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Most stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic collapsed in the early 1990s, with little sign of recovery since then. In the southern Gulf of St. Lawrence (sGSL), the failed recovery is due to severe increases in the natural mortality of adult Atlantic cod. We examined the role of predation by grey seals (Halichoerus grypus) in this failed recovery by directly incorporating grey seal predation in the population model for Atlantic cod via a functional response. Estimated predation mortality of adult Atlantic cod increased sharply during the cod collapse and has continued to increase, comprising the majority of mortality since the late 1990s. While predation by grey seals appeared to play a minor role in the collapse of Atlantic cod, we found it to be the main factor preventing recovery. Our results are consistent with the hypothesis that failed recovery is due to predation-driven Allee effects, a demographic effect due to the decline in cod abundance and an emergent effect resulting from increasing grey seal abundance. Under current conditions, extirpation of sGSL Atlantic cod appears likely unless there is a large decline in the abundance of grey seals.La plupart des stocks de morue (Gadus morhua) dans le nord-ouest de l'ocean Atlantique se sont effondres au debut des annees 1990, montrant peu d'indices de retablissement depuis. Dans la partie sud du golfe du Saint-Laurent (sGSL), cette absence de retablissement est due a d'importantes augmentations de la mortalite naturelle des morues adultes. Nous examinons le role de la predation par les phoques gris (Halichoerus grypus) dans ce processus en incorporant directement cette predation dans le modele demographique pour la morue sous la forme d'une reaction fonctionnelle. La mortalite par predation estimee des morues adultes a connu une augmentation marquee durant l'effondrement des stocks de morue et continue d'augmenter, expliquant la majorite de la mortalite depuis la fin des annees 1990. Si la predation par les phoques gris semble avoir joue un role limite dans l'effondrement de la morue, nous constatons qu'il s'agit du principal facteur empechant son retablissement. Nos resultats concordent avec l'hypothese voulant que l'absence de retablissement soit due a des effets d'Allee induits par la predation, un effet demographique decoulant de la baisse d'abondance des morues et un effet emergent decoulant de l'abondance croissante des phoques gris. Dans les conditions actuelles, la disparition de la morue du sGSL semple probable a moins d'une importante diminution de l'abondance des phoques gris. [Traduit par la Redaction], IntroductionMost stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic collapsed to very low levels of abundance in the late 1980s and early 1990s. Since then, abundance of these [...]

Marine ecosystems can undergo regime shifts, which result in nonstationarity in the dynamics of the fish populations inhabiting them. The assumption of time-invariant parameters in stock-recruitment models can lead to severe errors when forecasting renewal ability of stocks that experience shifts in their recruitment dynamics. We present a novel method for fitting stockrecruitment models using the Bayesian online change point detection algorithm, which is able to cope with sudden changes in the model parameters. We validate our method using simulations and apply it to empirical data of four demersal fishes in the southern Gulf of St. Lawrence. We show that all of the stocks have experienced shifts in their recruitment dynamics that cannot be captured by a model that assumes time-invariant parameters. The detected shifts in the recruitment dynamics result in clearly different parameter distributions and recruitment predictions between the regimes. This study illustrates how stock-recruitment relationships can experience shifts, which, if not accounted for, can lead to false predictions about a stock's recovery ability and resilience to fishing. Les ecosystemes marins peuvent subir des changements de regime, entrainant une non-stationnarite de la dynamique des populations de poissons qu'ils renferment. L'hypothese de l'invariabilite dans le temps des parametres de modeles de recrutement au stock peut mener a de graves erreurs dans la prevision de la capacite de renouvellement de stocks caracterises par des changements de la dynamique de recrutement. Nous presentons une nouvelle methode de calage de modeles de recrutement au stock qui fait appel a un algorithme bayesien en ligne de detection de points de changement, capable d'integrer des changements brusques des parametres de modelisation. Nous validons la methode a l'aide de simulations et l'appliquons a des donnees empiriques pour quatre poissons demersaux du sud du golfe du Saint-Laurent. Nous demontrons que tous les stocks ayant subi des changements de dynamique du recrutement ne peuvent etre representes par un modele reposant sur l'hypothese de parametres invariables dans le temps. Les changements detectes de la dynamique de recrutement se traduisent par des differences evidentes des distributions de parametres et predictions du recrutement entre les regimes. L'etude illustre les changements qui peuvent caracteriser les relations de recrutement au stock qui, s'ils ne sont pas pris en consideration, peuvent mener a de fausses predictions concernant la capacite de retablissement et la resilience a la peche d'un stock. [Traduit par la Redaction], Introduction Reproduction forms a vital link between current and future generations. The sustainability of fishing is largely attributable to the ability of a fish stock to renew itself such that [...]

