Stock Recruitment Research Articles

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.

Quirky patterns in time-series of estimates of recruitment could be artefacts

Abstract The accessibility of databases of global or regional stock assessment outputs is leading to an increase in meta-analysis of the dynamics of fish stocks. In most of these analyses, each of the time-series is generally assumed to be directly comparable. However, the approach to stock assessment employed, and the associated modelling assumptions, can have an important influence on the characteristics of each time-series. We explore this idea by investigating recruitment time-series with three different recruitment parameterizations: a stock–recruitment model, a random-walk time-series model, and non-parametric “free” estimation of recruitment. We show that the recruitment time-series is sensitive to model assumptions and this can impact reference points in management, the perception of variability in recruitment and thus undermine meta-analyses. The assumption of the direct comparability of recruitment time-series in databases is therefore not consistent across or within species and stocks. Caution is therefore required as perhaps the characteristics of the time-series of stock dynamics may be determined by the model used to generate them, rather than underlying ecological phenomena. This is especially true when information about cohort abundance is noisy or lacking.

Productivity dynamics of Atlantic cod

Productivity is a central determinant of population dynamics with consequences for population viability, resilience to exploitation, and extinction. In fish, the strength of a cohort is typically established during early life stages. Traditional approaches to measuring productivity do not allow for interannual variation in the maximum reproductive rate, a parameter governing population productivity. Allowing such process variation provides the ability to track dynamic changes instead of assuming a static productivity regime. Here we develop and evaluate a multivariate stock–recruitment state-space model to simultaneously estimate time-varying stock productivity and synchronicity of dynamics across populations. We apply the method to North Atlantic cod (Gadus morhua) populations, showing that the productivity of early life stages has varied markedly over time, with many populations at historically low productivity. Trends in productivity were similar in some adjacent populations but less regionally coherent than previously thought, particularly in the Northwest Atlantic. Latitudinal variation in the Northeast Atlantic suggests a differential response to environmental change. We conclude that time-varying productivity provides a useful framework that integrates across many dimensions of environmental change affecting early life history dynamics.

Recruitment and survival rate variability in fish populations: density-dependent regulation or further evidence of environmental determinants?

Recently, Minto et al. (2008) , based on a fishery data set including marine, estuarine, and freshwater fishes, described higher variability in the survival rates of juveniles at low rather than at high parental density in an inversely density-dependent fashion and suggested density-dependent mechanisms underpinning those patterns. This study, based on a long-term study of brown trout (Salmo trutta; a species and habitat not included in the Minto et al. (2008) analysis), documents that survival rates in these stream-living populations exhibit a pattern that matches exactly those reported by Minto et al. (2008) . Nevertheless, hypothesis testing rejected the occurrence of stock–recruitment relationships and the operation of density-dependent recruitment regulation. The patterns elucidated for these brown trout populations can be entirely explained by the operation of two single environmental factors, namely, stream discharge in March determining annual survival rates across streams and sites and site-specific depth determining site-specific survival rates. It is open to question that exactly the same patterns can be generated by two sets of opposing factors, density-dependent (i.e., Minto et al. 2008 ) and environmental factors (i.e., this study). The consistency of this pattern suggests that survival rates and recruitment are probably determined by environmental factors across fish populations and habitats.

Age-specific natural mortality rates in stock assessments: size-based vs. density-dependent

Abstract Natural mortality (M) rates are difficult to measure empirically and are often specified in stock assessments based on life history characteristics. More recently, these specifications have included M as a function of the size or age of a fish. However, natural mortality is a dynamic parameter that will change with the suite of predators and, thus, indirectly with cohort size and age. As an alternative, a density-dependent M rate function is derived and compared with the commonly used Lorenzen model, where M at age forms an allometric relationship with weight-at-age. The density-dependent model expresses M as a function of two parameters: one density dependent and one density independent. Properties of the two models (size-based vs. density-dependent) were explored to indicate conditions where the results are and are not similar. Associated catch equations, equilibrium analyses, and non-linear replacement lines in stock–recruitment theory are examined. Just as with density-independent values of M, most assessment data are not sufficient to provide precise estimates of density-dependent M parameters. However, the density-dependent model provides a basis for incorporating ecological variability into single-species assessments, noting the differing dynamics between short- and long-lived species. The incorporation of dynamic natural mortality has implications when estimating abundance trends and stock status, and ultimately setting management reference points.

