North Sea Stocks Research Articles

Recruitment regime shifts and nonstationarity are widespread phenomena in harvestable stocks experiencing pronounced climate fluctuations

AbstractMethods to reliably identify jump discontinuities in biological time series and to assess the specific contribution of various covariates are rapidly progressing. Here, we took advantage of these statistical improvements as well as those seen in complementary, down‐scaled climate and biogeochemical models to investigate causes of the substantial interannual variability observed in recruitment strength in hindcast analyses. This systematic meta‐analysis included 23 data‐rich, commercially valuable, warm‐ and cold‐temperate stocks in the North, Norwegian and Barents Seas. Since this study focuses on recruitment strength variability, we have used the term “recruitment regime shift” to distinguish from the concept of ecosystem regime shift. The breakpoint analysis revealed that the former criterion applied to more than half of the time series, mainly with respect to North Sea stocks but also to those in the Norwegian Sea. The exploratory analysis using vcGAM indicated that 1–3 shifts per stock were real, when using five drivers spanning spawning stock biomass to large‐scale climatic processes. Thus, non‐stationary relationships were extensively prevalent, indicating that each stock is uniquely adapted to its locally varying conditions. Outputs from the stationary GAM resembled those from the vcGAM but not after the threshold year. In‐depth case studies showed that the proxy of a given driver for the process which was to be included should be critically considered in a spatiotemporal context. Furthermore, the stock‐specific uncertainty associated with the given recruitment figures as such should also be an in‐built component of the model construct and thereby in the evaluation of the output.

The interactions between fishing mortality, age, condition and recruitment in exploited fish populations in the North Sea

A quantity of studies have documented how greater condition and older age of individual fish enhances reproductive success via a variety of processes from egg production to larval survival. If these relations translate into relationships between stock demography and recruitment, maintaining a “healthy” age composition may be a key step towards ensuring stock productivity. We examined the correlation between indicators of the age and size composition and the residual-variation from the stock-recruitment relationship (i.e. the variation not explained by the spawning stock biomass) in eleven commercially exploited fish stocks in the North Sea. We also analyzed to what extent spawner age and size composition responded to variations in fishing mortality. Before detrending to accommodate any temporal patterns, recruitment residuals correlated negatively with the relative age composition in all eleven stocks (plaice, whiting, and saithe being significant). However, after detrending the recruitment residuals, the correlations weakened and only nine out of eleven stocks correlated negatively. Lastly, we also conducted a delay difference analysis, which yielded a more or less even distribution between negative and positive correlations, and in addition two stocks (cod and Norway pout) came out showing significant positive correlations between recruitment residuals and age composition. In contrast, eight of the eleven stocks showed positive correlations between spawner weight and recruitment residuals and correlations were significant for plaice, cod, and saithe. After detrending the data, the significant correlations disappeared, but the overall tendency towards positive correlations between spawner weight and recruitment residuals prevailed. Overall, these results suggest that fish condition is more likely to show positive relationships with recruitment success than age composition. Lastly, it was found that the majority of stocks showed a strong link between fishing mortality and the two indicators of age composition, and when looking across stocks, the age composition appears to have returned to the same level as in the 1960 s. Conversely, spawner weight at age have not recovered, and considering the indications of a positive link between spawner weight and recruitment, recruitment success of the North Sea stocks may remain low in spite of the apparently successful rebuilding of the age composition.

Changes in functioning of the largest coastal North Sea flatfish nursery, the Wadden Sea, over the past half century

