, Scomber australasicus, Emmelichthys nitidus, and Sardinops sagax (5, 6). There is a particular need to understand aspects of the reproductive biology of these species because the adopted stock assessment methodology underpinning harvest strategies and general fisheries management is the Daily Egg Production Method (DEPM). These DEPM assessments are done at regular periods to estimate spawning biomass (7–9) and require information on the reproductive biology of the species. They are preferred to more traditional stock assessments as they are independent of the fishery and well suited to the life-history of small pelagic species.

The two commercially and recreationally important Trachurids in Australian waters are T. declivis and Trachurus novaezelandiae, commonly called yellowtail scad. Trachurus declivis and T. novaezelandiae have similar Australian distributions from roughly south-eastern Queensland south to Western Australia; however, T. declivis tend to be more abundant in cooler more southern waters than T. novaezelandiae (10). Trachurus novaezelandiae are most abundant, and support their largest fisheries, along the inshore coastal waters of eastern Australia (10). Both T. declivis and T. novaezelandiae also occur in New Zealand waters (11), however, are considered to be separate stocks from those in Australian waters (12). Species within the genus Trachurus play crucial roles within pelagic ecosystems, and the fluctuations in their populations can significantly impact the broader marine food web (13).

Unlike T. declivis, for which there is a reasonable understanding of key reproductive parameters (14, 15), almost nothing has been reported on the reproductive biology of T. novaezelandiae. Work relevant to deriving spawning biomass estimates from future egg-surveys has been done in developing a molecular method to discriminate the eggs of T. declivis and T. novaezelandiae (16). No other published information is available on their reproductive biology except for a single report that estimated the size at sexual maturity in a single embayment during the late 1970s (SPCC, 1981).

The objectives of the current study were to estimate and describe some key reproductive characteristics of T. novaezelandiae off eastern-Australia. Specifically, we investigated the lengths at sexual maturity, sex ratios in the commercial landed catch, and periods of peak reproductive activity using a long-term dataset derived mainly from commercial fishery monitoring in New South Wales (NSW), Australia. Such baseline information is critical for improving spawning biomass estimates under DEPM and for supporting sustainable management.

Sampling design

Data on the reproductive characteristics of T. novaezelandiae were available for 4,586 individuals sampled through 25 years, being 2001 to 2025 from the NSW commercial fishery. The vast majority (~92 %) of samples were obtained during routine monitoring of the landed catch from the local purse-seine fleet that targets T. novaezelandiae for human consumption. One hundred and seventy six T. novaezelandiae smaller than those generally retained for human consumption were obtained on an ad hoc basis from bycatch from trawlers targeting Eastern School Whiting (Sillago flindersi) and purse-seine fishers targeting Australian Sardine (Sardinops sagax).

Biological measurements

Individual fish were measured as fork length (FL) to the nearest 0.1 cm rounding down and weighed to the nearest 0.1 g. When possible, sex was determined based on a macroscopic examination of the gonads, with a 5-stage developmental scale assigned based on the characteristics of the gonad (Supplementary Table 1). A similar macroscopic staging schedule developed by the FAO (17) has been used for other Trachurids (18).

Gonads were weighed to the nearest 0.01 g and a gonadosomatic index (GSI) calculated as gonad weight/gonad free body weight × 100.

Statistical analysis

Sexual maturation was estimated based on the macroscopic gonad stage assigned to each fish. Individuals were considered to be capable of spawning during the following spawning period, and so mature, if they possessed gonads staged at 2 (developing) or greater. Length at maturity was estimated from these binary maturity data using a logistic ogive with binomial error structure and logit link function using the AquaticLifeHistory package in R v4.3.1 (19, 20). We estimated maturity ogives for males and females separately and compared them using the bivariate form of Wald's F-test in R v4.3.1 (19, 21) following the methods of Hughes and Stewart (22).

Chi-squared tests were done using R v4.3.1 (19) to test if the sex ratios in commercial landings were significantly different from 1:1. These samples were available between 2005/06 and 2023/24 (Supplementary Table 2). All fish that had a gonad stage of 1 or were smaller than the estimated length at 50% maturity (15 cm FL), were removed to limit the potential for bias in sex ratios due to an inability to assign either males or females for undeveloped fish.

