Pacific herring, Clupea pallasi, recruitment in the Bering Sea and north-east Pacific Ocean, II: relationships to environmental variables and implications for forecasting

Previous studies have shown that Pacific herring populations in the Bering Sea and north-east Pacific Ocean can be grouped based on similar recruitment time series. The scale of these groups suggests large-scale influence on recruitment fluctuations from the environment. Recruitment time series from 14 populations were analysed to determine links to various environmental variables and to develop recruitment forecasting models using a Ricker-type environmentally dependent spawner–recruit model. The environmental variables used for this investigation included monthly time series of the following: southern oscillation index, North Pacific pressure index, sea surface temperatures, air temperatures, coastal upwelling indices, Bering Sea wind, Bering Sea ice cover, and Bering Sea bottom temperatures. Exploratory correlation analysis was used for focusing the time period examined for each environmental variable. Candidate models for forecasting herring recruitment were selected by the ordinary and recent cross-validation prediction errors. Results indicated that forecasting models using air and sea surface temperature data lagged to the year of spawning generally produced the best forecasting models. Multiple environmental variables showed marked improvements in prediction over single-environmental-variable models.     

Variation in the distribution of yellowfin sole Limanda aspera larvae in warm and cold years in the eastern Bering Sea

Multiyear periods of relatively cold temperatures (2007–2013) and warm temperatures (2001–2005 and 2014–2018) altered the eastern Bering Sea ecosystem, affecting ocean currents and wind patterns, plankton community, and spatial distribution of fishes. Yellowfin sole Limanda aspera larvae were collected from the inner domain (≤50 m depth) of the eastern Bering Sea among four warm years (2002, 2004, 2005, 2016), an average year (2006), and three cold years (2007, 2010, 2012). Spatial distribution and density of larvae among those years was analyzed using generalized additive models that included timing of sea-ice retreat, areal coverage of water ≤0°C, and water temperature as covariates. Analyses indicated a combination of temperature effects on the location and timing of spawning, and on egg and larval survival, may explain the variation in larval density and distribution among years. During warm years, higher density and wider spatial distribution of larvae may be due to earlier spawning, an expansion of the spawning area, and higher egg and larvae survival due to favorable temperatures. Larval distribution contracted shoreward, and density was lower during cold conditions and was likely due to fish spawning closer to shore to remain in preferred temperatures, later spawning, and increased mortality. Predicted drift trajectories from spawning areas showed that larvae would reach nursery grounds in most years. Years when the drift period was longer than the pelagic phase of the larvae occurred during both warm and cold conditions indicating that settlement outside of nursery areas could happen during either temperature condition.     

Eastern Bering Sea pollock recruitment,abundance, distribution and approach to fishery management

Eastern Bering Sea pollock have two distinctly different stable spawning grounds—along the shelf and in the eastern and central Aleutian Islands between 400 and 500 m water columns. Pollock spawning behavior supports the hypothesis that the shelf and deepwater “basin” spawning pollock are completely independent reproductive stocks. Deepwater pollock inhabit the shelf and, once mature at age 5–6 years, migrate from the shelf onto the continental slope into the Zhemchug, Pribilof, and Bering canyons by the end of winter. Bering Sea pollock recruitment and year class abundance have high annual variability, but there are no clear relationships between pollock year class strength and water temperature, ice distribution or survival on early ontogenesis stages (eggs and larvae). Young-of-the-year fish survival varies dramatically during winter supporting the hypothesis that the Bering Sea pollock recruitment and strength of year class have high annual variability depending on young-of-the-year fish survival during winter. The annual change of physical oceanography condition, productivity and species composition of zooplankton community are associated with great differences in pollock seasonal migrations and distribution, reproduction, survival of recruits at early stages of development and finally with abundance of year classes and total biomass. Implementation of ecosystem-based fishery management most important for application of pollock research both of Russian national program and on base of International Agreements.     