Density-dependence can be revealed by modelling the variance in the stock–recruitment process: an application to flatfish

Abstract Recruitment success in marine species is mostly driven by the high and variable mortality of first life stages, and the relationships between stock and recruitment are then largely dominated by residual variability. We show that analysing the residual variability may provide insights on the density-dependence process occurring during the recruitment. Following the seminal formulation of Minto et al. (Survival variability and population density in fish populations. Nature, 2008), we show that when recruitment is considered as a sequence of a pelagic stage with stochastic density-independent mortality followed by a second stage with stochastic density-dependent mortality, then the variability of the recruitment rate per spawning biomass (RPSB) should be a decreasing function of the spawning biomass. Using stock–recruit data of 148 stocks from the RAM legacy database, we provide a test of this hypothesis by showing that the variability of RPSB is lower for fish species with the higher concentration during juvenile stages. Second, a hierarchical Bayesian model (HBM) is built to derive a meta-analysis of stock–recruit data for 39 flatfish stocks, characterized by a high concentration of juveniles in coastal nursery habitats. Results of the HBM show that the variance of the RPSB decreases with the spawning biomass for almost all stocks, thus providing strong evidence of density-dependence during the recruitment process. Finally, we attempt to relate patterns in recruitment variance to relevant life-history traits of flatfish species.

Reproductive potential and stock recruitment of the Caribbean and Brazilian metapopulations of the spiny lobster, Panulirus argus (Latreille, 1804)

The spiny lobsterPanulirus argus(Latreille, 1804) is the most important commercial fishing resource in the western central Atlantic and Brazil. Field studies covering the waters off southwestern Cuba and northern Brazil have improved our understanding of the variations in the reproductive potential (number of eggs), stock recruitment and reproductive efficiency of spiny lobsters according to location, depth and size class. Using the spawner-recruitment model, the reproductive potential index was correlated with the index of subsequent recruitment based on field sampling. Spiny lobster habitats in deeper waters need special attention in order to protect the species from overfishing of the recruitment. Considering the longevity and absence of reproductive senility in spiny lobsters, management strategies should ideally include the creation of spawner sanctuaries (marine protected areas) capable of restoring and maintaining the biomass of the spawning stock and the establishment of a maximum catch size of 135 mm (CL) for both sexes along the entire Brazilian coast. Based on our findings, we propose to establish spiny lobster sanctuaries (50-100 m) on the continental shelf off northern Brazil, from Amapá (5°25′N 51°0′W) to the western reaches of the coast of Pará (1°11′N 46°27′W, 0°42′N 46°45′W), covering a total surface area of 64 230 km2.

Accounting for escape mortality in fisheries: implications for stock productivity and optimal management

Few studies have considered the management implications of mortality to target fish stocks caused by non-retention in commercial harvest gear (escape mortality). We demonstrate the magnitude of this previously unquantified source of mortality and its implications for the population dynamics of exploited stocks, biological metrics, stock productivity, and optimal management. Non-retention in commercial gillnet fisheries for Pacific salmon (Oncorhynchus spp.) is common and often leads to delayed mortality in spawning populations. This represents losses, not only to fishery harvest, but also in future recruitment to exploited stocks. We estimated incidence of non-retention in Alaskan gillnet fisheries for sockeye salmon (O. nerka) and found disentanglement injuries to be extensive and highly variable between years. Injuries related to non-retention were noted in all spawning populations, and incidence of injury ranged from 6% to 44% of escaped salmon across nine river systems over five years. We also demonstrate that non-retention rates strongly correlate with fishing effort. We applied maximum likelihood and Bayesian approaches to stock-recruitment analyses, discounting estimates of spawning salmon to account for fishery-related mortality in escaped fish. Discounting spawning stock estimates as a function of annual fishing effort improved model fits to historical stock-recruitment data in most modeled systems. This suggests the productivity of exploited stocks has been systematically underestimated. It also suggests that indices of fishing effort may be used to predict escape mortality and correct for losses. Our results illustrate how explicitly accounting for collateral effects of fishery extraction may improve estimates of productivity and better inform management metrics derived from estimates of stock-recruitment analyses.

Comparative Recruitment Dynamics of Alewife and Bloater in Lakes Michigan and Huron

AbstractThe predictive power of recruitment models often relies on the identification and quantification of external variables, in addition to stock size. In theory, the identification of climatic, biotic, or demographic influences on reproductive success assists fisheries management by identifying factors that have a direct and reproducible influence on the population dynamics of a target species. More often, models are constructed as one‐time studies of a single population whose results are not revisited when further data become available. Here, we present results from stock recruitment models for Alewife Alosa pseudoharengus and Bloater Coregonus hoyi in Lakes Michigan and Huron. The factors that explain variation in Bloater recruitment were remarkably consistent across populations and with previous studies that found Bloater recruitment to be linked to population demographic patterns in Lake Michigan. Conversely, our models were poor predictors of Alewife recruitment in Lake Huron but did show some agreement with previously published models from Lake Michigan. Overall, our results suggest that external predictors of fish recruitment are difficult to discern using traditional fisheries models, and reproducing the results from previous studies may be difficult particularly at low population sizes.

