Hake Stock Research Articles

Deepening our understanding of the environmental influences on the reproductive dynamics of the main fishery stock of Argentine hake, Merluccius hubbsi, in the southwestern Atlantic Ocean

AbstractThis study analyzed the inter‐annual variability in the reproductive dynamics of the Argentine hake from Patagonian waters during the last 20 years concerning the environmental conditions and the nutritional status of females. Macroscopic and histological information on gonadal maturity, female condition factor (K), and oceanographic data at different sampling sites throughout the studied region were obtained from research surveys carried out during the main spawning season of this species. Four stages, according to the maturity ovarian phases composition, were assigned annually at each sampling site to assess reproductive dynamics: vitellogenesis, ovulation, regression, and resting. The spatial pattern of inter‐annual variability in reproductive dynamics was described, and the influence of temperature and female condition on the probability of ovulation and regression was assessed using generalized linear models. The area with the lowest inter‐annual variability in the hake reproductive dynamics was observed between 43 and 45°S (50 to 80 m depth) in relation to ovulation events. The vitellogenesis stage was more frequently found adjacent to the reproductive area, whereas the regression was mainly observed in deeper waters. Both bottom and surface temperatures explained the ovulation and regression stages, showing a positive and negative relationship, respectively. Lower values of the K factor, associated with low food availability, were further linked to the occurrence of regression. Finally, it is worth noting that the relationship observed between the ovulation/spawning dynamics of Argentine hake and water temperature suggests that the reproductive success of this species could be affected in the future scenario of climate change.

Enhancing the utility of known-biomass production models: a case study of the Bay of Biscay and Iberian Coast ecoregion

Our general purpose is to support the use of known-biomass production models (KBPMs), illustrating their usefulness by addressing the evolution of surplus production (SP) over time and the factors affecting it (e.g. environment). We also demonstrate the utility of KBPMs for multispecies management objectives or for estimating maximum sustainable yield reference points without a stock recruitment function, among other worthwhile applications. To do so, we present different uses of KBPMs, illustrating their application on demersal species in the International Council for the Exploration of the Sea (ICES) area, specifically for megrim, white anglerfish and European hake stocks. The proposed analytical approach involved fitting single-species and multispecies KBPMs, conducting retrospective analyses and assessing the effects of environmental variability on SP. The findings show that, in general, stock SP increased after a decline in biomass and SP, except for white anglerfish in the southern area. Megrim stocks are the least productive, while hake and northern anglerfish are the most productive. Retrospective analysis revealed SP shifts in northern hake stock for reasons other than biomass variability. Hence, the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation (AMO), two key climate variability modes in the North Atlantic, were tested for their links to SP, revealing a positive connection between SP and AMO, although further research is necessary. Beyond the specific results of our particular KBPM application, our main conclusion is that KBPMs can serve as a tool complementary to more complex assessment models for resolving unaddressed issues and crosschecking available assessment results.

A novel statistical approach to deal with spatial bias in maturity ogive estimation

The proportion of mature fish at length is one of the most important population attributes when evaluating reproductive potential for fish stock assessment purposes. Bias in maturity ogive parameters can lead to fishery management decisions based on misspecified biological reference points. These parameters can vary spatially and temporally, and this variability should be understood and included in the assessment models. However, integrating this variability becomes challenging when specific spatial-dependent ogives cannot be used in the stock assessment model. Hence, this study proposes a novel use of a multivariate response Bayesian regression model, employing an integrated nested Laplace approximation to estimate a single global maturity ogive using data from various spatial areas. This model explicitly accounts for differences in the sampling process and combines information from different areas to estimate shared maturity ogive parameters using joint-likelihood procedures. The model is applied to the European hake stock in ICES (International Council for the Exploration of the Sea) Divisions 27.8.c and 27.9.a, serving as a practical guide. In this model, we have considered different predictors to handle the relationship between the probability of being mature and the length and year covariates. Our results suggest that the logistic formulation correctly captures the relationship between the probability of being mature and length. For year variability, including a year factor covariate or year random effect in the predictor model produces similar values of goodness of fit measures.

