Spawning Stock Biomass Estimates Research Articles

Reduction in size‐at‐maturity in unprecedentedly strong cohorts of redfish (Sebastes mentella and S. fasciatus) in the Gulf of St. Lawrence and Laurentian Channel

AbstractLife‐history traits, such as size‐at‐maturity, are key parameters to model population dynamics used to inform fisheries management. Fishery‐induced evolution, density‐dependent effects, and global warming have been shown to affect size‐ and age‐at‐maturity, and resulting spawning stock biomass (SSB) in a wide range of commercial fish stocks. Marked changes in redfish biomass and environmental conditions in the Gulf of St. Lawrence and Laurentian Channel over the past decade called for a review and update of size‐at‐maturity for commercially important deepwater redfish Sebastes mentella and Acadian redfish Sebastes fasciatus stocks. Following a 25‐year moratorium, local redfish biomass has recently reached unprecedented levels, co‐occurring with an overall warming of bottom water temperatures. Our objectives were (1) to perform a histological assessment of redfish reproduction stages, including the validation and fine‐tuning of a robust visual chart to facilitate monitoring of size‐at‐maturity and SSB in a transforming environment, and (2) to evaluate changes in size‐at‐maturity in unprecedentedly strong cohorts of redfish, and consequences for stock status assessment and fisheries management. Each specimen was genetically identified to species, and gonad reproduction stages were determined by histology and macroscopic appearances. The present study enabled a robust visual chart for continued and cost‐effective monitoring of redfish reproduction stages to be refined and validated, and has shown a large decrease in redfish length when 50% of the individuals are considered mature that led to an increase in estimates of SSB during the 2011–2021 period for S. mentella and S. fasciatus. These changes modified the perception of stock status, thus having significant implications for fisheries management. Given that fishery‐induced evolution and community structure changes along with global warming are affecting numerous stocks worldwide, the present study outlines a major and global challenge for scientists and resources managers. As shown by our results, the monitoring and frequent updates of life‐history traits in transforming environments are needed to provide reliable science advice for sustainable fisheries.

Perspective comes with time: What do long‐term egg and juvenile indices say about Chesapeake Bay Striped Bass productivity?

AbstractObjectiveThree hypotheses about poor recruitment and collapse of Striped Bass Morone saxatilis in Chesapeake Bay during the 1970s and 1980s (excessive larval mortality related to water quality, recruitment overfishing, or a combination) emerged from intense investigations during the mid‐1980s into the early 1990s. Stock collapse during the 1970s and 1980s and recovery in 1995 were largely attributed to fishing mortality and this premise largely drives management. Stock and larval habitat assessments were not available before the early 1980s and habitat assessments were discontinued after recovery. I used egg and juvenile index time‐series that extended back to 1955 to test these three hypotheses about changes in productivity.MethodsLong‐term (1955–2019) indices of recruitment (juvenile index [JI]), spawning dispersion in time and space (Ep, proportion of plankton net samples with eggs), and an index of relative larval survival (RLS = JI/Ep) for spawning areas in Maryland’s portion of Chesapeake Bay were used to investigate whether larval survival patterns were stable (supported the overfishing hypothesis) or changed (supported the larval mortality hypothesis).ResultBaywide Ep estimates were within similar higher ranges during 1957–1981 and 1989–2019, and were low enough to affect recruitment during 1982–1988. While there was a strong relationship of baywide Ep to spawning stock biomass (SSB) estimated by the stock assessment during recovery (1982–1995), estimates of Ep went from depleted to a high level earlier than SSB estimates. Estimates of postrecovery Ep and SSB were both high, but did not have a relationship. A decline in SSB that began in 2012 was not evident with Ep. A shift to low JIs in the early 1970s was followed by a decline in baywide Ep after 1979. Estimates of RLS were higher in 1960s, declined in the early to mid‐1970s, were mostly low into the early 1990s, and became higher afterward. Recovery of Ep after 1988 was accompanied by recovery of larval survival a few years later. Strong correlations of juvenile indices of Striped Bass, White Perch Morone americana, and Yellow Perch Perca flavescens further supported the larval survival hypothesis; these anadromous fish share larval nurseries in space and time, but have different life histories and fisheries that make simultaneous overfishing unlikely.ConclusionDeterioration of larval survival initiated the collapse of Chesapeake Bay Striped Bass and improvement contributed to recovery. The hypothesis that collapse and recovery was solely driven by fishing was not supported, but excessive fishing pressure lowered Ep and SSB and contributed to poor recruitment during 1982–1988. Long‐term climate patterns and warming, deterioration and improvement in acidic deposition, concurrent increases in freshwater salinization and alkalinization, and shifts in agriculture and watershed management indicated shifts in larval habitat suitability were plausible.