Linking Northeast Pacific recruitment synchrony to environmental variability

We investigated the hypothesis that synchronous recruitment is due to a shared susceptibility to environmental processes using stock–recruitment residuals for 52 marine fish stocks within three Northeast Pacific large marine ecosystems: the Eastern Bering Sea and Aleutian Islands, Gulf of Alaska, and California Current. There was moderate coherence in exceptionally strong and weak year‐classes and correlations across stocks. Based on evidence of synchrony from these analyses, we used Bayesian hierarchical models to relate recruitment to environmental covariates for groups of stocks that may be similarly influenced by environmental processes based on their life histories. There were consistent relationships among stocks to the covariates, especially within the Gulf of Alaska and California Current. The best Gulf of Alaska model included Northeast Pacific sea surface height as a predictor of recruitment, and was particularly strong for stocks dependent on cross‐shelf transport during the larval phase for recruitment. In the California Current the best‐fit model included San Francisco coastal sea level height as a predictor, with higher recruitment for many stocks corresponding to anomalously high sea level the year before spawning and low sea level the year of spawning. The best Eastern Bering Sea and Aleutian Islands model included several environmental variables as covariates and there was some consistent response across stocks to these variables. Future research may be able to utilize these across‐stock environmental influences, in conjunction with an understanding of ecological processes important across early life history stages, to improve identification of environmental drivers of recruitment.     

Effects of temperature, wind and spawning stock biomass on recruitment of Arcto-Norwegian cod

Sea temperature has earlier been shown to have a large influence on the recruitment of Arcto-Norwegian cod, Gadus morhua. We here hypothesize that this linkage is partly due to the direct effect of temperature on larval and juvenile growth. Secondly, temperature acts as a proxy for both biotic and abiotic factors influencing recruitment. Indices of abundance of early juvenile cod (2–3 months old), 0-group cod (4–5 months old) and 3-year-old cod are analysed in more detail against the environmental temperature, wind stress components, wind-induced turbulent energy and the spawning stock biomass. To deal with autocorrelation, non-stationar-time and nun-normality, which complicate a statistical time series analysis, randomization and Box-Jenkins methods are applied. In addition to the important effect of high sea temperature during the early life stage in forming strong year classes, the results show that the spawning stock biomass is nearly as important. Also, alongshore southerly wind stress anomalies during the period of pelagic drift (from April through summer) and offshore wind stress anomalies during egg and early larval stages (in April) act favourably on recruitment. The beneficial effect of southerly wind anomalies could he linked partly to high temperature, but the flux of zooplankton-rich water from the Norwegian Sea into the feeding areas of the Barents Sea may also be increased. The favourable influence of offshore winds in April is less predominant and causal links are also less clear; possible explanations for this might be increased offshore spreading of eggs and early larvae, resulting in reduced risk of predation, and increased compensation inflow of intermediate Norwegian Sea water which, in this restricted period of time, has a high concentration of spawning copepods suitable as prey for the developing cod larvae.     

The effects of temperature on growth,development and settlement of northern rock sole larvae (Lepidopsetta polyxystra)

Northern rock sole (Lepidopsetta polyxystra) is a commercially important fish in the North Pacific and a focal species in understanding larval transport to nursery grounds in the Bering Sea. However, the temperature‐dependent vital rates and settlement dynamics for this species have not been described in detail. We reared northern rock sole larvae in the laboratory to measure growth, condition, development and settlement parameters across four temperatures (2, 4, 7 and 10°C). Both length and mass‐measured growth rates increased with temperature and were best described by non‐linear regression. Residuals of the length–mass relationships were positively related to temperature, indicating larval condition also increased with temperature. Larval development and settlement were largely size dependent, resulting in reduced larval stage duration and earlier settlement at higher temperatures owing to more rapid growth at elevated temperatures. However, larvae at colder temperatures were less developed at a given size, but more likely to settle at smaller sizes than larvae reared in warmer conditions. These temperature–response parameters can be used to refine current and future transport models for northern rock sole larvae under changing environmental conditions in the North Pacific.     