Mature thorny (Amblyraja radiata), winter (Leucoraja ocellata), and smooth (Malacoraja senta) skates have declined to very low abundance in the southern Gulf of St. Lawrence (SGSL) and on the eastern Scotian Shelf (ESS). We used stage-structured state-space models to examine decadal patterns in mortality rates in these skates. Mortality at early life stages (embryos in egg cases, hatchlings, and (or) small juveniles) appeared to decrease between the 1970s and the 2000s. In contrast, estimated mortality rates increased for larger individuals over this period. Although potentially confounded in models with effects of any changes in juvenile growth, the estimated increases in mortality could not instead be attributed solely to changes in growth. Increases in the mortality of large individuals appeared to reflect increases in natural mortality, possibly due to predation by grey seals. Increases in natural mortality were not evident for skates on the neighbouring western Scotian Shelf, where grey seal abundance has remained lower. Even in the absence of fishing, recovery of skates is unlikely under current ecosystem conditions in the SGSL and on the ESS. Les raies epineuses (Amblyraja radiata), tachetees (Leucoraja ocellata) et a queue de velours (Malacoraja senta) matures sont devenues tres peu abondantes dans le sud du golfe du Saint-Laurent (SGSL) et l'est du plateau neo-ecossais (ESS). Nous avons utilise des modeles d'espaces d'etats structures selon les etapes du cycle de vie afin d'examiner les patrons decennaux des taux de mortalite chez ces raies. La mortalite durant les premieres etapes du cycle de vie (embryons dans les capsules d'oeufs, alevins vesicules et petits juveniles) semble diminuer entre les annees 1970 et les annees 2000. En revanche, pour les individus plus grands, les taux de mortalite estimes ont augmente au cours de cette periode. Si, dans les modeles, les augmentations estimees de la mortalite peuvent potentiellement se confondre avec les effets de changements de la croissance des juveniles, elles ne peuvent toutefois etre entierement attribuees a de tels changements. Les augmentations de la mortalite des grands individus semblent refleter des augmentations de la mortalite naturelle possiblement dues a la predation par les phoques gris. Aucune augmentation claire de la mortalite naturelle n'a ete notee pour les raies de la partie ouest du plateau neo-ecossais, ou l'abondance des phoques gris est demeuree plus faible. Meme en l'absence de peche, un retablissement des raies est peu probable dans les conditions ecosystemiques actuelles dans le SGSL et le ESS. [Traduit par la Redaction], Introduction Elasmobranch fishes, especially the larger species, are considered to be among the most vulnerable fishes to exploitation because of their life-history characteristics, in particular late maturation, large adult size, [...]