The international Wadden Sea is an important flatfish nursery. Information from the Dutch Wadden Sea indicates that the flatfish nursery function of the area has been affected during the last decades. Increased seawater temperature has affected settling, habitat suitability for and growth performance of the various flatfish species. Settling of plaice, flounder and to a lesser extent sole larvae occurs earlier nowadays. In the 1960s, 0-, I-, II- and III-group plaice were present, but since 2000, II-group has disappeared and densities of I-group have decreased. For juvenile flounder, II-group almost disappeared, and for dab, a decline in densities of all age groups was observed from the 1990s onwards. Summer temperatures exceed the optimum for the cold-water species (plaice, flounder and dab) with increasing frequency, level and duration. Only for 0-group sole, the period with optimal growth conditions has become longer and has resulted in increased growth. Mortality rates in 0-group plaice have increased, coinciding with an increase in water temperatures and an increase in the abundance of predators. The decrease in density of juvenile plaice and dab in the Wadden Sea has not affected recruitment to North Sea stocks, suggesting that other areas have taken over part of the nursery function. The predicted increase in seawater temperature in the next decades will continue to improve the conditions for sole. The temperature tolerance of plaice and dab and to a lesser extent flounder will further reduce their scope for growth and may ultimately result in their disappearance from the Wadden Sea.

Reconstruction of Atlantic herring (Clupea harengus) recruitment in the North Sea for the past 455 years based on the δ13C from annual shell increments of the ocean quahog (Arctica islandica)

AbstractUnderstanding the recruitment variability of the Atlantic herring North Sea stock remains a key objective of stock assessment and management. Although many efforts have been undertaken linking climatic and stock dynamic factors to herring recruitment, no major attempt has been made to estimate recruitment levels before the 20th century. Here, we present a novel annually resolved, absolutely dated herring recruitment reconstruction, derived from stable carbon isotope geochemistry (δ13C), from ocean quahog shells from the Fladen Ground (northern North Sea). Our age model is based on a growth increment chronology obtained from fourteen shells. Ten of these were micromilled at annual resolution for δ13C analysis. Our results indicate that the anthropogenically driven relative depletion of 13C, the oceanic Suess effect (oSE), became evident in the northern North Sea in the 1850s. We calculated a regression line between the oSE‐detrended δ13C results (δ13CṠ) and diatom abundance in the North Sea, the regression being mediated by the effect of phytoplankton on the δ13C of the ambient dissolved inorganic carbon. We used this regression to build an equation mediated by a nutritional link to reconstruct herring recruitment using δ13CṠ. The reconstruction suggests that there were five extended episodes of low‐recruitment levels before the 20th century. These results are supported by measured recruitment estimates and historical fish catch and export documentation. This work demonstrates that molluscan sclerochronological records can contribute to the investigation of ecological baselines and ecosystem functioning impacted by anthropogenic activity with implications for conservation and stock management.

Exploring the Influence of Food and Temperature on North Sea Sandeels Using a New Dynamic Energy Budget Model

The zooplanktivorous schooling fish sandeels A. marinus comprise a quarter of North Sea fish biomass and are essential food for a number of marine mammals and birds. However, in recent decades there has been a decline in the abundance of older sandeels, with increased overwinter mortality proposed as a contributor. To survive winter animals require a minimum energy reserve which is determined by temperature, prey abundance, and possibly prey size. Here a new dynamic energy budget model is created to determine the influence of food and temperature on sandeel energy dynamics. The model hindcasts changes in energy and survival between 2000 and 2008, a period of pronounced stock decline in the northwestern North Sea. Overwinter starvation mortality was a large contributor towards a recent decline in sandeels in northern UK waters. Highest over-winter mortality rates were recorded for juveniles and not individuals aged 1 or over due to the effect of weight-specific metabolism. However, a sensitivity analysis of the model suggests that mortality rates are more sensitive to changes in copepod abundance in the build up to overwintering rather than temperature during overwintering. Further, projections show that temperature rises are negated by increases in large, but not small copepods. The conclusion is that food-driven size-selective starvation mortality contributed to the northwestern North Sea stock decline and that indirect food web effects of climate change are greater than direct physiological effects on sandeels.