Seasonality of spawning was assessed from combined monthly trends across all years sampled from the relative occurrence of each gonad stage and GSIs. For these analyses all fish with stage 1 gonads (immature) were removed.

Maturity stages were assigned to 3,706 T. novaezelandiae ranging in size from 6.9 to 32.3 cm FL. The samples comprised 1,853 females, 1,362 males, 271 juveniles (sex not assigned as gonads too undeveloped) and 220 individuals with an unknown sex. Logistic curves were fitted to the binary variable of immature (macroscopic gonad stage 1) or mature (macroscopic gonad stages 2, 3, 4, 5) for the female and male datasets separately, with both datasets including the juveniles. The logistic curves describing maturity as a function of length for males and females were not significantly different (Wald's test, W = 3.83, P > 0.05). The data were subsequently combined and a single length at maturity ogive was estimated for T. novaezelandiae (Figure 1). The length (with standard error) at which 50% of T. novaezelandiae were mature was 15.5 (0.2) cm FL and 95% were mature at 23.2 (0.2) cm FL.

The sex ratio in the total sample of 2,668 T. novaezelandiae collected from the commercial purse-seine fishery through a 10 year period was 0.72:1 males to females (42%−58%, respectively) (Supplementary Table 2). This skewed sex ratio was observed during every year and was significantly different from 1:1 (χ 2 = 33.39, d.f. = 9, P < 0.001).

A predominance of females was apparent in most months; however, the sex ratio each month was statistically different (χ 2 = 49.66, d.f.= 11, P < 0.001) with a post-hoc test with a Bonferroni correction showing that April had a statistically different sex ratio from other months, having a greater proportion of males (P < 0.001). The month of May also had a greater proportion of males in samples, but the sex ratio was not significantly different to other months (P = 0.06; Figure 2).

The relative abundance of gonad stages in the landed catch each month indicated that the highest proportions of females and males with reproductively active gonads (Stages 3 and 4) were found between July and March, peaking during October (Figure 3).

The mean and maximum GSI values indicated a similar pattern, with relatively larger gonads during the austral spring and summer (Figure 4). The mean and maximum GSIs for females were consistently greater than for males, with the exception of the month of May when females had relatively low GSI values (Figure 4). Female GSIs peaked at just under 9% (Figure 4).

Trachurus novaezelandiae off eastern Australia were sexually mature at approximately 15.5 cm FL, with most (95%) being mature by 23.2 cm FL. The length at 50% maturity corresponds closely to that predicted from the empirical relationship between the asymptotic length and length at maturity presented in Fishbase (23). Using an asymptotic length of 30.41 cm FL (24) for T. novaezelandiae the estimated length at maturity was 18.3 cm FL and within the range of data used to generate the relationship (23). These results suggest that sexual maturity in T. novaezelandiae occurs at a smaller size than reported in the only other published study, done during the late 1970s (25). That study was done in the embayment of Botany Bay (33.99° S, 151.17° E) and reported that males matured, on average, at 20 cm and females at 22 cm FL. There was, however, some uncertainty around this estimate due to relatively small sample sizes, a complex 7-stage macroscopic maturity schedule and the 50% maturity levels estimated by eye.

We found no significant differences between the logistic maturity curves for males and females. Kerstan (47) reported maturity in T. trachurus to be related to growth rate, and given that male and female T. novaezelandiae grow at similar rates (24), it is perhaps unsurprising that we found no differences in their lengths at maturity. Generally, females have been reported to mature at slightly larger sizes and older ages than males within the Trachuridae, including T. trachurus (18, 45, 46) and T. murphyi (26). However, studies have also demonstrated substantial variability in sex-related lengths at maturity between populations of the same species (46), with T. trachurus in some populations showing no sex-related differences in lengths at maturity (27, 47). The length at sexual maturity within the Trachuridae is governed by various environmental and genetic factors (28), making regional assessments of maturity important for regional management.

Several Trachurids have had sex ratios reported at approximately 1:1, including T. murphyi (29) T. capensis (30) and T. trachurus (31). The consistently skewed sex ratio toward females (~60 %) through 10 years of sampling commercial landings suggests that the population sex ratio of T. novaezelandiae may be skewed. Interestingly, George (32) also reported a persistent sex ratio skewed toward females for T. murphyi off the northern coast of Chile that changed to be male dominated in the month when females exhibited the highest presence of postovulatory follicles. Similarly, El Achi et al. (18) reported an overall predominance of female T. Trachurus in the north Atlantic but an approximately equal sex ratio during summer and immediately following the end of the spawning seasons. Costa (33) reported a predominance of female T. picturatus mainly during the spawning season. Despite the predominance of females overall, landings of T. novaezelandiae were slightly skewed toward males during April and May, coinciding with the 2 months immediately following the spawning period.