Large-scale fish movement affects metrics of management importance as indicated by quantitative stock assessment

Since achieving population recovery targets, management of the Saginaw Bay stock of walleye in Lake Huron is informed by a statistical-catch-at-age (SCAA) model providing estimates of abundance, spawning stock biomass, mortality rates, and exploitation rates. Movement was examined by an acoustic telemetry study and indicated that 37% of the adult population spends much of spring, summer, and fall in the main basin of Lake Huron and unavailable to the fishery in the bay. The current SCAA model used by managers accounts for this movement by including harvest estimates from main basin fisheries in addition to those within the bay. To quantify the effect of the inclusion of the migration information into the model, we constructed a reduced model version that was limited only to the bay harvest and data sources, and then compared these estimates to the full model estimates. All estimates from the reduced model deviated significantly from the full Lake Huron model’s estimates. Significantly different by year were 61% of population size estimates, 25% of total annual mortality estimates, 18% of recreational exploitation rates, and 52% of spawning stock biomass estimates. Differences between the two models were greatest in years of high walleye abundance (i.e., after recovery) and retrospective analysis indicated that this departure was not an estimation artifact. Generally, the reduced model underestimated predicted abundance. We concluded that incorporation of knowledge of movement of Saginaw Bay walleye from the acoustic telemetry study resulted in better informed stock assessment estimates.

Can spawning origin information of catch or a recruitment penalty improve assessment and fishery management performance for a spatially structured stock assessment model?

We used simulations based on lake whitefish (Coregonus clupeaformis) populations to explore the benefits of using spawning origin information for parsing catch to spawning populations in stock assessments for intermixed fisheries exhibiting an overlapping movement strategy. We compared this origin-informed assessment model with a standard assessment model that did not parse catch. We additionally evaluated the influence of including annual recruitment penalties. For standard assessment models, spawning stock biomass estimates could be unstable and biased (sometimes by more than 50%), depending upon population mixing and productivity, and in some cases estimated near average zero recruitment in the terminal year. Incorporating information on population-specific harvest age composition improved spawning stock biomass estimation (e.g., by sometimes essentially removing 50% biases and improving accuracy). Assessments with recruitment penalties produced less biased terminal recruitment estimates (sometimes a 100% bias was removed). Under status quo target mortality rates, improvements in assessments did not necessarily translate to improved fishery management performance (e.g., avoiding depletion of spawning biomass), but such improvements, and overall better performance, were seen at lower target mortality rates.

Towards a spatial integrated stock assessment model for European hake northern stock

European hake (Merluccius merluccius) is a key species in the management of several mixed fisheries in the north-east Atlantic where regional spatial management plans are being set up. The wide spatial distribution of hake and the lack of knowledge of some processes involved in its spatial dynamics could hamper the assessment and management of hake.To help understand the processes, we implemented a spatial version of the Stock Synthesis stock assessment model for northern hake, including available data on this stock and the associated fisheries. For the 1978–2012 time series, the model distinguished the Bay of Biscay, the Celtic Sea and a northern area aggregating the West of Scotland and the North Sea. The model considered an age-structured population for each age class and area, a seasonal migration, global recruitment allocated to areas and fishing fleets for each area. Parameters for stock and exploitation spatio-temporal dynamics were estimated by likelihood maximization for each quarter and area.The comparison of spatial and non spatial models goodness of fit showed they were close, although the fit to abundance indices slightly improved with the spatial model.Fishing mortalities, spawning stock biomasses and recruitments were slightly different from the current assessment model estimates, essentially at the beginning and the end of the time series. On average over the time series, fishing mortality estimates relative to the abundance were higher in the Bay of Biscay than elsewhere, spawning stock biomass estimates were higher in the Celtic Sea than elsewhere, and recruitment occurred mainly in the Bay of Biscay. Finally, the final spatial model estimated a strong movement from the northern area to the Celtic Sea. The final spatial model is a step towards a spatial assessment and management of the stock.