A spatial age-structured model for describing sea lamprey (Petromyzon marinus) population dynamics

The control of invasive sea lampreys (Petromyzon marinus) presents large-scale management challenges in the Laurentian Great Lakes. No modeling approach has been developed that describes spatial dynamics of lamprey populations. We developed and validated a spatial and age-structured model and applied it to a sea lamprey population in a large river in the Great Lakes basin. We considered 75 discrete spatial areas, included a stock–recruitment function, spatial recruitment patterns, natural mortality, chemical treatment mortality, and larval metamorphosis. Recruitment was variable, and an upstream shift in recruitment location was observed over time. From 1993 to 2011, recruitment, larval abundance, and the abundance of metamorphosing individuals decreased by 80%, 84%, and 86%, respectively. The model successfully identified areas of high larval abundance and showed that areas of low larval density contribute significantly to the population. Estimated treatment mortality was less than expected but had a large population-level impact. The results and general approach of this work have applications for sea lamprey control throughout the Great Lakes and for the restoration and conservation of native lamprey species globally.

Biotic and Abiotic Factors Influencing Cisco Recruitment Dynamics in Lake Superior during 1978–2007

AbstractHistorically, the Cisco Coregonus artedi was abundant throughout the Great Lakes basin, but anthropogenic influences caused the collapse of stocks during the mid‐1900s, and fishery managers are currently exploring options for restoration. To increase understanding of biotic and abiotic factors influencing Cisco recruitment dynamics throughout the Great Lakes, we used Ricker stock–recruitment models with rearing‐habitat‐weighted indices of recruitment and adult spawning stock size in Lake Superior to identify and quantify (1) the appropriate spatial scale for modeling age‐1 recruitment dynamics and (2) the effects of biotic and abiotic factors on age‐1 recruitment dynamics within the regions identified for modeling. Cisco recruitment variation in Lake Superior was best described by a regional model with separate stock–recruitment relationships for western, southern, eastern, and northern regions. The spatial scale for modeling was approximately 260 km. Age‐1 recruitment was negatively correlated with adult spawning stock size in all four regions. Multifactor models suggested (1) a positive correlation between age‐1 recruitment and the interaction between wind speed and air temperature on a lakewide scale and (2) a negative correlation between age‐1 recruitment and the biomass of Rainbow Smelt Osmerus mordax on a regional scale. Large‐scale abiotic factors are beyond the control of fishery managers, so harvest of adult Ciscoes and potential predators and competitors should be carefully managed to achieve desired goals in Lake Superior. Although our study was limited to Cisco stocks in Lake Superior, we believe that our general findings can be more broadly applied (albeit with caution) to the restoration and management of remnant stocks throughout the Great Lakes basin.Received June 4, 2013; accepted August 16, 2013

Comparisons of meta-analytic methods for deriving a probability distribution for the steepness of the stock–recruitment relationship

The steepness parameter of the stock–recruitment relationship (the proportion of unfished recruitment when spawning biomass is reduced to 20% of its unfished level) is a key parameter in stock assessment models, and hence in the provision of scientific management advice for many fisheries. Prior probability distributions for steepness have been used when conducting assessments of US west coast groundfish in the absence of data to estimate steepness reliably. These priors have been developed by applying meta-analytic methods to the results from stock assessments, but the performances of these methods have not been evaluated. Three potential methods for applying meta-analysis to construct steepness priors are available: non-linear mixed models, Bayesian hierarchical methods, and a novel method which approximates marginal likelihoods using likelihood profiles. These methods are evaluated using simulation. The profile method is found to perform best. Estimates of the parameters which define the steepness prior are uncertain owing primarily to uncertainty associated with the results of the stock assessments which provide the input for the meta-analysis methods, and because of the small number of stocks available for inclusion in the meta-analysis.