What the Hakes? Correlating Environmental Factors with Hake Abundance in the Gulf of Maine

According to the latest estimates, the Gulf of Maine is currently warming faster than 99% of the world’s oceans. As a result, this region has become an ideal location for research into the effects that warming has on the historical fisheries that make up this ocean basin. Both white hake (Urophycis tenuis) and red hake (Urophycis chuss) are common Gulf of Maine groundfish species, distributed both inshore and offshore. While these two species are closely related phycid hakes, white hake stocks are recognized in the Gulf of Maine as rebuilding, while red hake are above target biomass levels. As a species commonly found throughout the Gulf of Maine that prefers cooler waters (4–12°C), we hypothesize the effects of climate change might influence stock behavior, such as changes in species distribution. We used generalized additive models (GAMs) to describe the relationship between hake abundance and environmental conditions using bottom temperature, bottom salinity, depth, and catch data contributed by the Maine Department of Marine Resources during their Maine – New Hampshire Inshore Trawl Surveys of the last 22 years (2000 – 2021). Our results reveal species-specific preferences for bottom temperature (white hake ~9 to ~13℃, red hake < 12℃) and depth (white hake ~55 to ~100m, red hake > ~65m), with no significant correlation to bottom salinity. Spatially over time, white hake abundance displayed a gradual center of gravity northward, while red hake rapidly increased inshore. Overall, these results highlight species-specific density changes in inshore distribution, consistent with previous studies, with considerable implications on future management strategies in this region.

Sustainable management of a new fishing stock: Bioeconomic approach under biological and market uncertainty

Potential and emerging fisheries create challenges and opportunities for fishery managers who need to decide how to sustainably manage a fishery that does not yet exist. A new Pacific hake stock off the west coast of Mexico has been identified. The Mexican government is interested in assessing the feasibility of a new commercial fishery. This work proposes and analyzes alternative potential fishery management measures for this possible new fishery under biological and market uncertainties. Results indicate that a new fishery could be biologically sustainable and economically profitable under a set of management strategies and control rules. A limited access strategy with low effort is recommended because it is most profitable per vessel and biologically cautious, considering the high uncertainty associated with the exploitation of an unfished stock. Despite the combination of high operating costs and low prices the fishery could still be profitable in the long-term, although there is risk of overexploitation if high fishing effort is allowed. Nevertheless, our results suggest low risk of fishing down the dwarf hake stock if the fishery is managed under low effort levels, allowing for sufficient opportunities for data collection and adaptive management. This work addresses the rare opportunity of assessing a fishery previous to its operation, with all the associated data limitations and uncertainty.

Stage‐specific drivers of Pacific hake (Merluccius productus) recruitment in the California Current Ecosystem

AbstractUnderstanding environmental drivers of recruitment variability in marine fishes remains an important challenge in fish ecology and fisheries management. We developed a conceptual life‐history model for Pacific hake (Merluccius productus) along the west coast of the United States and Canada to generate stage‐specific and spatiotemporally‐specific hypotheses regarding the oceanographic and biological variables that likely influence their recruitment. Our model included seven life stages from pre‐spawning female conditioning through pelagic juvenile recruitment (age‐0 fish) for the coastal Pacific hake stock. Model‐estimated log recruitment deviations from the 2020 hake assessment were used as the dependent variable, with predictor variables drawn primarily from a regional ocean reanalysis for the California Current Ecosystem. Indices of prey and predator abundance were also included in our analysis, as were predictors of local‐ and basin‐scale climate. Five variables explained 59% of the recruitment variability not accounted for by the stock–recruitment relationship in the hake assessment. Recruitment deviations were negatively correlated with May–September eddy kinetic energy between 34.5° and 42.5°N, the North Pacific Current Bifurcation Index, and Pacific herring (Clupea pallasii) biomass during the spawner preconditioning stage, alongshore transport during the yolk‐sac larval stage, and the number of days between storm events during the first‐feeding larval stage. Other important predictors included upwelling strength during the preconditioning stage, the number of calm periods during the first‐feeding larval stage, and age‐1 hake predation on age‐0 pelagic juveniles. These findings suggest that multiple mechanisms affect Pacific hake survival across different life stages, leading to variability in population‐level recruitment.

Investigating the Applicability of Ichthyoplanktonic Indices in Better Understanding the Dynamics of the Northern Stock of the Population of Atlantic Hake Merluccius merluccius (L.)