Improving assessment of Pandalus stocks using a seasonal, size-structured assessment model with environmental variables. Part II: Model evaluation and simulation

Integrated, size-structured stock assessment models are now being used widely for assessment and management of hard-to-age species. However, few studies have attempted to evaluate their performance. A seasonal, size-structured assessment model with environmental covariates has been developed for hermaphroditic Pandalidae. We conducted simulations to evaluate its sensitivity to model configuration and performance with various misspecifications. Ignoring the seasonal fishing pattern (half-year closure) led to risk-prone assessment results of overestimating spawning stock biomass (SSB) and recruitment (R) and underestimating fishing mortality (F). Failure to incorporate environmental signals when the recruitment dynamics was environmentally driven led to bias in recent estimates of SSB, R, and F in the simulation. Ignoring annual variability in growth resulted in large estimation bias. Failing to account for time-varying natural mortality (M) led to strong biases; however, misspecifying size-specific M produced even stronger estimation bias. This result may depend on the variation of M among size classes. Under no model misspecifications, an unbiased estimate of M could be obtained by taking advantage of the seasonal fishery closure. Annual growth parameters were also estimable, but the large number of parameters with annual growth made it difficult for the model to converge.

Interannual and Spatial Variability in Maturity of Walleye Pollock Gadus chalcogrammus and Implications for Spawning Stock Biomass Estimates in the Gulf of Alaska.

Catch quotas for walleye pollock Gadus chalcogrammus, the dominant species in the groundfish fishery off Alaska, are set by applying harvest control rules to annual estimates of spawning stock biomass (SSB) from age-structured stock assessments. Adult walleye pollock abundance and maturity status have been monitored in early spring in Shelikof Strait in the Gulf of Alaska for almost three decades. The sampling strategy for maturity status is largely characterized as targeted, albeit opportunistic, sampling of trawl tows made during hydroacoustic surveys. Trawl sampling during pre-spawning biomass surveys, which do not adequately account for spatial patterns in the distribution of immature and mature fish, can bias estimated maturity ogives from which SSB is calculated. Utilizing these maturity data, we developed mixed-effects generalized additive models to examine spatial and temporal patterns in walleye pollock maturity and the influence of these patterns on estimates of SSB. Current stock assessment practice is to estimate SSB as the product of annual estimates of numbers at age, weight at age, and mean maturity at age for 1983-present. In practice, we found this strategy to be conservative for a time period from 2003–2013 as, on average, it underestimates SSB by a 4.7 to 11.9% difference when compared to our estimates of SSB that account for spatial structure or both temporal and spatial structure. Inclusion of spatially explicit information for walleye pollock maturity has implications for understanding stock reproductive biology and thus the setting of sustainable harvest rates used to manage this valuable fishery.

Incorporating stochasticity in the study of exploited fish population dynamics: Implications for the study of post-recruitment harvest strategies

Previous work has shown that an effective method of maintaining spawning stock biomass (SSB), the biomass of fish that are reproductively mature, within an exploited stock is to regulate harvest so that maximum fishing mortality rates occur after peak spawning during the year. This is known as post-recruitment harvest. The goal of this work is to examine if the advantages of post-recruitment harvest hold when reported stochasticity in the age and time distribution of harvest rates, known as selectivity, is considered. A hybrid dynamical systems model, one in which both continuous-time and discrete-time processes operate simultaneously, was derived, and recursive solutions were found. Results from other studies indicating the benefit of post-recruitment harvest were verified using this hybrid model when selectivity was considered fixed. Simulations were repeated including variance in selectivity using a Markov Chain Monte Carlo (MCMC) procedure. Results show that the benefits of post-recruitment harvest to the preservation of SSB were considerably less advantageous when each age class was assumed to be subject to annual stochastic selectivity. Furthermore, the stochastic scenarios gave estimates of SSB that were lower than their fixed selectivity analogs, indicating that the benefits of theoretical post-recruitment harvest may be diminished to some extent when stochasticity plays a large role in the dynamics.