A stochastic surplus production model in continuous time

AbstractSurplus production modelling has a long history as a method for managing data‐limited fish stocks. Recent advancements have cast surplus production models as state‐space models that separate random variability of stock dynamics from error in observed indices of biomass. We present a stochastic surplus production model in continuous time (SPiCT), which in addition to stock dynamics also models the dynamics of the fisheries. This enables error in the catch process to be reflected in the uncertainty of estimated model parameters and management quantities. Benefits of the continuous‐time state‐space model formulation include the ability to provide estimates of exploitable biomass and fishing mortality at any point in time from data sampled at arbitrary and possibly irregular intervals. We show in a simulation that the ability to analyse subannual data can increase the effective sample size and improve estimation of reference points relative to discrete‐time analysis of aggregated annual data. Finally, subannual data from five North Sea stocks are analysed with particular focus on using residual analysis to diagnose model insufficiencies and identify necessary model extensions such as robust estimation and incorporation of seasonality. We argue that including all known sources of uncertainty, propagation of that uncertainty to reference points and checking of model assumptions using residuals are critical prerequisites to rigorous fish stock management based on surplus production models.

Is spawning stock biomass a robust proxy for reproductive potential?

AbstractReference points based on fishing mortality (F) and spawning stock biomass (SSB) are a requirement of many fisheries management frameworks. SSB is assumed to be a proxy for stock reproductive potential (SRP). Limit reference points based on SSB are used to indicate the level of biomass below which productivity is affected. SSB fails to account for changes in fecundity, egg viability and sex ratio, and it has been argued that total egg production (TEP) provides a better reflection of SRP. We explore how accounting for TEP impacts limit reference points and evidence for a relationship between stock and recruit. Time series of SSB and TEP are compared for three North Sea stocks: cod (Gadus morhua), herring (Clupea harengus) and plaice (Pleuronectes platessa). Dynamics based on TEP are different from those based on SSB for cod and plaice, but the stock–recruit relationships were not ‘improved’ using TEP. Shifts in productivity (spawner per recruit) occur in all three time series and SSB underestimated uncertainty. Yet again, it was shown that assumptions of stationarity about fish population productivity are incorrect. We argue that the use of TEP does improve the realism in our understanding of stock dynamics, and demographically, more complex management strategy evaluation is required to develop management procedures that are robust to uncertainty and integrate F and the demographic health of a stock. Empirical feedback control systems based on fisheries independent indices including surveys of eggs, larvae, recruits, juveniles or spawning adults should be evaluated and compared to traditional approaches.

Minimising unsustainable yield: Ten failing European fisheries

Ten European fish stocks recognised by the European Union as “outside safe biological limits” are considered in light of widespread reforms to fisheries legislation in 2013, particularly the legal responsibility to exploit these resources sustainably. Given that some of these stocks are – as of 2013 – fished at over 150% the recommended intensity and many have been outside of these limits for the entirety of their assessment history, the utilisation of traditional fisheries management measures of sustainability are questioned and tougher approaches such as “zero-catch” and long-term, expansive spatial closure scenarios are considered. Finally, the pervasive issue of data-deficiency (the status of 54% of European stocks) is briefly considered, with specific reference to the understudied West of Scotland and North Sea stock of European seabass (Dicentrarchus labrax).

Spatio-Temporal Variability of the North Sea Cod Recruitment in Relation to Temperature and Zooplankton

The North Sea cod (Gadus morhua, L.) stock has continuously declined over the past four decades linked with overfishing and climate change. Changes in stock structure due to overfishing have made the stock largely dependent on its recruitment success, which greatly relies on environmental conditions. Here we focus on the spatio-temporal variability of cod recruitment in an effort to detect changes during the critical early life stages. Using International Bottom Trawl Survey (IBTS) data from 1974 to 2011, a major spatio-temporal change in the distribution of cod recruits was identified in the late 1990s, characterized by a pronounced decrease in the central and southeastern North Sea stock. Other minor spatial changes were also recorded in the mid-1980s and early 1990s. We tested whether the observed changes in recruits distribution could be related with direct (i.e. temperature) and/or indirect (i.e. changes in the quantity and quality of zooplankton prey) effects of climate variability. The analyses were based on spatially-resolved time series, i.e. sea surface temperature (SST) from the Hadley Center and zooplankton records from the Continuous Plankton Recorder Survey. We showed that spring SST increase was the main driver for the most recent decrease in cod recruitment. The late 1990s were also characterized by relatively low total zooplankton biomass, particularly of energy-rich zooplankton such as the copepod Calanus finmarchicus, which have further contributed to the decline of North Sea cod recruitment. Long-term spatially-resolved observations were used to produce regional distribution models that could further be used to predict the abundance of North Sea cod recruits based on temperature and zooplankton food availability.