Abundance indices are essential data for the application of stock assessment models to obtain fish abundance estimates. Abundance indices have usually been derived from fishery-dependent data, yet the increase in fisheries-independent surveys is now offering new opportunities for these calculations. In this study, we explored the usefulness of ichthyoplankton indices derived from scientific surveys in estimating spawning biomass. In addition, we also investigated whether the strength of the year–class of the commercial cohort of Atlantic hake, as a determinant, could be defined at an early life stage. We used samples collected during the triennial mackerel and horse mackerel egg surveys (MEGS), which cover the hake spawning area in the Bay of Biscay. The biomass indices were determined as the abundance of eggs in the early development stage (stage 1) when transformed into egg production (EP) from 1995 to 2019 in the months of March and April—which is considered a period of high spawning activity for hake in this area. Additionally, we built a metric for larval abundance and converted larval length into age. This was in addition to constructing a pre-recruit year-class index (YCI) while using the EVHOE bottom trawl abundance database for hake for the period of 1997 to 2016. The results of regression analysis of egg production and spawning stock biomass indicate that both parameters are significantly correlated (r = 0.76). By connecting the abundance of eggs and larvae in the adjoining stages, we are able to identify two periods of high mortality associated with the transition from “yolk-sac-first” to “feeding larvae” and “late larvae-YCI10”, but we were unable to discover when the strength of the recruitment year–class is determined. As such, it appears that for the northern stock of hake, recruitment is established in the late juvenile stages.

Assessing changes in size at maturity for the European hake (Merluccius merluccius) in Atlantic Iberian waters

European hake (Merluccius merluccius) is a commercially important resource in Iberian Atlantic waters. Despite the recovery plan implemented in 2006 and the multiannual management plan for western waters, fishing mortality is still higher than that corresponding to the maximum sustainable yield for the southern European hake stock. The biological processes underlying the dynamics of this stock and its life history traits are essential for assessing population productivity and resilience, making them basic information for management. We analysed the temporal variability of size at maturity (L50) of this species and the main factors influencing it in Atlantic Iberian waters from 1982 to 2019. The annual variability of L50 for each sex was modelled with generalized additive models, considering explanatory environmental variables (Atlantic Multidecadal Oscillation, North Atlantic Oscillation and sea surface temperature) and biological variables (biomass, spawning biomass at length and relative condition factor). The results showed that the L50 of males decreased by a total of 12.9 cm and L50 of females decreased by a total of 10.9 cm from 1982 to 2019. For females the significant explanatory variables were year, spawning biomass at length, biomass and the North Atlantic Oscillation, while for males only year was an explanatory variable. These results are important for understanding the status of the European hake population, signalling that L50 is a good indicator for predicting future population dynamics.

The Aegean Sea demersal fishery under four climatic and socio-political futures

The future of the Aegean Sea demersal fishery is projected under four climatic and socio-political scenarios, namely: “World Markets“, “Global Sustainability”, “National Enterprise” and “Local Stewardship”. The scenarios combine two IPCC climatic futures (RCP4.5 and RCP8.5) with four socio-political systems that foresee changes in economic, technological as well as management aspects of the fishery corresponding to four contrasting futures. The future projection of a large number of physico-chemical variables per climatic scenario, made available through CERES project (http://ceresproject.eu/), gave the opportunity to model the effects of Climate Change (CC) on the productivity of demersal stocks. The projections foresee an increase in primary productivity in the Aegean Sea waters, which is projected to become mesotrophic from currently oligotrophic, while statistical analysis suggests that mullet stocks might be favored, while the European hake stock is more likely to be in decline. The combined, as well as the individual, effects of the different components of change on the fishery are explored. With respect to management, the results demonstrate the key role of technical interactions in the attempt to achieve sustainable and optimally harvested fisheries. It is found that a minor reduction of effort can achieve sustainability and good yield (PGY) from key stocks. A main source of uncertainty stems from the large number of stocks that remain unassessed. The ongoing tropicalization of the Aegean Sea and the response of these other stocks to CC increases the uncertainty and creates new challenges but also new opportunities for the Aegean Sea demersal fishery.

