Northern cod, comprising populations of Atlantic cod (Gadus morhua) off southern Labrador and eastern Newfoundland, supported major fisheries for hundreds of years. But in the late 1980s and early 1990s, northern cod underwent one of the worst collapses in the history of fisheries. The Canadian government closed the directed fishing for northern cod in July 1992, but even after a decade-long offshore moratorium, population sizes remain historically low. Here we show that, up until the moratorium, the life history of northern cod continually shifted towards maturation at earlier ages and smaller sizes. Because confounding effects of mortality changes and growth-mediated phenotypic plasticity are accounted for in our analyses, this finding strongly suggests fisheries-induced evolution of maturation patterns in the direction predicted by theory. We propose that fisheries managers could use the method described here as a tool to provide warning signals about changes in life history before more overt evidence of population decline becomes manifest.
By estimating probabilistic reaction norms for age and size at maturation, we show that maturation schedules of Atlantic cod (Gadus morhua) off Labrador and Newfoundland shifted toward earlier ages and smaller sizes during the late 1980s and early 1990s, when these populations underwent a severe collapse in biomass and subsequently were closed for directed commercial fishing. We also demonstrate that this trend towards maturation at younger ages and smaller sizes is halted and even shows signs of reversal during the closure of the fisheries. In addition, our analysis reveals that males tend to mature earlier and at a smaller size than females and that maturation age and size decrease with increasing latitude. Importantly, the maturation reaction norms presented here are robust to variation in survival and growth (through phenotypic plasticity) and are thus strongly indicative of rapid evolutionary changes in cod maturation as well as of spatial and sex-specific genetic variation. We therefore suggest that maturation reaction norms can provide helpful reference points for managing harvested populations with evolving life histories.Résumé : L'estimation de normes probabilistes de réaction pour l'âge et la taille à la maturation nous permet de démontrer que les calendriers de maturation de la morue franche (Gadus morhua) au large du Labrador et de TerreNeuve ont glissé vers des âges plus précoces et des tailles plus basses au cours de la fin des années 1980 et du début des années 1990; à ce moment, ces populations ont connu un effondrement important de leur biomasse, après quoi il y a eu interdiction de la pêche commerciale ciblée. Nous démontrons aussi que la tendance vers une maturation à des âges plus précoces et à des tailles plus petites s'est arrêtée et a même montré des signes de retour en arrière durant la période d'interdiction de la pêche. De plus, notre analyse démontre que les mâles tendent à atteindre la maturité plus jeunes et plus petits que les femelles et que l'âge et la taille à la maturation décroissent en fonction de l'augmentation de la latitude. Il est important de noter que les normes de réaction de maturation que nous présentons sont robustes vis-à-vis la variation de la survie et de la croissance (via la plasticité phénotypique) et qu'elles indiquent ainsi fortement l'existence de changements évolutifs rapides dans la maturation des morues, ainsi que des variations génétiques en fonction de l'espace et du sexe. Nous croyons donc que les normes de réaction de la maturation peuvent fournir des points de référence utiles pour la gestion de populations exploitées à cycle biologique en cours de modification.[Traduit par la Rédaction] Olsen et al. 823
As the world’s oceans continue to undergo drastic changes, understanding the role of key species therein will become increasingly important. To explore the role of Atlantic cod (Gadus morhua Gadidae) in the ecosystem, we reviewed biological interactions between cod and its prey, predators and competitors within six ecosystems taken from a broad geographic range: three are cod‐capelin (Mallotus villosus Osmeridae) systems towards cod’s northern Atlantic limit (Barents Sea, Iceland and Newfoundland–Labrador), two are more diverse systems towards the southern end of the range (North Sea and Georges Bank–Gulf of Maine), and one is a species‐poor system with an unusual physical and biotic environment (Baltic Sea). We attempt a synthesis of the role of cod in these six ecosystems and speculate on how it might change in response to a variety of influences, particularly climate change, in a fashion that may apply to a wide range of species. We find cod prey, predators and competitors functionally similar in all six ecosystems. Conversely, we estimate different magnitudes for the role of cod in an ecosystem, with consequently different effects on cod, their prey and predator populations. Fishing has generally diminished the ecological role of cod. What remains unclear is how additional climate variability will alter cod stocks, and thus its role in the ecosystem.
We analyzed 47 yr (1946–92) of research trawl data and 5 yr (1964–68) of research gillnet data to identify spawning locations of Atlantic cod, Gadus morhua, in the Newfoundland–Labrador region. Offshore, spawners are common on the continental shelf but generally rare on the slope. Relative abundance of spawning individuals on the shelf is comparably high off northeast Newfoundland, within 100 km of the Newfoundland coast from Cape Freels to Cape Race, on central Grand Bank, and on St. Pierre Bank. Slope spawning is largely restricted to the eastern slope of Hamilton Bank, a small section of northern Grand Bank, and Flemish Cap. Inshore spawning is evident in southeastern Labrador and southeastern Newfoundland, particularly in St. Mary's, Placentia, Trinity, and Bonavista bays. Trajectories of satellite-tracked drifter buoys indicate that it is highly improbable that eggs spawned on the slope of Grand Bank and much of the northeastern Newfoundland slope will be transported into shelf and coastal waters. We conclude that cod spawn in areas in which their eggs and larvae are likely to be retained and that inshore spawning populations may provide a considerably larger contribution to cod recruitment in coastal Newfoundland than has previously been believed.
Biological changes in the ecosystem of the Northeast Newfoundland Shelf during the late-1980s and 1990s included a collapse in the biomass of cod (Gadus morhua), a substantial increase in the biomass of northern shrimp (Pandalus borealis), and an expansion in the area fished for shrimp. The timing and magnitude of changes in cod biomass and the quantity of shrimp consumed by cod were explored to determine if they were consistent with the hypothesis that the increase in shrimp biomass was a consequence of a reduction in predation pressure from cod. Results are equivocal because the timing of both the increase in the shrimp stock and the decline in the cod stock remain unclear and there is considerable uncertainty in the estimates of consumption of shrimp by cod. Nevertheless, it appears that an initial increase in shrimp biomass must have occurred during the early to mid-1980s and was not related to changes in the cod, whereas a larger increase in shrimp biomass in the 1990s was related at least in part to the collapse of the cod.
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