A fish stock assessment model using survey data when estimates of catch are unreliable

Methods of assessment that depend upon commercial catch data can be undermined by misreporting or where parts of the catch, such as discards, are not accounted for. An age-structured model that makes use of survey data alone, and avoids this problem, is developed within a Bayesian framework so that routine stock summary statistics such as fishing mortality, recruitment and spawning stock biomass can be estimated with associated levels of uncertainty. It is also possible to estimate catch on a relative scale which can be compared to reported catches. The model is applied to West of Scotland haddock (Melanogrammus aeglefinus), a stock with suspected high catch misreporting. Stock trends derived from the model are consistent with conventional assessments that use catch data during periods of low misreporting. Estimated proportions of fish at each age in the catch correspond closely with observed values. Model estimates of total catches suggest substantial misreporting in some years, though the precision of the estimates is very low. Revised estimates of natural mortality are obtained from the model that are higher than conventional values used for this stock. These new values are generally consistent with those obtained from multispecies predation modelling for the adjacent North Sea stock. The model provides many of the basic quantities used for management advice. It should not be regarded as a replacement for more comprehensive analyses, but an additional tool to explore available data when catch information is unreliable.

Suitability of otolith microchemistry for stock separation of Baltic cod

Microchemical otolith analyses have been shown to provide valuable information on the life history, dispersal and stock characteristics of teleost fish. In the present study, the suitability of this technique for identifying the origin and distribution of Atlantic cod Gadus morhua L. from the Baltic Sea was examined using laser ablation-ICPMS. The capacity to distinguish individuals from different Baltic Sea stocks and from the adjacent North Sea stock based on incoporation of stock-specific elemental fingerprints along otolith growth axes was investigated. It was further tested if different origins led to spawning-site specific element concentrations in otolith cores. The results indicate that microchemical analyses of Baltic cod otoliths are applicable for differentiating individuals of different stocks. Analyses of similarities including 12 element/calcium ratios resulted in significant differences between individuals from the eastern and the western Baltic Sea and between North Sea and Baltic Sea samples. Sr/Ca, Ba/Ca, Y/Ca, Mg/Ca, Zr/Ca and Mn/Ca ratios had the strongest discriminatory power. A further separation of individuals caught in 3 different spawning grounds of the eastern Baltic, however, was not possible. Elemental compositions from the core regions of otoliths from young of the year cod caught in eastern and western Baltic Sea spawning grounds showed significant differences in Sr/Ca, Ba/Ca and Mg/Ca concentrations. Analyses of similarities again showed significant differences between these areas for juveniles. This study demonstrates the potential of otolith microchemical analyses to provide important information about the stock structure and connectivity of G. morhua in the Baltic Sea.

Target strength and tilt-angle distribution of the lesser sandeel (Ammodytes marinus)