Regime shifts and recruitment dynamics of snow crab,Chionoecetes opilio,in the eastern Bering Sea

Shifts in climate regime are prominent features of the physical environment of the eastern Bering Sea and in recent years have been documented in approximately 1977 and 1989. Average snow crab (Chionoecetes opilio) recruitment decreased sharply after the 1989 fertilization year. Models in which control of snow crab recruitment shifts between drivers dependent on climate ‘regime’ are presented. These models are evaluated using cross‐validation and retrospective analysis, both of which indicate that the relationships are relatively robust to varying levels of information. Larval survival as influenced by food availability in the pelagic phase and advection to suitable nursery grounds are the hypothesized mechanisms driving recruitment dynamics.     

Density‐independent and density‐dependent factors affecting temporal changes in spatial distributions of eastern Bering Sea flatfish

The general warming of the eastern Bering Sea (EBS) and the wide range of abundance exhibited by several eastern Bering Sea flatfish motivated an examination of how density‐dependent and density‐independent factors may influence the spatial distributions of EBS flatfish. In this study, EBS trawl survey data from 1982 to 2006 were used to examine how temporal changes in the distributions of six flatfish species groups [yellowfin sole (Limanda aspera), rock sole (Lepidopsetta sp.), flathead sole (Hippoglossoides sp.), Alaska plaice (Pleuronectes quadrituberculatus), arrowtooth flounder (Atheresthes sp.), and Greenland turbot (Reinhardtius hippoglossoides)] are related to temporal changes in the location of the ‘cold pool’ (bottom water < 2°C), and how the area occupied by flatfish are related to the cold pool and population abundance. Rock sole and flathead sole distributions have generally moved northwest since 1982 and are significantly correlated with the movement of the cold pool, whereas arrowtooth flounder avoid the cold pool and their area occupied is inversely related to the size of the cold pool. The area occupied by arrowtooth flounder and rock sole are also significantly related to stock abundance. Multivariate statistical models indicate that the location of rock sole is more strongly related to stock abundance than to the cold pool, whereas the area occupied by arrowtooth flounder is more strongly related to the area of the cold pool rather than abundance. The temperatures occupied by several flatfish stocks indicate a substantial variability in suitable temperatures. These results suggest that a complex suite of density‐dependent and density‐independent factors may determine the response of EBS flatfish spatial distributions to increasing temperatures.     

Causes of walleye pollock (Theragra chalcogramma) recruitment decline in the northern Sea of Japan: implications for stock management

Recruitment of the northern Japan Sea stock (JSS) of walleye pollock has been decreasing since around 1990. In this study, I analyzed the factors causing this decrease in recruitment by investigating the relationship between recruitment, spawning stock biomass (SSB) and environmental factors using a generalized additive model (GAM). GAM fit to the data showed the importance of SSB, sea surface temperature (SST), ocean current strength (Tsushima Warm Current) and wind intensity (Asian monsoon) in determining the recruitment. Of these, the relationship between SSB and recruitment was positive and not negatively density‐dependent. On the other hand, the recruitment was negatively related to SST and ocean current strength, and a dome‐shaped relationship was observed between wind intensity and recruitment. Since around 1990, the values of SST and ocean current strength have mostly been high and that of wind intensity mostly low. In addition, SSB has been decreasing since the late 1990s. It is likely that the recruitment decline of JSS after approximately 1990 has been caused by warm water temperature, strong Tsushima Warm Current and weak Asian monsoon, and that the recent decrease in SSB has amplified this recruitment decline. According to the model’s estimation, a recruitment recovery due to environmental improvement will be highly restricted as long as SSB remains at its current low level. Significant recovery of SSB is urgently needed for JSS.     