The purse-seine fishery for T. novaezelandiae off eastern-Australia is largely non-selective by size and targets the adult portion of the population (34). The observed sex ratios are therefore likely unrelated to fishery selectivity. Purse-seine vessels target surface schools, often after using berley to get the school feeding in a concentrated area, so do not target sub-surface schools. Sex-related behavioral differences associated with spawning seasons have been reported in other species (35), and it is possible that following the main period of reproduction that some adult female T. novaezelandiae become less vulnerable to purse-seine fishing by generally occupying slightly deeper waters, or by moving outside of the fishing area. Future research through assessing sex ratios from mid-water and bottom trawls, and from areas outside of the main purse-seine fishing grounds, may be useful in understanding the drivers of the observed catch compositions.

Trachurids are reported to be indeterminate batch spawners with protracted reproductive periods (15, 32). Trachurus novaezelandiae off eastern Australia exhibited reproductively active gonads mainly between July and March, with a peak during October. Trachurus novaezelandiae conform to the general pattern for small pelagic species off south-eastern Australia that exhibit peak spawning during the austral spring, with reproduction beginning earlier at lower latitudes, Sardinops sagax (36), Scomber australasicus (37), T. declivis (38), and Emmelichthys nitidus (39).

Female T. novaezelandiae had greater relative gonad sizes than males during the spawning season. This greater investment into gonad development by females is similar to some other of the Trachuridae, T. Trachurus (18, 40), T. picturatus (33), and of species with batch, broadcast and pelagic spawning strategies where investment into multiple batches of eggs with lower rates of fertilization and subsequent survival is required (41).

Stewart and Ferrell (24) demonstrated substantial latitudinal variation in growth of T. novaezelandiae, with fish from the main fishing area in central NSW growing faster than those in the cooler more southern waters. Given that life-history traits, including those relating to reproduction vary with environmental conditions and therefore commonly latitude (42), it is highly likely that T. novaezelandiae reproductive traits will vary along eastern Australia. The vast majority (~92%) of samples in the present study were obtained from the commercial purse-seine fleet that spans less than one degree of latitude (Wollongong to Ulladulla), whereas the species distribution spans approximately 18 degrees of latitude along eastern Australia (43). The purse-seine fleet operates roughly in the center of the species range, and we had insufficient samples beyond this area to assess latitudinal variability. Nevertheless, we consider that because the present study represents the reproductive biology of T. novaezelandiae where the main fishery occurs it is currently the most relevant for management of the fishery.

Fishery implications and future research

We have successfully described important reproductive traits of a key ecological and fishery-important small-pelagic species off eastern-Australia T. novaezelandiae. The species' reproductive schedule in terms of length at maturity, spawning periodicity and sex ratios are broadly consistent with other members of the Trachuridae globally. Our estimates of the lengths at maturity demonstrate that the offshore purse-seine fishery only harvests fish of mature sizes (24, 44). This commercial fishery is managed under an annual total allowable catch quota regime and the stock is assessed as a sustainable stock (10). Nevertheless, estimation of mortality rates from fishery-dependent age samples suggests low fishing mortality has been applied to the population (34). Under an environment of increasing commercial harvests of small-pelagic species in Australia to service developing markets (6), it is possible that T. novaezelandiae will be fished more intensively in future. Our results show that T. novaezelandiae 's peak spawning time coincides with those of similar co-occurring small-pelagic species off eastern-Australia including S. sagax, S. australasicus, T declivis, and E. nitidus and as such are a candidate for inclusion in egg-surveys designed to optimize efficiency by estimating spawning biomass of several species concurrently through single DEPM surveys (8). Such an approach is more likely given recent work to develop a molecular technique to discriminate between eggs of T. declivis and T. novaezelandiae (16). Our baseline estimates of reproductive biology and sex ratio, in addition to future work on fecundity and spawning fraction, will facilitate application of robust DEPM-derived estimates of spawning biomass and sustainable increases in allowable catch.