European hake (Merluccius merluccius) stock structure in the Mediterranean as assessed by otolith shape and microchemistry

The European hake Merluccius merluccius is the third most valuable species for the North-East Atlantic and the Mediterranean fisheries. European hake has been rated as overexploited in the Mediterranean, thus careful management is advisable. Mediterranean hake is well-differentiated from Atlantic hake, but sub-population structure within the Mediterranean, and how this structure could be translated into stocks (operative management units), is still an elusive topic. Otolith shape and chemistry (concentration of trace elements) have been systematically used to distinguish fish stocks. Our aim was therefore to assess the discrimination capability (assigning fish to the correct geographical unit) of otolith shape and microchemistry at two geographical scales within the Mediterranean: (1) the official geographical subareas (GSAs), and (2) three larger units previously suggested by genetic markers (i.e., Western Mediterranean, Adriatic with Central Mediterranean, and Eastern Mediterranean). Two complementary analyses were completed because shape is more easily analyzed than chemistry. First, a large sample of juvenile hake (n = 1656) from 40 Mediterranean GSAs subunits was used for shape analysis. Second, a subsample of those fish (n = 154) from 10 GSAs was analyzed for both otolith shape and microchemistry. Irrespective of the type of data (shape and/or chemistry) and geographical scale (GSAs versus the 3-units), between-unit differences were always statistically significant. However, according to the large within-unit variability, discrimination capability was always poor but better at the GSA scale, and even better when both shape and microchemistry were combined. Moreover, unsupervised clustering methods (the number and limits of the units are data-driven and not a priori defined as above) failed to find an optimal structure. Overall, these results are fully compatible with the hypothesis of a continuous gradient, within which discrete spatial units cannot be safely recognized.

Exploring Changes in Fishery Emissions and Organic Carbon Impacts Associated With a Recovering Stock

International objectives for sustainable development and biodiversity conservation require restoring fish populations to healthy levels and reducing fishing impacts on marine ecosystems. At the same time, governments, retailers, and consumers are increasingly motivated to reduce the carbon footprint of food. These concerns are reflected in measures of the EU Common Fisheries Policy (CFP) and the CFP Reform Regulation, which highlighted a need to move from traditional single-stock management toward an ecosystem approach to fisheries management (EAF). Using publicly available landings and effort data combined with estimates of adult population biomass, we develop methods to explore the potential for lowering emissions intensity and impacts on organic carbon stocks through ending overfishing and rebuilding stocks. We use the recent recovery of European hake (Merluccius merluccius) stocks in the Northeast Atlantic as a case study. With a focus on the hake fisheries of France, Spain, and the United Kingdom, we compare 2008 and 2016 fishing years. We make an initial estimate of the influence of changing stock status on greenhouse gas emissions during the fishery phase from fuel use and investigate the potential disturbance of organic carbon in the ecosystem, specifically via identification of bottom trawling overlap with organic-rich muddy sediments, and directly on storage in hake biomass. Our findings indicate that recovery of the hake stock was associated with reductions in overall emissions intensity from fuel and proportional impact on hake populations, however, total emissions from both fuel and landings increased, as did likely disturbance of sedimentary organic carbon in surface sediments due to benthic trawling. Ultimately, the aims of this analysis are to further explore the climate impacts of fisheries and overfishing, and to inform development of EAF in the EU.

Effects of a fishing closure area on the structure and diversity of a continental shelf fish assemblage in the NW Mediterranean Sea

Bottom trawling is the most extensive fishing activity affecting the continental shelf in Mediterranean waters. This gear has caused negative effects on the communities and topography of the seafloor. Temporal or spatial fishing closures have been proposed as strategies to reduce the disturbances caused by overfishing and for biodiversity recovery and restoration of ecosystems. The present study used various indicators to analyze and compare the differences between the demersal fish assemblages in a fishing closure area (FCA) established by the fishers of the Roses port (NW Mediterranean) and those on a fishing ground (FG) to assess the efficiency of this strategy two years after the cessation of fishing. Our findings demonstrated a noticeable increase in the abundance and biomass of all species in the FCA, especially species of small and medium size. Thus, our findings demonstrated that there were detectable shifts in the community (composition, rank abundance plots, ABC curves and diversity metrics) in a short time, evidencing slight disturbance effects on ecosystems. The present study also showed positive effects on the population structure, which had an increase in larger individuals, although the pattern varied between species. In particular, the European hake stock showed an increase in recruits, and the presence of large adults supported the suitability of this protection measure. Consequently, long time periods are not necessary to perceive noticeable benefits in terms of biodiversity recovery and ecosystem restoration in some deep marine ecosystems, and monitoring from the first year of fishing cessation is very important.