Temperature-dependent stock-recruitment model for walleye pollock (Theragra chalcogramma) around northern Japan

Changes in fish year‐class strength have been attributed to year‐to‐year variability in environmental conditions and spawning stock biomass (SSB). In particular, sea temperature has been shown to be linked to fish recruitment. In the present study, I examined the relationship between sea surface temperature (SST), SSB and recruitment for two stocks of walleye pollock (Theragra chalcogramma) around northern Japan [Japanese Pacific stock (JPS) and northern Japan Sea stock (JSS)] using a temperature‐dependent stock‐recruitment model (TDSRM). The recruitment fluctuation of JPS was successfully reproduced by the TDSRM with February and April SSTs, and February SST was a better environmental predictor than April SST. In addition, the JPS recruitment was positively related to February SST and negatively to April SST. The JSS recruitment modeled by the TDSRM incorporating February SST was also consistent with the observation, whereas the relationship between recruitment and February SST was negative, that is the opposite trend to JPS. These findings suggest that SST in February is important as a predictor of recruitment for both stocks, and that higher and lower SSTs in February act favorably on the recruitment of JPS and JSS respectively. Furthermore, Ricker‐type TDSRM was not selected for either of the stocks, suggesting that the strong density‐dependent effect as in the Ricker model does not exist for JPS and JSS. I formulate hypotheses to explain the links between SST and recruitment, and note that these relationships should be considered in any future attempts to understand the recruitment dynamics of JPS and JSS.     

Coupling hydrodynamic and individual‐based models to simulate long‐term larval supply to coastal nursery areas

For many marine fish species, recruitment is strongly related to larval survival and dispersal to nursery areas. Simulating larval drift should help assessing the sensitivity of recruitment variability to early life history. An individual‐based model (IBM) coupled to a hydrodynamic model was used to simulate common sole larval supply from spawning areas to coastal and estuarine nursery grounds at the population scale in the eastern Channel on a 14‐yr time series, from 1991 to 2004. The IBM allowed each particle released to be transported by currents from the hydrodynamic model, to grow with temperature, to migrate vertically giving stage development, and possibly to die according to drift duration, representing the life history from spawning to metamorphosis. Despite sensitivity to the larval mortality rate, the model provided realistic simulations of cohort decline and spatio‐temporal variability of larval supply. The model outputs were analysed to explore the effects of hydrodynamics and life history on the interannual variability of settled sole larvae in coastal nurseries. Different hypotheses of the spawning spatial distribution were also tested, comparing homogeneous egg distribution to observation and potential larval survival (PLS) maps. The sensitivity analyses demonstrated that larval supply is more sensitive to the life history along larval drift than to the phenology and volume of spawning, providing explanations for the lack of significant stock–recruitment relationship. Nevertheless, larval supply is sensitive to spawning distribution. Results also suggested a very low connectivity between supposed different sub‐populations in the eastern Channel.     

Spatial and temporal patterns in summer ichthyoplankton assemblages on the eastern Bering Sea shelf 1996–2007

Larval and early juvenile fishes were sampled from the eastern Bering Sea (EBS) shelf from 2001 to 2005, and in 2007. Data from these collections were used to examine spatial and temporal patterns in species assemblage structure and abundance. The years 2001–2005 were unusual because the EBS water temperature was ‘warm’ compared with the long‐term mean temperature. In contrast, 2007 was a ‘cold’ year. The abundance of the five most numerous taxa at 12 stations common to all years sampled (1996–2005, 2007) were significantly different among years. Larval and early juvenile stage Theragra chalcogramma (walleye pollock), a commercially important gadid, were by far the most abundant fish in all years. Bottom depth alone best explained assemblage structure in most years, but in others, bottom depth and water column temperature combined and percent sea‐ice coverage were most important. Abundance of T. chalcogramma larvae increases with water column temperature until 5°C and then becomes level. Higher abundances of Gadus macrocephalus (Pacific cod) larvae occur in years with the greatest percent sea‐ice cover as indicated by GAM analysis. Larvae of Lepidopsetta polyxystra (northern rock sole) increase in abundance with increasing maximum wind speed, but decrease at a later date during the last winter storm. The data are consistent with the hypothesis that oceanographic conditions, specifically water temperature and sea‐ice coverage, affect the spatial and temporal pattern of larval abundances. In general, ichthyoplankton species assemblages can be important early indicators of environmental change in the Bering Sea and potentially other subarctic seas as well.     