The stock of Atlantic cod (Gadus morhua) on the eastern Scotian Shelf (ESS) collapsed in the early 1990s and showed no sign of recovery during a 15-year fishing moratorium, but has recently increased in abundance. Both the prolonged lack of recovery and the recent improvement have been attributed to changes in the biomass of forage fishes through effects of predation and competition by these fishes on early life stages of cod. An examination of the relationships between forage fish biomass and the population dynamics of ESS cod provided no support for this hypothesis. Contrary to expectations under this hypothesis, cod recruitment rate was unrelated to forage fish biomass. The main factor delaying recovery was high natural mortality (M) of adult cod. The recent improvement in ESS cod is due to the strong 2004 year class and a decline in M. These factors cannot be attributed to an effect of forage fishes. Both the delay in recovery and recent improvement of the ESS cod stock appear to be due to factors other than interactions with forage fishes. Le stock de morue (Gadus morhua)de l'est de la plate-forme neo-ecossaise (ESS) s'est effondre au debut des annees 1990 et n'a montre aucun signe de retablissement durant les 15 annees d'un moratoire sur la peche. Son abondance connait toutefois, depuis peu, une augmentation. L'absence prolongee d'un retablissement et l'amelioration recente ont toutes deux ete attribuees a des changements dans la biomasse de poissons fourrage qui se refletent dans des effets de la predation et de la concurrence par ces poissons sur les premieres etapes du cycle de vie de la morue. Un examen des relations entre la biomasse de poissons fourrage et la dynamique des populations de morue de l'ESS n'a fourni aucun appui a cette hypothese. Contrairement a ce que cette derniere predit, le taux de recrutement de morues n'etait pas relie a la biomasse de poissons fourrage. Le principal facteur ayant retarde le retablissement est une mortalite naturelle (M) elevee des morues adultes. L'amelioration recente du stock de morue de l'ESS est le fait de la forte classe d'age de 2004 et d'une diminution de la M. Ces facteurs ne peuvent etre attribues a aucun effet des poissons fourrage. Tant le retablissement tardif que l'amelioration recente du stock de morue de l' ESS semblent etre attribuables a des facteurs autres que les interactions avec les poissons fourrage. [Traduit par la Redaction], Introduction Most stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic collapsed in the early 1990s, predominantly because of overfishing. Nearly 20 years later, most of these stocks have [...]

Worldwide, many commercial fish stocks have experienced dramatic declines due to overfishing. Such fisheries-induced population reductions could potentially erode the genetic diversity of marine fish populations. Based on analyses of DNA extracted from archived and contemporary samples, this paper compares the genetic variability at nine microsatellite loci in a Canadian population of Atlantic cod (Gadus morhua) over 80 years, spanning from before the fishery intensified to now when the population is at historically low abundance. Extensively validated genetic data from the temporally spaced samples were used to estimate the effective population size. Over the period, we observed no loss of either heterozygosity or allelic diversity. Several of the estimation methods applied could not distinguish the effective population size from infinity, and the lower 95% confidence limit on estimates was generally >500, suggesting that the effective population size is probably considerably larger than this. Hence, it appears that the southern Gulf of St. Lawrence cod stock has maintained genetic variability to sustain future evolution despite a dramatic population decline Resume: A lechelle globale, plusieurs stocks commerciaux de poissons ont connu des declins spectaculaires a` cause de la surpeche. De telles reductions demographiques dues a` la peche pourraient potentiellement diminuer la diversite gene-tique des populations de poissons marins. Sappuyant sur des analyses d'ADN faites sur des echantillons archives et contemporains, notre etude compare la variabilite genetique a` neuf locus microsatellites dans une population canadienne de morues franches (Gadus morhua) sur une periode de 80 ans couvrant le temps d'avant l'intensification de la peche jusqua maintenant alors que la population connait une abondance historiquement basse. Des donnees genetiques solidement validees provenant d'echantillons etales dans le temps ont servi a estimer la taille effective de la population. Durant cette periode, nous n'observons de perte ni d'heterozygotie, ni de diversite allelique. Plusieurs des methodes d'estimation utiliseesne peuvent pas distinguer la taille effective de la population de l'infini et la limite inferieure de l'intervalle de confiance de 95 % des estimations est generalement >500, ce qui laisse croire que la taille effective est probablement beaucoup plus elevee que ce nombre. Il apparait donc que le stock de morues du sud du golfe du Saint-Laurent a conserve la variabilite genetique necessaire pour assurer son evolution future, malgre son declin demographique spectaculaire. [Traduit par la Redaction], Introduction In recent years many commercial fish stocks have declined dramatically as a result of overfishing (Hutchings and Reynolds 2004; Worm et al. 2006). A growing body of literature has [...]