A comparison of fisheries biological reference points estimated from temperature-specific multi-species and single-species climate-enhanced stock assessment models

Multi-species statistical catch at age models (MSCAA) can quantify interacting effects of climate and fisheries harvest on species populations, and evaluate management trade-offs for fisheries that target several species in a food web. We modified an existing MSCAA model to include temperature-specific growth and predation rates and applied the modified model to three fish species, walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias), from the eastern Bering Sea (USA). We fit the model to data from 1979 through 2012, with and without trophic interactions and temperature effects, and use projections to derive single- and multi-species biological reference points (BRP and MBRP, respectively) for fisheries management. The multi-species model achieved a higher over-all goodness of fit to the data (i.e. lower negative log-likelihood) for pollock and Pacific cod. Variability from water temperature typically resulted in 5–15% changes in spawning, survey, and total biomasses, but did not strongly impact recruitment estimates or mortality. Despite this, inclusion of temperature in projections did have a strong effect on BRPs, including recommended yield, which were higher in single-species models for Pacific cod and arrowtooth flounder that included temperature compared to the same models without temperature effects. While the temperature-driven multi-species model resulted in higher yield MBPRs for arrowtooth flounder than the same model without temperature, we did not observe the same patterns in multi-species models for pollock and Pacific cod, where variability between harvest scenarios and predation greatly exceeded temperature-driven variability in yield MBRPs. Annual predation on juvenile pollock (primarily cannibalism) in the multi-species model was 2–5 times the annual harvest of adult fish in the system, thus predation represents a strong control on population dynamics that exceeds temperature-driven changes to growth and is attenuated through harvest-driven reductions in predator populations. Additionally, although we observed differences in spawning biomasses at the accepted biological catch (ABC) proxy between harvest scenarios and single- and multi-species models, discrepancies in spawning stock biomass estimates did not translate to large differences in yield. We found that multi-species models produced higher estimates of combined yield for aggregate maximum sustainable yield (MSY) targets than single species models, but were more conservative than single-species models when individual MSY targets were used, with the exception of scenarios where minimum biomass thresholds were imposed. Collectively our results suggest that climate and trophic drivers can interact to affect MBRPs, but for prey species with high predation rates, trophic- and management-driven changes may exceed direct effects of temperature on growth and predation. Additionally, MBRPs are not inherently more conservative than single-species BRPs. This framework provides a basis for the application of MSCAA models for tactical ecosystem-based fisheries management decisions under changing climate conditions.

Estimates of Skipped Spawning in Lean and Siscowet Lake Trout in Southern Lake Superior: Implications for Stock Assessment

AbstractWe found that some lean (12.2%) and siscowet (58.0%) Lake Trout Salvelinus namaycush in populations in southern Lake Superior do not reproduce each year (i.e., display “skipped spawning”) even though they have reached puberty. We measured skipped spawning in Lake Trout in southern Lake Superior and assessed its impact on reproduction by developing length‐based and age‐based ogives (i.e., probabilities of being mature). In populations in which skipped spawning is negligible, a maturity ogive can be considered equivalent to a reproduction ogive (probability of being reproductive). However, in populations in which skipped spawning is present, such as in lean and siscowet Lake Trout in southern Lake Superior, maturity and reproduction ogives are very different. We evaluated the influence of skipped spawning on a key stock assessment quantity—spawning stock biomass—by applying maturity and reproduction ogives to simulated populations. Furthermore, we evaluated whether skipped spawning was associated with lower energy reserves by testing for differences in the hepatosomatic index (HSI) according to maturity status. Our population simulations indicate that spawning stock biomass can be overestimated (8.3% for leans and 475% for siscowets) when skipped spawners are assumed to be a part of the spawning stock. An inflated reproductive potential of the population could allow for excessive fishing if quotas are based on these biased spawning stock biomass estimates. Siscowet Lake Trout that were skipping spawning had significantly lower energy reserves (assessed by HSI) than mature fish. The high incidence of skipped spawning in siscowets may be related to the reported slowed somatic growth, high population abundance, and low prey resources in southern Lake Superior, which supports the hypothesis that skipped spawning is a density‐dependent response to limited energy resources.