Mortality in the initial ontogeny of Paralabrax maculatofasciatus (Actinopterygii, Perciformes, Serranidae) caused by Chattonella spp. (Raphidophyceae)

Fish are particularly sensitive to metabolites produced by Raphidophyte species and these have caused intensive fish kills in several countries. However, the effects on embryos of marine fish are unknown but could probably provoke an important impact on new stock recruitment and hence on fisheries. We evaluated the toxic effects of Chattonella spp. strains from the Gulf of California on three development stages of spotted sand bass (Paralabraxmaculatofasciatus): embryo in segmentation stage (ES), embryo (EM), and eleutheroembryo (EL). Embryos (ES) were exposed to different cell concentrations of Chattonella subsalsa, Chattonella marina, Prorocentrum micans, and f/2 medium as control. Also, one set of embryos was tested with cell-free media for C. subsalsa cultures. Incubation lasted until embryos reached apterolarva phase. The ES was the most sensitive stage reaching 98% mortality with C. subsalsa, followed by cell-free media of C. subsalsa cultures, with mortalities close to 90%, whereas EM and EL phases presented mortalities below 60%. This work demonstrates that larval stages of P. maculatofasciatus are highly sensitive to short time exposure to all Chattonella spp. strains tested, that direct physical contact with cells is not required to cause mortality, and that the toxic effect is more pronounced when embryos hatch.

Reference point based management of Norwegian Atlantic salmon populations

SUMMARYWhile management according to biological reference points is well established for many commercial marine fisheries, similar systems for more leisure based fisheries for freshwater fishes are less common. This paper describes the scientific foundation for management according to conservation limits and management targets for Norwegian populations of Atlantic salmon, a highly valued and heavily exploited anadromous fish species. Based on stock recruitment relationships during the freshwater phase, the biomass of females necessary to attain the carrying capacity (yielding average maximum recruitment) has been established as conservation limits for each of the 439 Norwegian populations. Using a simulation model based on reported catch and estimates of exploitation rates, the probability and percentage attainment of the conservation limits have been assessed annually since 2008, and exploitation advice provided for 176 of the largest populations. The number of populations that attained their conservation limits increased substantially after the new management scheme was introduced, despite that the number of returning salmon remained at historical low levels. Overall the populations evaluated in 2011 were at 95% of their conservation limits compared to 91% in 2008 and 85% in 2005. The improvement could largely be attributed to reduced exploitation rates, due to new restrictions in both the marine and river fisheries. The new management scheme also improved the catch statistics and stimulated data acquisition for management. Implementation of management according to conservation limits has been a success in terms of attaining the main management goal of protecting the Atlantic salmon populations by ensuring that an increasing number of the populations likely are at their maximum reproductive capacity. Long-term increases in fisheries yield, the secondary management goal, are likely to be attained, but remain to be documented. Reference point based management of Atlantic salmon exemplifies management within the intersection of fisheries management and conservation biology, borrowing principles from both sides.

Cost–Benefit Analysis of Alternative Techniques for Rehabilitating Abalone Reefs Depleted by Abalone Viral Ganglioneuritis

The relative cost–benefit of alternative methods for rehabilitating blacklip abalone (Haliotis rubra) reefs depleted by viral ganglioneuritis was evaluated using quantitative population and bio-economic modeling. Fishery assessment models were adapted to describe population dynamics and economics of reseeding, translocation, and natural rebuilding. Past and current literature on reseeding, translocation, and mortality rates informed a stakeholders’ workshop that agreed on the parameters for the analysis. Comparative economic performance was quantified as the impaired value of the individual transferable quota from the area until the stock recovered to the level of maximum sustainable yield, as it was prior to the abalone viral ganglioneuritis impact. When the cost of capital is considered, none of the intervention scenarios produced any cost–benefit above that estimated to accrue from an unfished natural recovery. The translocation of adults and reseeding of juveniles were found to be similarly effective largely due to the productivity of the assumed stock–recruitment relationship at low stock densities. Across the scenarios, translocation was estimated to always at least pay for direct costs but rarely covered capital costs while reseeding only covered direct costs if the price of abalone was >$25/kg and seed was <$0.50 per individual. When a depensatory form of stock–recruitment relationship, related to adult biomass, was assumed, translocation was estimated to be an order of magnitude more effective due to the time required to augment the breeding biomass with reseeded juveniles.

Stock–recruitment relations controlled by feeding interactions alone

Stock–recruitment (S–R) relationships are the centrepiece of fisheries management aimed at achieving maximum sustainable yield. Here we consider the possibility that the density dependence evident in S–R relations is controlled by feeding interactions alone. We simulate a food-web model with dynamic representations of intra- and inter-specific size structure and a linear relation between food intake and hatchling production of adults. Population sizes of individual stocks are modified by imposing additional mortality. The predominant functional forms and the steepness of resulting S–R relationships agree well with observations. We conclude that recruitment is plausibly regulated by feeding interactions alone.