Atlantic cod (Gadus morhua) was the dominant demersal fish and most important predator in the southern Gulf of St. Lawrence ecosystem as recently as the 1980s. However, productivity of southern Gulf cod has declined, and the population is no longer viable even in the absence of fishing. We conducted population projections taking into account uncertainty in current abundance-at-age and uncertainty or variability in each of the components of population productivity (i.e., rates of recruitment, individual growth, and adult natural mortality). We defined extirpation as a spawning stock biomass less than 1000 t ( Aussi recemment que durant les annees 1980, la morue franche (Gadus morhua) etait le poisson benthique dominant et le plus important predateur dans l'ecosysteme du sud du Golfe Saint-Laurent. Cependant, la productivite des morues du sud du golfe a diminue et la population n'est plus viable, meme en l'absence de peche. Nous avons fait des projections demographiques qui tiennent compte de l'incertitude au sujet des abondances actuelles en fonction de l'age, ainsi que de l'incertitude ou de la variabilite associees a chacune des composantes de la productivite de la population (c'est-a-dire les taux de recrutement, de croissance individuelle et de mortalite naturelle des adultes). Nous avons defini l'extirpation comme correspondant a une biomasse du stock reproducteur inferieure a 1000 t ( [Traduit par la Redaction], Introduction Atlantic cod (Gadus morhua) has been the dominant demersal fish in continental shelf ecosystems of the Northwest Atlantic since Europeans first fished these waters in the sixteenth century or [...]

We describe dramatic shifts in the species composition of the marine fish community of the southern Gulf of St. Lawrence using a 35-year time series of catch rates in an annual bottom-trawl survey. We attempt to understand the causes of these changes using a traits-based approach that relates the similarity among species in their abundance trends to similarities in their ecological traits. We selected traits based on a priori beliefs of how each should reflect susceptibility to changes in a different external factor potentially affecting the community. We found evidence for an effect of ocean climate and top-down effects of fishing and seal predation, but not for bottom-up effects of prey availability on adult fishes. Mean body length in the community decreased dramatically in the 1990s. This reflected the removal of large-bodied fishes by fishing and sharp increases in the abundance of small fishes. The biomass of small fish was inversely correlated with an index of predation on those fish by larger fish, suggesting strong predator control of the abundance of small-bodied fishes. Our results suggest that changes in ocean climate combined with direct and indirect effects of harvesting can dramatically and rapidly alter the composition of marine fish communities. Nous decrivons des changements importants de composition specifique dans les communautes de poissons marins du sud du golfe du Saint-Laurent a partir d'une serie chronologique de 35 annees de taux de captures dans des inventaires annuels an chalut de fond. Nous tentons de comprendre les causes de ces changements a 'aide d'une methodologie basee sur les traits qui met en rapport les similartises des tendances d'abondance chez les especes et les similarites de leurs traits ecologiques. Nous avons choisi les traits d'apres notre perception a priori de comment chaque trait devrait refleter la susceptibility aux changements d'un facteur externe particulier qui affecte potentiellement la communaute. Nous trouvons des indications de 'existence d'un effet du climat oceanique et d'effets descendants de la peche et de la predation par les phoques, mais non d'effets ascendants de la disponibilite des proies sur les poissons adultes. La longueur corporelle moyenne dans la communaute a diminue de fargon importante durant les annees 1990. Cela reflete le retrait des poissons de grande taille par la peche et un accroissement marque des petits poissons. 11 existe une correlation inverse entre la biomasse des petits poissons et un indice de predation des gros poissons sur les petits, ce qui laisse croire a un important controle de 1'abondance des poissons de petite taille par les predateurs. Nos resultats indiquent que les changements dans le climat oceanique combines aux effets directs et indirects de la recolte peuvent modifier de fargons importante et rapide la composition des communautes de poissons maros. [Traduit par la Redaction], Introduction There is increasing recognition of the need to consider fisheries in the context of the ecological communities and ecosystems in which they are prosecuted (e.g., Link 2002; Pikitch et [...]