Migrations and hydrography determine the abundance fluctuations of blue whiting (Micromesistius poutassou) in the Barents Sea

The Barents Sea is the north‐eastern fringe of the distribution of blue whiting (Micromesistius poutassou). Fluctuations in distribution and abundance of blue whiting in the area have been marked. Two hypotheses are put forward to explain these fluctuations. First, rich year classes in the main Atlantic stock of blue whiting may contribute to increased abundance in the Barents Sea. Second, variations in hydrography, such as influx of warm Atlantic water, may be particularly important in this fringe area. We investigated these hypotheses using data from bottom trawl surveys conducted during the period 1981–2006. Variations in abundance (measured either as incidence or density) and distribution were correlated with recruitment in the Atlantic stock of blue whiting as well as hydrographic conditions. Regression analyses indicated that the abundance fluctuations are primarily determined by variations in recruitment of Atlantic blue whiting, a strong year class leading to high abundance in the Barents Sea the year after spawning. However, salinity anomaly in the Fugløya–Bear Island transect during the previous year, an indicator of high inflow of Atlantic water, had also a significant, positive effect. Thus, the data suggested a climatic modulation of dynamics that were primarily determined by recruitment of blue whiting in the main Atlantic stock. Analyses of size structure as well as earlier studies on population genetics supported this conclusion.     

Stock–environment–recruitment models for North Atlantic albacore (Thunnus alalunga)

Different stock–recruitment models were fitted to North Atlantic albacore (Thunnus alalunga) recruitment and spawning stock biomass data. A classical density dependence hypothesis, a recent environmental‐dependence hypothesis and a combination of both were considered. For the latter case, four stock–environment–recruitment models were used: Ricker, Beverton‐Holt, Deriso's General Model (modified to take into account environmental effects) and conditioned Neural Networks. Cross‐validation analysis showed that the modified Deriso model had the best predictive capability. It detected an inverse effect of the North Atlantic Oscillation (NAO) on recruitment, a Ricker‐type behaviour with density dependent overcompensation when environmental conditions are unfavourable and a Beverton–Holt‐type behaviour towards an asymptotic recruitment carrying capacity with favourable environmental conditions. The Neural Network model also detected that under favourable environmental conditions high spawning stock biomass does not necessarily have a depensatory effect on recruitment. Moreover, they suggest that under extremely favourable environmental conditions, albacore recruitment could increase well above the asymptotic carrying capacity predicted by Beverton–Holt‐type models. However, the general decrease in spawning stock biomass in recent years and increasing NAO trends suggest that there is low probability of exceptionally large recruitment in the future and instead there is a danger of recruitment overfishing.     

Oceanographic drivers of sablefish recruitment in the California Current

Oceanographic processes and ecological interactions can strongly influence recruitment success in marine fishes. Here, we develop an environmental index of sablefish recruitment with the goal of elucidating recruitment‐environment relationships and informing stock assessment. We start with a conceptual life‐history model for sablefish Anoplopoma fimbria on the US west coast to generate stage‐ and spatio‐temporally‐specific hypotheses regarding the oceanographic and biological variables likely influencing sablefish recruitment. Our model includes seven stages from pre‐spawn female condition through benthic recruitment (age‐0 fish) for the northern portion of the west coast U.S. sablefish stock (40°N–50°N). We then fit linear models and use model comparison to select predictors. We use residuals from the stock‐recruitment relationship in the 2015 sablefish assessment as the dependent variable (thus removing the effect of spawning stock biomass). Predictor variables were drawn primarily from ROMS model outputs for the California Current Ecosystem. We also include indices of prey and predator abundance and freshwater input. Five variables explained 57% of the variation in recruitment not accounted for by the stock‐recruitment relationship in the sablefish assessment. Recruitment deviations were positively correlated with (i) colder conditions during the spawner preconditioning period, (ii) warmer water temperatures during the egg stage, (iii) stronger cross‐shelf transport to near‐shore nursery habitats during the egg stage, (iv) stronger long‐shore transport to the north during the yolk‐sac stage, and (v) cold surface water temperatures during the larval stage. This result suggests that multiple mechanisms likely affect sablefish recruitment at different points in their life history.     

