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An age-structured, spatial survey-based assessment model (SSURBA) is developed and applied to the Grand Banks stock (NAFO Divisions 3LNO) of American plaice (Hippoglossoides platessoides) in Newfoundland and Labrador. The state-space model is fit to annual spatial (i.e., three divisions) stock size-at-age research vessel (RV) survey indices that are assumed to be proportional to abundance. We model index catchability (q) as a logistic function of fish length, which varies with age, cohort, and the time of the survey; therefore, the model facilitates the estimation of q values that change spatially and temporally following changes in fish growth and survey gears. The SSURBA model produces division-level estimates of fishing mortality rates (F), stock productivity, and stock size relative to the logistic catchability assumption with q = 1 for fully selected ages. The spatial model allows us to include additional survey information compared with the space-aggregated assessment model (all of 3LNO) that is currently used to assess stock status. The model can provide estimates of relative catch, which we compare with reported catch trends to partially validate the model. Un modele devaluation base sur des releves spatiaux structures par age (SSURBA) est mis au point et utilise pour le stock des Grands Bancs (divisions 3LNO de l'OPANO) de plies canadiennes (Hippoglossoidesplatessoides) a Terre-Neuve-et- Labrador. Le modele d'espace d'etats est cale sur des indices de releves par navire de recherche (NR) de l'age selon la taille du stock qui sont presumes proportionnels a l'abondance. Nous modelisons la capturabilite indexee (q) en tant que fonction logistique de la longueur des poissons, qui varie selon l'age, la cohorte et le moment du releve; ainsi le modele facilite l'estimation de q qui varient dans l'espace et le temps a la suite de changements de la croissance des poissons et des engins utilises pour les releves. Le modele SSURBA produit des estimations au niveau de la division des taux de mortalite par peche (F), de la productivite du stock et de la taille du stock au vu de l'hypothese de capturabilite logistique pour q = 1 pour des ages entierement selectionnes. Le modele spatial nous permet d'inclure plus d'information tiree des releves que ce que permet le modele devaluation spatialement regroupe (ensemble des 3LNO) actuellement utilise pour evaluer l'etat du stock. Le modele peut fournir des estimations des prises relatives que nous comparons aux tendances des prises rapportees afin de valider partiellement le modele. [Traduit par la Redaction], Introduction Canadian fisheries management has a long history of conducting scientific surveys to monitor fish populations, with some of the earliest surveys dating back to the 1970s (Halliday and Fanning [...]

The relationship between stock size and recruitment is an essential element in the understanding of the productivity of a population. However, predicting the number of recruits produced by a population has proven to be a difficult challenge. This may in part be a result of poor estimation of reproductive potential (RP). We determined if including increased information on reproductive biology in indices of RP results in better predictions of recruitment. We investigated some of the conditions that lead to better (or worse) recruitment prediction when more biologically complex indices of RP are used. Data from four populations in the Northwest Atlantic were examined: southern Grand Bank (NAFO Division 3NO) cod (Gadus morhua), Flemish Cap (NAFO Division 3M) cod, Newfoundland (NAFO Division 3LNO) American plaice (Hippoglossoides platessoides), and Greenland halibut (Reinhardtius hippoglossoides) (NAFO Subarea 2 + Division 3KLMNO). Stock-recruit models paired with complex indices of RP gave a better estimate of recruitment in slightly more than half of the tests conducted. When there were larger trends in the reproductive biology (maturity at age, sex ratio and egg production), more complex indices of RP were more likely to provide a better estimate of recruitment. La relation entre la taille d'un stock et le recrutement est un element essentiel pour la comprehension de la productivite d'une population. Cependant, la prediction du nombre de recrues produites par la population s'est averee etre un defi difficile. Ce peut en partie etre le resultat d'une mauvaise estimation du potentiel reproductif (RP). Nous avons determine si l'addition d'information supplementaire sur la biologie de la reproduction dans les indices de RP produit de meilleures predictions du recrutement. Nous avons etudie certaines des conditions qui entrainent une meilleure (ou moins bonne) prediction du recrutement lorsque des indices plus biologiquement complexes de RP sont utilises. Nous avons examine les donnees provenant de quatre populations du nord-ouest de l'Atlantique, les morues (Gadus morhua) du Grand Banc sud (NAFO Division 3NO), les morues de Flemish Cap (NAFO Division 3M), les plies canadiennes (Hippoglossoides platessoides) de Terre-Neuve (NAFO Division 3LNO) et les fletans du Groenland (Reinhardtius hippoglossoides) (NAFO sous-region 2 + Division 3KLMNO). Les modeles stocks-recrues apparies a des indices complexes de PR donnent une meilleure estimation du recrutement dans un peu plus de la moitie des tests. Lorsqu'il y a des tendances plus marquees dans la biologie de la reproduction (maturite en fonction de l'age, proportion des sexes, production des oeufs), les indices plus complexes de PR sont plus susceptibles de fournir une meilleure estimation du recrutement. [Traduit par la Redaction], Introduction Predicting the number of recruits produced by a population has proven to be a difficult challenge in fisheries science. Yet the relationship between stock size and recruitment is an [...]