The stock of Atlantic Cod (Gadus morhua) in the southern Gulf of St. Lawrence (sGSL) (NAFO Division 4T and Subdivision 4Vn (November to April)) supported landings averaging 30,000 tonnes (t) annually between 1917 and 1940 and 56,000 t annually between 1941 and 1992. The stock collapsed in the early 1990s due to high mortality. Moratoria on directed fshing for southern Gulf cod were put in place in 1994-1997, 2003 and since 2009. Reported by-catch in fsheries for other groundfsh have averaged 110 t annually since 2009, with catches near 60 t in 2017 and 2018. Estimated fshing mortality since 2009 has averaged 0.2% annually for cod aged 5-8 years and 0.7% for cod 9 years and older. Spawning stock biomass (SSB) and abundance are at the lowest levels observed in the 69-year record and are declining. Estimated SSB at the start of 2018 was 13,947 t, 12% of the already low value in 2000 and 4% of the 1985 value (380,000 t). SSB is estimated to be 17% of the limit reference point (LRP = 80,000 t) with no chance of recovering to the LRP over the next fve years, even with no fshing mortality. The probability that SSB will decline between 2018 and 2023 is 90% with annual catches of 0 or 100 t and 99% with catches of 300 t. SSB is projected to decline to 9,400 t at the start of 2023 with no fshery catch and to 9,100 t with annual catches of 300 t. If current conditions were to persist, the population would be expected to decline to extinction by mid-century (SSB < 1,000 t was used as a proxy for local extinction). The ongoing decline of this population is due to the high natural mortality of adult cod (i.e., ages 5 years and older). Natural mortality (M) of about 18% annually (an instantaneous rate of M=0.2) is considered normal for adult cod. In this population, natural mortality of adults has increased over the past 35 years and is now estimated to be about 55% annually (M=0.81-0.85). Predation by grey seals appears to currently be the main cause of this mortality. Based on population models, this predation can account for all of the adult M above the normal level (0.2) since about the year 2000. This cod population appears to be experiencing a predation-driven Allee effect. An Allee effect occurs when the per capita rate of population growth decreases as abundance decreases. This is opposite to the expected increase in population productivity at low abundance due to reduced intraspecifc competition. In addition, southern Gulf cod are experiencing a strong Allee effect with a production defcit since 2001. If they persist, strong Allee effects will drive a population to extinction. This Allee effect is caused by the severely depleted abundance of southern Gulf cod and the high and increasing abundance of their grey seal predators. At the current high abundance of grey seals, this cod population is expected to continue to decline toward extinction. [ABSTRACT FROM AUTHOR]

The factors affecting herring recruitment are still poorly understood, complicating the prediction of stock dynamics and the choice of operational management strategies. We investigated effects of intrinsic (SSB) and extrinsic factors (physical and biological environments, including competition and predation) on recruitment of the spring and fall spawning components of each of the two herring stocks occurring in the Gulf of the St. Lawrence between 1971 and 2014. Effects of potential explanatory factors on recruit (age 2) abundance were tested using Generalized Additive Models. Model fit was significantly improved by incorporating both physical and biological environmental variability, but effects of herring SSB and predation were not significant. Indices of zooplankton abundance and phenology explained more variance in recruitment than physical indices. Our results emphasize the dominance of bottom‐up processes over SSB in the regulation of herring recruitment. Environmental variability did not seem to act uniformly on the recruitment of either stock or their respective spawning components. A long‐term trend of decreasing recruitment in spring spawners was associated with a long‐term decline in abundance of cold water copepods. In fall spawners, optimal recruitment was dependent on warmer environmental conditions combined with an adequate supply (species composition and phenology) of zooplankton. These results provide the first empirical evidence that spring and fall spawning herring are adapted to contrasting environmental conditions and shed light on the potential mechanisms linking herring recruitment to key zooplankton community characteristics and phenology. Management strategies can be improved by incorporating this new knowledge on environmental drivers of herring recruitment. [ABSTRACT FROM AUTHOR]

Fisheries-induced evolution has been hypothesized to delay the recovery of collapsed fish stocks through effects on their productivity. The cod stock in the southern Gulf of St. Lawrence (SGSL) collapsed in the early 1990s and has shown no recovery since then, due mainly to high natural mortality of adult cod. Age and size at maturation of SGSL cod decreased sharply over time in cohorts produced in the 1950s and 1960s, likely reflecting an evolutionary response to intensified fishing, and have remained low since then, despite severe reductions in fishing mortality over the past 15 years. A predicted consequence of early maturation is increased natural mortality due to higher costs to reproduction. Early maturation may be a cause of increases in natural mortality of SGSL cod in the 1970s but does not appear to be related to the much larger increases since then. Instead, the current high natural mortality of SGSL cod appears to be primarily a cause, rather than a consequence, of the continued early maturation in this population, now replacing fishing mortality as the agent of selection favouring early maturity. This striking example of the failure to reverse fisheries-induced evolution by relaxing fishing pressure emphasizes the need for management strategies that minimize the chances of harvest-induced genetic change. [ABSTRACT FROM AUTHOR]