Abstract Kubilius, R., and Ona, E. 2012. Target strength and tilt-angle distribution of lesser sandeel (Ammodytes marinus) – ICES Journal of Marine Science, 69: 1099–1107. North Sea stocks of lesser sandeel have recently become depleted, and improved methods for abundance estimation are sought. This paper focuses on the acoustic target strength (TS) and orientation of sandeel, measured simultaneously in several field experiments. A specially designed cubic cage, fitted with an echosounder and a video camera, was lowered onto the sea bottom, trapping wild sandeel inside. Methods for manually selecting valid echotraces from individual sandeel are described. Scattered mean TS values from several experiments are reported. These are, in spite of the observed variability, summarized in a TS–fish length (cm) relationship as TS = 20logL – 93.1 (dB) at 200 kHz. We believe that the accuracy of the relationship may still be debated; incorporating larger uncertainty in the overall mean TS will increase the total uncertainty of the stock biomass estimate from sandeel acoustic surveys. This uncertainty is now, using standard narrow-beam echosounders, dominated by the fish patchiness relative to the survey coverage. Results from pilot investigations of the sandeel swimming orientation using video cameras are also presented, showing that sandeel usually has an anguilliform swimming pattern with substantial positive (head-up) tilt. The spread of the tilt-angle distribution is also larger than for more neutrally buoyant fishes.

Long-Term Retrospective Analysis of Mackerel Spawning in the North Sea: A New Time Series and Modeling Approach to CPR Data

We present a unique view of mackerel (Scomber scombrus) in the North Sea based on a new time series of larvae caught by the Continuous Plankton Recorder (CPR) survey from 1948-2005, covering the period both before and after the collapse of the North Sea stock. Hydrographic backtrack modelling suggested that the effect of advection is very limited between spawning and larvae capture in the CPR survey. Using a statistical technique not previously applied to CPR data, we then generated a larval index that accounts for both catchability as well as spatial and temporal autocorrelation. The resulting time series documents the significant decrease of spawning from before 1970 to recent depleted levels.Spatial distributions of the larvae, and thus the spawning area, showed a shift from early to recent decades, suggesting that the central North Sea is no longer as important as the areas further west and south. These results provide a consistent and unique perspective on the dynamics of mackerel in this region and can potentially resolve many of the unresolved questions about this stock.

Reproductive investment in the North Sea haddock: temporal and spatial variation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 432:149-160 (2011) - DOI: https://doi.org/10.3354/meps09168 Reproductive investment in the North Sea haddock: temporal and spatial variation Peter J. Wright*, Fiona M. Gibb, Iain M. Gibb, Colin P. Millar Marine Scotland Science, Marine Laboratory, Aberdeen AB11 9DB, Scotland, UK *Email: p.wright@marlab.ac.uk ABSTRACT: Maturation and fecundity have been examined since the 1970s in 2 putative sub-populations of North Sea haddock Melanogrammus aeglefinus. Reproductive investment within the western North Sea haddock sub-population significantly increased between the 1970s and 2000s, as was evident from both a decline in maturation probability and an increase in fecundity with size. A decline in maturation probability was also evident in males from the east North Sea, although the length at 50% probability of maturing (Lp50) at the beginning and end of the study period was much higher than in the west North Sea. The changes in reproductive traits could not be explained as a compensatory response, since the trend for increasing reproductive investment was not reversed when the North Sea stock recovered over the past decade. Indeed increased reproductive investment was accompanied by a reduction in somatic growth rate. Temperature immediately prior to secondary gametogenesis could partially explain inter-annual variation in female maturation probability and may have influenced the difference in sub-population trends. However, declines in maturation probability with cohort were still highly significant after accounting for a temperature effect, and differences in potential fecundity could not be explained by changes in somatic condition. Overall, it appears that west North Sea haddock have increased their reproductive effort, following decades of high mortality, consistent with an evolutionary response. KEY WORDS: Life-history theory · PMRN · Maturation · Fecundity · Haddock Full text in pdf format PreviousNextCite this article as: Wright PJ, Gibb FM, Gibb IM, Millar CP (2011) Reproductive investment in the North Sea haddock: temporal and spatial variation. Mar Ecol Prog Ser 432:149-160. https://doi.org/10.3354/meps09168 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 432. Online publication date: June 27, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Substock variation in reproductive traits in North Sea cod (Gadus morhua)