Impacts of seasonal stock mixing on the assessment of Atlantic cod in the Gulf of Maine

Atlantic cod (Gadus morhua) in the Northwest Atlantic off New England and southern Atlantic Canada exhibit a complex population structure. This region has three independently assessed stocks [Georges Bank, Gulf of Maine (GOM), and the 4X stock], all of which are known to mix with each other. Assessments of these stocks, however, assume no interpopulation mixing. Using simulations, we evaluated impacts of ignoring mixing resulting from seasonal migrations on the GOM assessment. The dynamics of the three stocks were simulated according to different scenarios of interstock mixing, and a statistical catch-at-age stock assessment model was fitted to the simulated GOM data with and without mixing. The results suggest that, while mixing causes measurable bias in the assessment, under the conditions tested, this model still performed well. Of the bias that does exist, spawning-stock biomass estimates are relatively sensitive to mixing compared with estimates of recruitment and exploitation rate. The relative timing of seasonal migration of the three stocks plays a critical role in determining the magnitude of bias. The scale and trends among years in the bias were driven by how representative the catch and survey data were for the GOM stock; this representation changed with the mixing rates.

Estimates of reproductive potential of Greenland halibut (Reinhardtius hippoglossoides) in East Greenland based on an update of maturity status

When estimating reproductive potential (RP), correct interpretation of the maturity status is essential. It has now become apparent the presence of vitellogenic oocytes within the ovary of Greenland halibut (Reinhardtius hippoglossoides) does not necessarily indicate they will spawn within the next twelve months. This has led to a revision of the interpretation of the maturity scale where fish which contain only a developing cohort (DC) of oocytes are considered immature. Comparisons were made of estimates of L50 of female Greenland halibut in East Greenland using the previous interpretation of maturity status where the leading cohort (LC) and DC oocytes are not differentiated with the new interpretation where they are. Differentiation led to an increase from 63.8 to 80.2cm and from 61.2 to 74.1cm for the northern (between 63°40′N and 67°00′N) and southern area (between 61°45′N and 62°40′N), respectively. Combining the maturity data with abundance data of Greenland halibut in East Greenland, spawning stock biomass (SSB) and total egg production (TEP) was estimated in four quadrants between 1998 and 2012 using both the previous and current interpretation of the maturity scale. Using the new interpretation of the scale led to a decrease in SSB estimates of 28–92% in specific areas and years, with an average of 56%. Estimates of TEP were directly proportional to SSB so this approach did not offer any advantages over SSB as a measure of reproductive potential. Length composition of Greenland halibut caught by Norwegian fishing vessels fishing in East Greenland indicate that 85 and 57% of the females caught by the trawl and longline fleet respectively in the northern area and 46% caught by the longline fleet in the southern area were immature.

Performance of stock assessments with misspecified age- and time-varying natural mortality

Natural mortality (M) in fish likely varies with age (size) and among years, and while M is often assumed constant over these factors in stock assessments, several methods have been applied to estimate age- or year-specific values. Including age- or time-varying M in stock assessments, however, may not produce less biased parameter estimates, despite the increased realism. Two generic fish populations (long- and short-lived) subject to size- and year-varying M were simulated, along with data used to fit statistical catch-at-age (SCAA) models. The SCAA models assumed M was constant, age-varying, or year-varying, and the bias in estimates of spawning stock biomass (SSB), fully selected fishing mortality (F), and recruitment was recorded. The scale and trends among years in the bias of parameter estimates were generally driven by how the true values of M varied among years and how M was treated in the SCAA model. Bias was unaffected by how fishing mortality rates varied among years. The scale and trends among years in the bias of parameter estimates were generally similar when specifying a constant M and an age-varying M in the SCAA model. Using an annually varying M in the SCAA model generally improved the scale, and reduced trends among years in the bias of parameter estimates relative to constant M and age-varying M. Trends among years in bias were generally more pronounced for estimates of SSB than recruitment for long-lived life history simulations, but the opposite was true for short-lived life history simulations. These results suggested that the biases induced by not accounting for age-variation in M are likely secondary to those caused by the misspecification of temporal trends (trend and annual fluctuations in this case) in M. More time should be dedicated to accounting for temporal shifts in M than accounting for age-variation in M. The consequences of misspecification of M may also partially depend on life history.

Does predation by grey seals (Halichoerus grypus) affect Bothnian Sea herring stock estimates?