Temperature, maternal effects and density-dependent processes during early life stages of Argentine hake as relevant recruitment drivers

We analyzed the annual variability in total egg production and recruitment of the Patagonian stock of Argentine hake since 1990 and correlated these indices with some environmental variables as temperature and larval density estimated in the spawning area. We used data from research surveys carried out between 2001 and 2018 during the reproductive peak of this stock and information from the assessment model. The optimal spawning area of Argentine hake was associated with a bottom thermal front, with temperature values between 9° and 13 °C. The study revealed that reproductive success is positively influenced by the oldest females of the stock and negatively influenced by the egg density in the spawning area. In the first case, a larger proportion of older spawning females (≥ 7 years-old) produces larvae with greater survival rates; while in the second case, a greater abundance of early larvae in a more limited optimum spawning area could produce a higher rate of mortality due to competition.

The recruitment dynamics of the nylon shrimp Heterocarpus reedi, effects of climate and predation off Chile

Climate variability and predation influence the fluctuations in the recruitment of exploited marine populations. This study analyses the dynamics of the recruitment of the nylon shrimp (Heterocarpus reedi Bahamonde 1955) over the period 1968 and 2015, considering the influence of climate variability and the biomass of hake (Merluccius gayi), as a proxy for predation in its distribution area. We collected the Humboldt Current Index (HCI) and Southern Oscillation Index (SOI) time series as climate variables and estimates of recruitment and spawning biomass of nylon shrimp, as well as biomass of hake. Annual deviations in nylon shrimp recruitment showed increased sensitivity to climate variability and hake biomass, expressed through a significant cumulative correlation over time. The recruitment drives the spawning biomass, implying an environmentally driven shrimp recruitment dynamic. Generalized Linear Models showed that hake biomass and climate indices affect negative and positively the shrimp recruitment during the recruitment year, respectively. A path diagram with Structural Equation Models showed that SOI impacted HCI, negatively impacting the hake stock. Hake negatively affected the shrimp stock biomass and recruitment, and SOI either directly or indirectly, through its effect on the hake. The hake impact on the shrimp's spawning stock and recruitment is due to predation under concurrent climate variability on longer-term periods.

Long-Term Stability in the Trophic Ecology of a Pelagic Forager Living in a Changing Marine Ecosystem

Natural or human-induced environmental changes can modify the structure of ecological communities and thus alter food web interactions. After the collapse of hake stocks (Merluccius hubbsi) provoked by fisheries over the Patagonian shelf in 1997 profound changes have taken place in the community, including long-term dietary shifts in some marine vertebrate predators. Stable carbon (δ13C) and nitrogen (δ15N) isotope ratios in bone collagen of subadult and adult male South American fur seals (Arctocephalus australis) were measured for the period 1976-2017 to investigate if the changes occurred in the marine community from northern and central Patagonia affected the diet of this predator. Suess corrected δ13C values and δ15N values in bone collagen of fur seals did not change significantly over the study period. According to this, male fur seals have remained as pelagic foragers without changing their trophic position nor switching their main preys in the Patagonian food web over the last four decades. This long-term stability in the diet of fur seals contrasts with dietary changes reported for benthic foragers in the region. Although long-term diet studies are restricted to a few marine predators in the region, current evidence suggests that benthic-demersal foragers were more prone to dietary shifts than pelagic ones after perturbations that occurred in the marine community of northern and central Patagonia.

New age-determination method based on fast growth rate of Cape hake Merluccius capensis in Namibia

Namibia’s most important commercial fisheries resource, the shallow-water Cape hake Merluccius capensis, is currently assessed using statistical catch-at-age analysis. Age data obtained from otoliths constitute an important data component of this model. Recent age-validation studies of M. capensis showed that growth was previously underestimated. We investigated this new fast-growth hypothesis (FGH) by using measurements and counts of translucent zones (T1 to T14, from otolith core to edge) on two survey otolith samples covering the entire range of fish lengths. We compared three hypotheses of periodicity of otolith zone formation and show that, if all zones are counted, T2 (at 9.0 mm otolith length), T5, T8, T11 and T14 are most likely to be annuli. A conversion from the slow-growth-hypothesis (SGH, currently used) age data was calculated as: FGH age group = round (0.41[SGH age group] + 0.25), and this formula should be applied to compute and test updated catch-at-age data in a future hake stock assessment. Additional adjustment for the hake stock assessment following the FGH, such as the timing of recruitment in winter and catches in summer, should be considered in future assessments.