Multispecies biomass dynamics models reveal effects of ocean temperature on predation of juvenile pollock in the eastern Bering Sea

Walleye pollock (Gadus chalcogrammus) supports one of the largest commercial fisheries in the world. Juvenile pollock are important forage fish in the eastern Bering Sea (EBS) ecosystem, often representing the largest fraction in the diets of major Bering Sea piscivores. Large variability in the EBS pollock stock biomass in recent years has been attributed primarily to fluctuations in recruitment. It has been hypothesized that predation rates on forage fishes increase when the cold pool (a body of cold water < 2°C) is extensive and covers much of the middle continental shelf, which tends to concentrate larger predatory fishes in the outer shelf and slope regions. In contrast, young pollock appear to tolerate colder temperatures than older fish and can stay in the cold pool, thereby reducing predation. We used a multispecies modeling approach to examine the effects of the cold pool size on predation of juvenile pollock. We found that predation on age‐1 pollock by age‐3+ pollock decreased, and predation on age‐1 and age‐2 pollock by arrowtooth flounder increased with increasing bottom temperature, which was used as a proxy for the cold pool size. These results suggest that the cold pool creates spatial separation between juvenile pollock and arrowtooth flounder, but not between adult and juvenile pollock. The model developed in this study could be used to examine the effects of other covariates on interspecific interactions, help explain observed changes in fish communities, and understand implications of climate change on ecosystems and their productivity.     

Comparison of factors affecting recruitment variability of walleye pollock Theragra chalcogramma in the Pacific Ocean and the Sea of Japan off northern Japan

The Japanese Pacific stock (JPS) and the northern Japan Sea stock (JSS) of walleye pollock Theragra chalcogramma are mainly distributed in the Pacific Ocean and the Sea of Japan off northern Japan, respectively. This paper summarizes and compares the factors affecting the recruitment variability of these two stocks. Spawning season is from December to March for both stocks. JPS recruitment has a positive relationship with the water temperature in January and February, whereas that of JSS has a negative relationship with the water temperature in January, February, and April. One possible reason for this is that pollock larvae have an optimum growth temperature of approximately 5 °C in the field. Drift of early life stages also appears to be an important influence on the recruitment of both stocks. Because the current generated by the northwest wind carries eggs of JPS into the main larval nursery ground, JPS recruitment is enhanced in years when the northwest wind is predominant in February. On the other hand, early life stages of JSS are transported into the nursery ground by the Tsushima Warm Current. However, this current also carries early life stages into the Sea of Okhotsk and offshore, resulting in poor JSS recruitment in years when this current is strong in March. In contrast to JPS, the recruitment of which is significantly impacted by cannibalism, young pollock have not been found in the stomachs of adult JSS. Warm temperatures in the Sea of Japan seem to induce the separation of young and adult pollock, and the shape of the stock–recruitment relationship also suggests that cannibalism is not important for JSS. Based on this knowledge, and on the hatch date distributions of larvae and juveniles, we propose mechanisms that can explain the recruitment fluctuations for JPS and JSS pollock.     

The influence of life history dynamics and environment on the determination of year class strength in North Sea herring (Clupea harengus L.)