Abstract Gårdmark, A., Östman, Ö., Nielsen, A., Lundström K., Karlsson O., Pönni, J., and Aho, T. 2012. Does predation by grey seals (Halichoerus grypus) affect Bothnian Sea herring stock estimates? – ICES Journal of Marine Science, 69: . Mortality of small pelagic fish due to marine mammals is generally considered to be low compared with other sources of mortality. With recent recoveries of marine mammal predators worldwide, this may no longer hold. The grey seal (Halichoerus grypus) population in the Bothnian Sea has increased fivefold since 1985. Its main prey, herring (Clupea harrengus), is a key species for fisheries in the region. Yet, current stock assessments assume constant natural mortality, leading to a risk of biased stock estimates with increasing predation and misleading analyses of herring population dynamics. We estimated grey seal predation from diet data and reanalysed herring spawning stock biomass (SSB) during 1973–2009. Accounting for predation increased the herring SSB 16% (maximum 19%), but this was within the confidence intervals when ignoring predation. Although mortality in older individuals was inflated when accounting for seal predation, this did not change the conclusions about drivers of herring dynamics. Accounting for grey seal predation is important for abundance estimates of old herring, but currently not for SSB estimates, given the great uncertainties in the standard assessment. The grey seal impact on Bothnian Sea herring will need to be reassessed if stock age composition, grey seal feeding preferences, or total stock development change.

Old Tales in Recent Context: Current Perspective on Yellow Perch Recruitment in Lake Michigan

AbstractTo evaluate factors affecting recruitment of yellow perch Perca flavescens in southwestern Lake Michigan, a Ricker stock–recruitment model was fitted to age‐0 abundance estimated from a fall bottom trawl survey (1989–2007) and spawning stock biomass estimates. Models were built for all possible combinations of four external variables: spring–summer water temperature, June nearshore zooplankton density, spring abundance of alewives Alosa pseudoharengus, and an index of wind‐induced transport of yellow perch larvae. Four relatively parsimonious models were deemed robust, and three external variables were represented (spring–summer water temperature, spring alewife abundance, and June zooplankton density). The Ricker stock–recruitment model with the lowest value of Akaike's information criterion (i.e., the best‐supported model) included spring–summer water temperature and spring alewife abundance. The basic Ricker stock–recruitment model ranked second, followed by the Ricker model that included June zooplankton density. The fourth Ricker model that was deemed robust included spring–summer water temperature. The four best models provided a good fit to age‐0 yellow perch abundance estimates based on fall seining catches along the Illinois shoreline of Lake Michigan during 1989–2007. Our findings indicate that over the last two decades, yellow perch recruitment benefited during years associated with relatively warm water, low alewife abundance, and high zooplankton density.

Inaccuracies in routinely collected Atlantic herring (Clupea harengus) maturity data and correction using a gonadosomatic index model

Macroscopic maturity staging data are widely used to distinguish between reproductive and non-reproductive individuals. The implicit assumption is that these data are accurate. The accuracy of macroscopic maturity staging of North Sea herring (Clupea harengus) has not been checked since the macroscopic scale was produced in 1961. The aim of this study was to assess the accuracy of macroscopic maturity staging of female North Sea herring by comparison to histological staging and the gonadosomatic index (GSI). Ovary samples were collected during the North Sea Herring Acoustic Survey in 2006 on-board FRV ‘Scotia’ (Scotland) and in 2007 on-board FRV ‘Scotia’ and RV ‘Johan Hjort’ (Norway). Commercial samples were also collected by Marine Scotland, Aberdeen in both years. The maturity staging error was relatively low in 2006 (21% error) but was much higher on-board FRV ‘Scotia’ (57%) and RV ‘Johan Hjort’ (47%) in 2007. There was estimated to be a 27% under-estimation of the spawning stock biomass (SSB) in 2007 due to the differences in the proportion mature but no change in SSB estimates in 2006. GSI cut-off scores, estimated by means of multinomial regression models were successfully able to separate immature females from both mature-active and recovering females; however, there was some overlap between the mature-active and recovering individuals. We conclude that an effective and low-cost means of reducing error in herring maturity studies is the combined use of a four-point macroscopic maturity scale with routinely collected GSI data, the latter acting to validate and fine tune macroscopic staging.

Abundance estimation of Northeast Atlantic mackerel based on tag recapture data—A useful tool for stock assessment?