Mitigating unwanted catches in the southern Iberian hake stock fisheries: Improving fishing technology vs market control policies

Unwanted catches can be reduced by improving fishing effectiveness in targeting species and sizes and by banning their sale for human consumption. The landing obligation introduced by the European Union can be seen as a combination of these two measures, and the aim of this paper is to analyse its effects on the Southern Iberian Hake Stock fishery. To this end, reference points for a mixed fishery are computed under the two measures as the steady-state solution of a dynamic optimal management problem. Our results show that measures that improve selectivity obtain better results than sales ban strategies in terms of increasing yields and stocks and reducing discards. In particular, we find that reducing the selectivity parameters by 90% for the three early ages leads to an almost six-fold increase in the hake yield and lowers the discard rate by more than 20 percentage points. Banning the sale of the two youngest ages also increases hake yield by 21% and the discard rate by 7 percentage points.

A bayesian estimation of the economic effects of the Common Fisheries Policy on the Galician fleet: A dynamic stochastic general equilibrium approach

What would have happened if a relatively looser fisheries policy had been implemented in the European Union (EU)? Using Bayesian methods a Dynamic Stochastic General Equilibrium (DSGE) model is estimated to assess the impact of the European Common Fisheries Policy (CFP) on the economic performance of a Galician (north-west of Spain) fleet highly dependant on the EU Atlantic southern stock of hake. Our counterfactual analysis shows that if a less effective CFP had been implemented during the period 1986–2012, fishing opportunities would have increased, leading to an increase in labour hours of 4.87%. However, this increase in fishing activity would have worsened the profitability of the fleet, dropping wages and rental price of capital by 6.79% and 0.88%, respectively. Welfare would also be negatively affected since, in addition to the increase in hours worked, consumption would have reduced by 0.59%.

Response of Scotian Shelf silver hake (Merluccius bilinearis) to environmental variability

AbstractCommercially‐exploited fish populations are not only shaped by fishing pressure, but also by the dynamics of their environment. By quantifying the influence of environmental variability, fisheries management advice can be improved and uncertainties reduced. To this end, we developed statistical models of the response of Scotian Shelf silver hake stock metrics to variability in zooplankton community composition and phenology over the past 18 years and in the physical environment since 1985. Dominant modes of variability in these pelagic habitat indicators were characterized using principal component analysis, and the relationships of silver hake condition, abundance, and recruitment to pelagic habitat variability were assessed using generalized additive models. Condition was largely modulated by the onset and duration of the spring bloom, which controls food availability. In contrast, adult abundance was governed by composition of the zooplankton community and bottom‐water temperature, which dictates the distribution of silver hake. Finally, recruitment was affected by both thermal conditions and food availability. Our results presented here form the basis for qualitative assessment of ecosystem attributes and the influence on silver hake stock productivity.

Maternal condition, fecundity and oocyte quality of Argentine hake (Merluccius hubbsi) from the Northern stock

Abstract The influence of the condition of spawning females on fecundity and oocyte quality, measured using morpho-physiological and biochemical indices, was analyzed for the Northern stock of Argentine hake ( Merluccius hubbsi ). First the relationships between morpho-physiological and biochemical indices were analyzed, and if they were influenced by female size. Samples of spawning-capable females were collected during the main reproductive peak of this stock in May 2011. The hepatosomatic index (HSI) in M. hubbsi was a good indicator of lipid concentration for both the liver and muscle. HSI and the relative condition factor (Kn) were independent of female size. Muscle protein content was positively associated with maternal attributes. Relative fecundity ranged between 287 and 916 hydrated oocytes g −1 and showed significant positive relationships with GSI and Kn. The dry weight of hydrated oocytes (n = 100) ranged between 2.65 and 4 mg and was positively related with the HSI. The oil droplet diameter of hydrated oocytes varied between 250 and 292 μm and was also positively related with HSI, Kn, liver and muscle lipid content, suggesting that it could be a good indicator of the nutritional condition for hake females of the Northern spawning stock.