The inter-annual variability in year class strength (1976–2000) of North Sea herring (Clupea harengus) was investigated using Paulik diagrams based on survey data and Virtual Population Analysis. The herring life cycle was split into five stages: spawning stock biomass (SSB), egg production, larvae, fish with 0 winter rings on the otolith (0-wr), 1-wr and 2-wr. Surveys were used as indices and Paulik analysis revealed relationships between stages. In 80% of the years, year class strength reflected SSB. Poorer than expected year classes were determined during the larva to 0-wr phase, whilst stronger than expected year classes were apparently determined during the 0-wr to 1-wr stage. There was no clear relationship between survival of young stages of herring and the abundance of Calanus finmarchicus but the year class strength of 0-wr and 1-wr had a negative relationship to bottom water temperature. Lower sea water temperatures in the North Sea are associated with higher Calanus abundance. The analysis shows that the strength of aberrant year classes of North Sea herring is determined between the pelagic larval and the juvenile stages.     

Modelling recruitment dynamics of hake, Merluccius merluccius, in the central Mediterranean in relation to key environmental variables

Hake recruitment has been examined in relation to environmental variables in two of the main reproductive areas of the central Mediterranean, the northern and central Tyrrhenian Sea. Seventeen years time series data from trawl surveys revealed high fluctuations in recruit abundance that could not be just explained by spawning biomass estimations. Generalized additive models were developed to investigate hake recruitment dynamics in the Tyrrhenian Sea in relation to spawner abundance and selected key oceanographic variables. Environmental data were explored in attempt to explain survival processes that could affect early life history stages of hake and that accounted for high fluctuations in its recruitment.Thermal anomalies in summer, characterised by high peaks in water temperature, revealed a negative effect on the abundance of recruits in autumn, probably due to a reduction in hake egg and larval survival rates. In the northern Tyrrhenian, recruitment was reduced when elevated sea-surface temperatures were coupled with lower levels of water circulation. Enhanced spring primary production, related to late winter low temperatures could affect water mass productivity in the following months, thus influencing spring recruitment. In the central Tyrrhenian a dome-shaped relationship between wind mixing in early spring and recruitment could be interpreted as an “optimal environmental window” in which intermediate water mixing level played a positive role in phytoplankton displacement, larval feeding rate and appropriate larval drift. Results are discussed in relation to the decline in hake stock biomass and within the present climate change and global warming context.     

Using a biophysical model to investigate connectivity between spawning grounds and nursery areas of King George whiting (Sillaginodes punctatus: Perciformes) in South Australia’s gulfs

Many demersal marine fish species depend on a dispersive larval stage that connects geographically discrete sub‐populations. Understanding connectivity between these sub‐populations is necessary to determine stock structure, which identifies the appropriate spatial scale for fishery management. Such connectivity is poorly understood for King George whiting (Sillaginodes punctatus; Perciformes) in South Australia's gulf system, even though spawning grounds and nursery areas are adequately defined. In response to declines in commercial catches and estimated biomass, this study aimed to determine the most important spawning grounds and nursery areas to recruitment, and the connectivity between them. A biophysical model was seeded with particles according to the distribution and density of eggs throughout the spawning area in 2017 and 2018. Despite inter‐annual differences in the origins of particles, dispersal pathways and predicted settlement areas remained consistent between years. Predicted settlement was generally highest to nursery areas only short distances from regional spawning grounds, consistent with previous hydrodynamic models. However, the model also predicted that spawning in one region could contribute to recruitment in an adjacent region later in the spawning season, which aligned with the breakdown of thermohaline fronts at the entrance of each gulf. The connectivity between spawning grounds and nursery areas predicted by the model is supported by spatio‐temporal patterns in the otolith chemistry of pre‐flexion larvae and settled juveniles. Consequently, the most parsimonious explanation is that the populations of King George whiting in South Australia's gulf system constitute a single, panmictic stock, which has implications for fishery management.