In the present study we utilize tag recapture data to estimate year class abundance and spawning stock biomass of mackerel ( Scomber scombrus L.) in the Northeast Atlantic for the period 1986–2008. On average 20,000 jigged mackerel have been tagged annually with internal steel tags in the spawning area west of Ireland and the British Isles, and the tags have been recaptured in commercial catches screened through metal detectors. The spawning stock biomass estimates derived from two different tag-based models were highly variable but were on average 2 and 2.3 times higher than the ICES official estimate. The official estimate is considered uncertain and most likely an underestimate of the actual biomass, due to unregistered mortality in the fisheries and lack of fishery-independent, age-disaggregated data. Hence, tag-based estimates could potentially improve the current assessment if included in the ICES stock assessment on a regular basis. These estimates also involve some uncertainty that needs consideration, especially related to variable tagging mortality, detector efficiency and migrations of the stock.

Estimates of batch fecundity and spawning fraction for the southern stock of horse mackerel (Trachurus trachurus) in ICES Division IXa

Abstract Gonçalves, P., Costa, A. M., and Murta, A. G. 2009. Estimates of batch fecundity, and spawning fraction for the southern stock of horse mackerel (Trachurus trachurus) in ICES Division IXa. – ICES Journal of Marine Science, 66: 617–622. Since 1995 the annual egg production method has been applied triennially to the southern stock of horse mackerel (Trachurus trachurus) in the Northeast Atlantic (ICES Division IXa). This method assumes that fecundity is determinate, but increasing evidence indicates that horse mackerel are indeterminate spawners. The daily egg production method (DEPM) does not rely on the assumption of determinate fecundity, making it the appropriate method for this species. Therefore, we reanalysed samples collected from previous surveys (2002, 2004, 2005, and 2007) to obtain estimates for batch fecundity and spawning fraction, which are important DEPM parameters. The estimates of batch fecundity are around 200 oocytes g−1 of female (total ovary-free weight). Several criteria were used to estimate spawning fraction (migratory nucleus stage, hydrated oocytes, and post-ovulatory follicles) and all showed the same trend among years, varying between 0.10 and 0.30 d−1. The estimates were significantly different among methods, but those differences were similar across surveys, indicating that a consistent bias would be reflected in the final spawning-stock biomass (SSB) estimates obtained from the DEPM. Until further information is available regarding the accuracy of the criteria used to estimate spawning fraction, the southern horse mackerel SSB estimates from the DEPM should only be taken as indicative of trends rather than measures of absolute abundance.

Decadal changes in the North Sea food web between 1981 and 1991 — implications for fish stock assessment

The North Sea ecosystem of the early 1980s differed substantially from that of the early 1990s. The current North Sea multispecies fisheries assessment models are parameterized by fish diet data sets that reflect both ecosystem states, as the stomachs were sampled in 1981 and 1991. In this study, multispecies virtual population analysis (MSVPA) was parameterized with either diet data set, leading to different model food webs, each representing the predator's diet selection behavior and spatiotemporal overlap with their prey in the two respective ecosystem states. The impact of these changes in predator preferences and spatiotemporal overlap on recruitment success and on stock developments could be demonstrated by using either stomach data set to estimate historic and future spawning stock biomass and recruitment trajectories. The observed changes in the food web mainly impacted the hindcasted recruitment trajectories, whereas spawning stock biomass estimates were quite robust. In the prediction runs, the differences in the survival rate of the recruits decided whether fish stocks of commercially important species (e.g., Gadus morhua, Merlangius merlangus) would recover or collapse in the near future.

Assessing Baltic herring stocks with a model that incorporates migration

An age-structured stock assessment model incorporating fish migration was developed to test the migration effects on the assessment and management of the western and central Baltic herring stocks. The results of parasitological analyses provide the best evidence of the exchange between these stocks. In the model, part of the western stock migrates to central waters in the first half of the year, while a part of the central stock migrates to the western area in the second. Survivors return to each stock at the end of 6-month periods. The model was fitted for a range of assumed migration rates (from 0.1 to 0.4 for the western stock and from 0 to 0.1 for the central stock) in the 1991–2003 period. Next, catches and biomasses were projected for various fishing mortality options. The model, which incorporates migration, produces higher spawning stock biomass estimates for the western stock and lower ones or the central stock, in comparison with the model that omits migration. The catch and biomass projections performed indicate that the migration effect on catch and biomass ranges from low (less than 5% change) to moderate (up to 20% change) in comparison with projections that omit migration.