Author(s): Esben M. Olsen (corresponding author) [1, 4]; Mikko Heino [1, 2]; George R. Lilly [3]; M. Joanne Morgan [3]; John Brattey [3]; Bruno Ernande [1]; Ulf Dieckmann [1] Northern [...]

Stock–recruitment relationships (SRRs) may vary over time due to ecological and anthropogenic impacts, challenging traditional approaches of calculating maximum sustainable yield (MSY)-based reference points that assume constant population traits. We compare seven methods to calculate MSY, F MSY and B MSY by modelling constant, stochastic (uncorrelated), and autocorrelated SRRs using simulations and two case studies [Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) on the Grand Bank off Newfoundland, Canada]. Results indicated that the method used to model SRRs strongly affected the temporal pattern of recruitment projection, and the variations generated by autocorrelated SRRs were more similar to observed patterns. When the population productivity had low-frequency and large-magnitude variations, stochastic SRRs generated greater MSY and F MSY estimates than constant or autocorrelated SRRs, while no consistent pattern of B MSY was detected. In the case studies, stochastic and autocorrelated SRRs produced asymmetric relationships between fishing mortality and yield, with higher risk of overfishing by going beyond F MSY. Overall, our results suggest that caution should be taken when calculating MSY-based reference points in highly dynamic ecosystems, and correctly accounting for non-stationary population dynamics could, therefore, lead to more sustainable fisheries. [ABSTRACT FROM AUTHOR]

Fish distribution is strongly influenced by temperature, with populations predicted to respond to ongoing changes in ocean climate by adapting distribution to maintain preferred thermal habitats. Over the last several decades, the Northwest Atlantic off Newfoundland, Canada has experienced wide variation in temperature and significant changes in the structure of the marine community. A period of particularly cold ocean conditions from the mid-1980s to mid-1990s saw Greenland halibut occupying deeper waters, and moving farther offshore and southward. Distributional shifts across periods of varying thermal conditions were most pronounced in the youngest ages. While no specific nursery areas were evident, these young fish typically occurred in shallower waters where temperatures were more variable, moving deeper with age. Sensitivity to changing ocean climate may vary with age. This adaptation to shifting temperatures suggests that this species is likely to alter its distribution in the face of continued changes in ocean climate. Age-specific differences in response to changing temperature have potential implications for ecosystem interactions and fisheries management. [ABSTRACT FROM AUTHOR]

The current stock assessment model for American plaice (Hippoglossoides platessoides) on the Grand Bank of Newfoundland (NAFO Divisions 3LNO) is a virtual population analysis (VPA). This model does not account for the considerable uncertainty about the landings data for this stock. Retrospective patterns have also been noted in the current assessment with overestimation of spawning stock biomass (SSB) and underestimation of fishing mortality (F). Via a thorough model selection process, we develop a state-space stock assessment model (SSM) for this stock that accounts for the uncertainties in the landings data and reduces the retrospective patterns. Our SSM fit the data well, with overall trends in SSB and average F (ages 9--14) similar to those estimated from the current VPA. The retrospective patterns for the SSM were reduced for both SSB and average F which should lead to the provision of better scientific advice for the management of this stock. An important result from our analysis suggests that the current assumption for natural mortality (M) in the stock assessment model may be too low. The lack of recovery of the stock of American plaice on the Grand Bank has often been attributed to overfishing, however fixing M within the model to be lower than is reasonable may be over-estimating the relative impact of F and subsequently over-stating the contribution of fishing mortality to the lack of recovery of the stock. [ABSTRACT FROM AUTHOR]

Much of my career has been focussed on exploring the causes and consequences of variation in the productivity of fish. The aim has been to provide better advice for fisheries management. We have realized that variation in the components of productivity, such as growth, maturation, and fecundity is substantial. Incorporating this variation into stock assessment leads to a significant change in the perception of reference points and stock status. If exploitation levels are not adjusted for varying productivity, they will not be sustainable. Although we have learned much about the causes and consequences of variation in productivity there is still much to learn. It will be a huge leap forward when we can explain the processes driving this variation and use this information in our population models. My contributions to the field have benefitted greatly from a series of collaborations that have fuelled my creativity, productivity and enjoyment. My direct involvement in stock assessment has resulted in my research being used directly in the provision of scientific advice and also opened up areas of research that I would not have otherwise pursued. Get involved. Be open to new opportunities and seize them. You never know where they will lead you! [ABSTRACT FROM AUTHOR]