The North Sea stock of Atlantic cod ( Gadus morhua ) is comprised of a number of subcomponents that differ both genetically and phenotypically. A potential cause for such spatial variability is adaptive divergence, which may be linked to differences in thermal environment and (or) historical fishing pressure. Here we present evidence that spatial variation in maturity–size relationships in the wild has a significant intrinsic component. Using a common-environment experiment on wild-caught juveniles raised through to maturity, we demonstrate that cod from the southern North Sea (SNS) mature at larger sizes than those from the northwestern North Sea (NWNS) despite broadly similar growth rates. Consistent with these experimental results, year-class-specific maturity ogives for recent maturing year classes (1999–2001) suggested that the length at which 50% of females reached maturity was 11 cm greater for SNS than for NWNS cod. Under a common environment, smaller female size at maturity partly reflected higher relative liver weight, with NWNS females having a higher relative liver weight than SNS females. By investigating maturation under controlled conditions, our study provides evidence for life history trade-offs in energy allocation between growth, energy storage, and reproduction that may underlie the spatial variation observed in the field.

The dangers of ignoring stock complexity in fishery management: the case of the North Sea cod

The plight of the marine fisheries is attracting increasing attention as unsustainably high exploitation levels, exacerbated by more extreme climatic conditions, are driving stocks to the point of collapse. The North Atlantic cod (Gadus morhua), a species which until recently formed a major component of the demersal fisheries, has undergone significant declines across its range. The North Sea stock is typical of many, with a spawning stock biomass that has remained below the safe biological limit since 2000 and recruitment levels near the lowest on record. Cod within the North Sea are currently managed as a single stock, and yet mounting empirical evidence supports the existence of a metapopulation of regionally variable, genetically distinct, sub-stocks. Applying the same management strategies to multiple stocks that differ in their resilience to exploitation inevitably results in the overfishing and likely collapse of the weaker components. Indeed, recent studies have identified two North Sea spawning stocks that have undergone disproportionally large collapses with very substantial reductions in egg production. Similarly affected cod stocks in the northwest Atlantic have shown little evidence of recovery, despite fishery closures. The possible implications of ignoring sub-structuring within management units for biocomplexity, local adaptation and ecosystem stability are considered.

Coupled use of data storage tags and otolith microchemistry to assess population dispersal and intra-specific diversity in migratory behaviour in North Sea plaice (Pleuronectes platessa L.)

In high dispersal fish species, large scale genetic homogeneity of populations is considered axiomatic. However, in these species, sampling artefacts due to the active migrations of adults might lead to seemingly unstructured populations while, in fact, inbreeding occurs. This seems to be the case in North Sea plaice (Pleuronectes platessa L.), for which previous studies have identified geographically discrete feeding aggregations (sub-stocks) during the summer, from which the adults migrate to spawn during the winter. In this context, otoliths of mature female plaice tagged with electronic data storage tags (DST) were examined for their potential to reconstruct population structure and fish lifetime migratory history. DST allow continuous records to be made of ambient conditions experienced by free swimming fish in their natural environment over periods of up to 18months, from which their annual geographical movements can be reconstructed. Most fish also record ambient conditions throughout their lifetimes by the accretion of calcium carbonate on their ear-stones (or “otoliths”), the chemical composition of which reflects the environmental history of individual fish. Here we use the otoliths from tagged plaice adults to illustrate how DST data can be linked with otolith microchemistry to allow the retrospective positioning of the fish in space throughout their lifetimes. These data could not easily be obtained using conventional techniques, and link for the first time pre and post recruitment fish behaviour. This unveils precious information about population structure in this commercially important species, which is essential for the sustainable management of its North Sea stocks.