Bio-economic management strategy evaluation of deepwater stocks using the FLBEIA model

Deepwater fish are characterized by long lifespans, late maturity and low productivity. This implies slow recovery from low biomass levels and it is, therefore, important to manage these stocks correctly to avoid overfishing. However, these stocks are generally data poor, which it makes difficult to apply quantitative assessment models on which to base their management. The management strategy evaluation (MSE) approach consists in evaluating the performance of management strategies by simulation before their implementation. In the evaluation, the main sources of uncertainty in a fishery system and its management process should be taken into account in order to find robust management strategies. Thus, the MSE approach is relevant to the management of data-poor stocks. We used the FLBEIA software, an FLR library coded in the R statistical language, to conduct a bio-economic impact assessment of fisheries management strategies. We evaluated the performance of several management strategies in three different deepwater case studies using the MSE approach: beaked redfish (Sebastes mentella) in the Norwegian Sea, blackspot seabream (Pagellus bogaraveo) in the Strait of Gibraltar, and French mixed deepwater fisheries in the North East Atlantic. In conclusion, we found that the S. mentella stock is very sensitive to the stock recruitment relationship chosen. For P. bogaraveo, simulations showed that the present constant catch management is as good as more sophisticated strategies. Finally, in mixed fisheries, case study fleet dynamics were key for the success of management strategies

Development of a Stock–Recruitment Model and Assessment of Biological Reference Points for the Lake Erie Walleye Fishery

AbstractWe developed an updated stock–recruitment relationship for Lake Erie Walleye Sander vitreus using the Akaike information criterion model selection approach. Our best stock–recruitment relationship was a Ricker spawner–recruit function to which spring warming rate was added as an environmental variable, and this regression model explained 39% of the variability in Walleye recruitment over the 1978 through 2006 year‐classes. Thus, most of the variability in Lake Erie Walleye recruitment appeared to be attributable to factors other than spawning stock size and spring warming rate. The abundance of age‐0 Gizzard Shad Dorosoma cepedianum, which was an important term in previous models, may still be an important factor for Walleye recruitment, but poorer ability to monitor Gizzard Shad since the late 1990s could have led to that term failing to appear in our best model. Secondly, we used numerical simulation to demonstrate how to use the stock recruitment relationship to characterize the population dynamics (such as stable age structure, carrying capacity, and maximum sustainable yield) and some biological reference points (such as fishing rates at different important biomass or harvest levels) for an age‐structured population in a deterministic way.Received September 21, 2012; accepted July 2, 2013

Can stock–recruitment points determine which spawning potential ratio is the best proxy for maximum sustainable yield reference points?

Abstract Legault, C. M., and Brooks, E. N. 2013. Can stock–recruitment points determine which spawning potential ratio is the best proxy for maximum sustainable yield reference points? – ICES Journal of Marine Science, 70: 1075–1080. The approach of examining scatter plots of stock–recruitment (S–R) estimates to determine appropriate spawning potential ratio (SPR)-based proxies for FMSY was investigated through simulation. As originally proposed, the approach assumed that points above a replacement line indicate year classes that produced a surplus of spawners, while points below that line failed to achieve replacement. In practice, this has been implemented by determining Fmed, the fishing mortality rate that produces a replacement line with 50% of the points above and 50% below the line. A new variation on this approach suggests FMSY proxies can be determined by examining the distribution of S–R points that are above or below replacement lines associated with specific SPRs. Through both analytical calculations and stochastic results, we demonstrate that this approach is fundamentally flawed and that in some cases the inference is diametrically opposed to the method's intended purpose. We reject this approach as a tool for determining FMSY proxies. We recommend that the current proxy of F40% be maintained as appropriate for a typical groundfish life history.

Integrated, age-structured, length-based stock assessment model with uncertain process variances, structural uncertainty, and environmental covariates: case of Central Baltic herring

We developed a generic, age-structured, state-space stock assessment model that can be used as a platform for including information elicited from stakeholders. The model tracks the mean size-at-age and then uses it to explain rates of natural and fishing mortality. The fishery selectivity is divided to two components, which makes it possible to model the active seeking of the fleet for certain sizes of fish, as well as the selectivity of the gear itself. The model can account for uncertainties that are not currently accounted for in state-of-the-art models for integrated assessments: (i) The form of the stock–recruitment function is considered uncertain and is accounted for by using Bayesian model averaging. (ii) In addition to recruitment variation, process variation in natural mortality, growth parameters, and fishing mortality can also be treated as uncertain parameters. The use of the model is exemplified in the context of participatory modelling where stakeholders have specified how environmental variables affect the stock dynamics of Central Baltic herring (Clupea harengus membras).