Growth and condition in fishes have been taken as indicating levels of energy available for survival and reproduction, major components of a population's productivity. After a rapid collapse in population size, northern (NAFO Division 2J3KL) cod (Gadus morhua) remained at a very low level of abundance for 20 years. We investigated the potential for poor growth and condition to have played a role in the collapse and lack of recovery of northern cod. Juveniles and adult males and females all showed similar patterns. Perceptions about the importance of growth and condition to population status depended on the metrics and area examined. When the northern cod population was declining, the northern areas of the population clearly had reduced growth and condition, while these metrics improved in the south. Results were equivocal as to the potential role of growth and condition in the continued low abundance of northern cod and, to some extent, depended on the nature of the metric being examined. Indices of condition, which included lipid storage in the form of liver weight, were generally lower in the north while the stock remained at a low level. Metrics associated with longer-term protein storage returned to precollapse levels quickly following the period of collapse. An index of food availability was more closely related to growth and condition than was temperature. These results point to the need both for studies of growth and condition in a population to have a comprehensive time-series of data covering the entire range of the population and the need for a better understanding of the causes and implications of changes in different metrics of condition. [ABSTRACT FROM AUTHOR]

We tested whether our perception of the performance of the adopted Harvest Control Rule (HCR) for Greenland halibut was robust to using different measures of Reproductive Potential (RP) combined with different assumptions about the stock recruit relationship to drive the stock dynamics in simulations. We tested the HCR using alternative stock recruitment functions (segmented regression, Ricker and modified Ricker) with different RP indices which vary in the level of biological complexity. The RP indices used in increasing order of biological information were: Biomass 10+, SSB with varying maturity at age (SSB), female SSB (FSB), and Total Egg Production (TEP). All Operating Models (OM) were based on the current accepted XSA assessment. Understanding the basis of uncertainty in the S/R relationships is generally the most difficult outstanding problem in fisheries assessment and management and it is a key problem in Management Strategy Evaluation (MSE). A Ricker stock recruitment function fits the Greenland halibut stock recruitment data better than the segmented regression for all the RP indices, except TEP. The results show that the inclusion of more biological information when estimating Reproductive Potential does not improve the stock recruitment fit for either (segmented regression or Ricker). The best fits in both cases were obtained in descending order with: 10+Biomass, SSB, FSB, and TEP. All the OMs based on the segmented regression have very similar results and seem to be robust to assumptions about RP. However, there was variability in the results of the different indices of RP in the Ricker and modified Ricker OMs and some impact on whether performance targets were met. The choice of stock recruit function had a greater impact than the inclusion of more biological information in the index of RP. The inclusion of alternative indices of RP is likely to have more of an impact for stocks with depleted reproductive capacity and/or where alternative indices have a greater effect on the S/R relationships. [ABSTRACT FROM AUTHOR]

Fisheries management frameworks based on maximum sustainable yield reference points have been put forward by all agencies and organizations responsible for providing scientific advice for the management of 22 major North Atlantic cod stocks. These frameworks provide a structure for rebuilding depleted cod stocks and for future sustainable fishing. Unfortunately, in most cases these frameworks have not been fully implemented. Although information is lacking for some stocks, it would appear that a large proportion of North Atlantic cod stocks are either experiencing a fishing mortality rate that exceeds that required to achieve maximum sustainable yield, are at a biomass level far below that which provides maximum sustainable yield, or have both occurring. Despite this general failure, there are some notable successes, which have led to stock rebuilding and substantial fisheries, providing an incentive for striving to achieve rebuilding and sustainable management objectives for other Atlantic cod stocks. Management frameworks based on harvest control rules that result in fishing mortality rates below those associated with maximum sustainable yield are essential when stock size is low in order to rebuild to levels capable of producing maximum sustainable yield. [ABSTRACT FROM AUTHOR]

In 1994, a directed fishing moratorium was declared for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea) stocks because both stocks showed severe declines in abundance from heavy exploitation during the mid 1980s and early 1990s. Four years later, the fishery for yellowtail re-opened while the plaice stock has shown little recovery and the moratorium is still in effect. To assess the possible causes of the differences in recovery between species, we examined the spatial structure and environmental characteristics of the continental shelf nursery habitats of plaice and yellowtail, and their relationship to recruitment variability and overall population size. Depth plays a major influential role determining the spatial pattern and the abundance of juveniles of both species and in the case of plaice the spatial structure of the adult population also determines the amount of nursery area utilised by juveniles. Recruitment variability was higher in plaice than in yellowtail. We found year class synchrony in both species indicating that common environmental conditions and/or biological processes are affecting recruitment in a similar manner. Density-dependent regulation appears to be more severe in yellowtail and this should contribute to a more stable population when compared to plaice. These results are discussed in terms of resiliency of both stocks to over-exploitation. [Copyright &y& Elsevier]