Otolith shape analysis as a tool for stock separation of horse mackerel ( Trachurus trachurus) in the Northeast Atlantic and Mediterranean

The geographic variability in otolith shape of horse mackerel ( Trachurus trachurus) was investigated as a tool for stock separation. The outlines of several thousand otoliths, collected in 20 sampling areas during two consecutive years and covering most of the distributional range of the species in the Northeast Atlantic and Mediterranean, were digitised and analysed for shape variation by elliptical Fourier analysis. Only fish of a medium total length (20–35 cm) were included in the data analysis to minimise size effects between areas. The extracted Fourier descriptors were corrected for fish size preceding multivariate analysis. Multidimensional scaling of the average Fourier descriptors by area, combining both sampling years, showed three distinct clusters of areas: a northern, an Ibero-Mauritanian and an eastern Mediterranean group. These patterns were supported by discriminant analysis of the individual Fourier descriptors, with correct classifications ranging from 31 to 95%. Similar groups could be observed when analysing the two sampling years separately. The discrimination between these groups was 81–88%, while individuals from the Northeast Atlantic and Mediterranean were correctly classified by around 88–91%. Average otolith shapes for these groups showed characteristic differences, especially in the dorsal part of the otolith outline. The results of this study indicate that the separation line between the western and the southern stock should be shifted to the coast of southern Galicia (Cape Finisterre). In contrast to previous genetic and morphometric studies, the observed variation in otolith shapes could not confirm a separation of a western horse mackerel stock from the North Sea stock. Further, fish from the areas around the Iberian Peninsula (including the western Mediterranean) appear to be very similar to the northwest African occurrences. By integration of the inferred information on stock structure into a currently pursued multidisciplinary approach to stock identification of horse mackerel, the stock-adaptive assessment and sustainable management of horse mackerel in the Northeast Atlantic and Mediterranean will be significantly improved.

The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod, haddock and whiting

Within commercial fisheries, particularly mixed fisheries, both target and non-target species are often discarded. Discarding represents a potentially significant loss to the productivity of fish stocks; it can have damaging ecological consequences, and is a potential cause of the failure of recovery plans. The Nephrops fishery in the North Sea is classified as a mixed fishery. Nephrops trawls are constructed with smaller meshes than trawls used to target whitefish; consequently, the bycatch of juvenile fish can be substantial. Several new Nephrops trawl designs have been tested in the North Sea. The data from these trials are used to investigate the potential impact of their implementation on cod, haddock and whiting stocks in the North Sea (including the Kattegat and Skagerrak). The model examines five trawl designs, and also the scenarios of a cessation of discarding in all North Sea fisheries and in just the Nephrops fishery. The model is deterministic, and evaluates the relative differences between scenarios assuming all other variables remain constant. If discarding of cod, haddock and whiting in the North Sea fisheries were eliminated, stocks would increase by 41%, 14% and 29%, respectively, within 10 years. Eliminating discarding in the Nephrops fishery alone would increase stocks by 2%, 1% and 13%, respectively, reflecting the relative proportion of catches of these species in the Nephrops fishery. For cod and haddock, the introduction of the Nephrops trawl with a grid with a square-mesh codend was the only scenario in which a notable increase in stock number was observed. This trawl design facilitates the escape of fish of all ages/sizes from the trawl, effectively making the Nephrops fishery a single-species fishery. For whiting, stock numbers and landings increased under all scenarios, but forecasted landings were lower than if current discard patterns continued in all except the no-discards scenario. The dependency of the results on the validity of the assumptions and on the accuracy of the input data is discussed.

Evaluating the science behind the management advice for North Sea cod

Abstract Reeves, S. A. and Pastoors, M. A. 2007. Evaluating the science behind the management advice for North Sea cod. – ICES Journal of Marine Science, 64: 671–678. Since 1974, annual assessments have constituted the core of the scientific advice underpinning management of the North Sea cod stock. Here, we base our evaluation of the assessments' performance on the quality of the advice and on the wider issue of how science is used to produce management advice. At the same time, we consider catch-at-age analyses and catch forecasts in the context of the amount of person-time and expertise available at the relevant working group meetings. The results indicate that, generally, the catch forecasts have been positively biased and that the bias has been driven by three main factors: (i) substantial overestimates of terminal stock size since the late 1990s, (ii) overestimates of incoming recruitment, and (iii) overestimates of growth. Information for other North Sea stocks (haddock, plaice, and sole) suggests that bias is a problem generally, not one unique to cod. Our results include a discussion of ways to use science more effectively in supplying management advice.