Ichthyoplankton community structure in the northwest Gulf of St. Lawrence (Canada): past and present (original) (raw)
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Marine Ecology, 2014
Species diversity is generally considered one of the key factors of ecosystem resilience in response to anthropogenic pressures, including fishing. In this context, the spatial and temporal changes in demersal fish assemblages and species diversity were investigated in the northern Gulf of St. Lawrence (Canada), over a 20-year period (1990-2010). Data were obtained from the summer research survey conducted by the Department of Fisheries and Oceans, and include commercial and non-commercial species. The study covers the period of groundfish fishery collapse, the moratorium period, and the post-moratorium period, and reflects various modifications in management. Multivariate statistical methods revealed two communities. A coastal community corresponds to strata located above 200 m depth and a deeper community located in the deep channels. Interannual differences in the composition of fish assemblages were observed and are mainly due to the changes in the relative biomass of some dominant species. Three diversity indices (Shannon-Wiener, Simpson's Index of Diversity and Motomura's constant) indicate a slight but significant increase of the diversity over time. This trend is due to the increase of the relative biomass of low-rank species, which may have been favoured by the prohibition of groundfish trawling after 1997 in that region. The geographical distribution of the Shannon-Wiener index also shows temporal dynamics reflecting the biomass distribution of dominant demersal species.
Status of fish communities of the freshwater and brackish waters of the St. Lawrence
2020
More than 80 fish species are found in the freshwater and brackish waters of the St. Lawrence River and evaluation of the status of these fish stocks is complex. In order to paint a current picture of our understanding of the fish communities and aquatic habitats of the St. Lawrence, and to judge the evolution of these elements over the last 25 years, different status indicators were used: (1) Index of Biotic Integrity, (2) status of the stock of certain species subject to recreational and commercial fishing, (3) status of species under threat, (4) coastal zone fish biodiversity index and (5) relative abundance index for submerged aquatic vegetation. Analysis of these indicators highlights what are sometimes very different findings between species and sectors. Several rapid changes that have arisen during the last two decades in the characteristics of aquatic habitats and in the structure of fish communities testify to an evolving ecosystem that is tending to degrade in certain sect...
No more detectable fishing effect on Northern Gulf of St Lawrence benthic invertebrates
ICES Journal of Marine Science
Trawling has been reported worldwide to alter seabed structure, and thus benthic habitats and ecosystems. Usually, a decrease in species richness and biomass is observed, and community structure is modified towards more opportunistic species. The Gulf of St Lawrence (Canada) has been intensely exploited since the 17th century, including net, longline, dredge and trawl fishing activities. Recently, the collapse of groundfish stocks induced a shift in fishing practices toward shrimp trawling, which is currently considered a sustainable fishing activity in the region. However, no long-term study has evaluated the potential effects of trawling disturbances on benthic mega-invertebrates. We investigated whether shrimp trawling had long-(ca. 20 years), mid-(ca. 10 years), and short-term (ca. 4 years) impacts on benthic mega-invertebrate taxa richness, biomass, and community structure. Scientific and fishery trawling data analyses showed that no significant long-, mid-, or short-term effec...
Marine Ecology Progress Series, 2018
Decapod shrimps typically occupy lower trophic levels, often as omnivores, and many have high population turnover rates, which makes shrimp populations susceptible to changes in predator abundance and environmental change. The increased biomass of northern shrimp Pandalus borealis in many parts of the NW Atlantic has been associated with the collapse of predatory fish populations and changes in water temperature. In this study, we show that biomass of many shrimp species in the Gulf of St. Lawrence (Canada) increased between 4-and 100-fold between the late 1980s and mid-2000s. Multivariate analyses based on trawl survey data during a period of relative stability (2004−2015) that followed the increase revealed species assemblages that were strongly spatially structured and which were associated with depth and bottom water temperatures. Univariate analyses of the associations between individual species' densities and depth and temperature revealed that approximately half of the species had narrow temperature associations, suggesting that these species may be susceptible to projected warming of the ecosystem. Furthermore, all species in the northern Gulf of St. Lawrence will be affected by recent and ongoing increases in predator biomass. In contrast, predator biomasses continue to decrease in the southern Gulf of St. Lawrence.
Frontiers in Marine Science
Redfish (Sebastes mentella and S. fasciatus) are back at spectacular record high levels in the Gulf of St. Lawrence (GSL) and the effects of this massive resurgence on other components of the food web remain largely unknown. To better understand the trophic implications of the surging redfish biomass within the GSL ecosystem, 3,690 stomachs containing food were collected during two periods: one characterised by low redfish abundance (1993–1999) and the other during a period of record abundance (2015–2019). Taxonomical analysis of stomach contents from individuals of different sizes from three subareas of the GSL was carried out to determine diet composition during both periods. Zooplankton represented the main prey category for small redfish (< 20 cm), which was driven by a predation on amphipods, mostly Themisto sp. in North-East Gulf, in the 1990s and on copepods of the genus Calanus in the deep channels and euphausiids in North-West Gulf in the 2010s. Themisto sp. still domina...
Estuarine Coastal and Shelf Science, 2007
Mass-balance models have been constructed using inverse methodology for the northern Gulf of St. Lawrence for the mid-1980s, the mid-1990s, and the early 2000s to describe ecosystem structure, trophic group interactions, and the effects of fishing and predation on the ecosystem for each time period. Our analyses indicate that the ecosystem structure shifted dramatically from one previously dominated by demersal (cod, redfish) and small-bodied forage (e.g., capelin, mackerel, herring, shrimp) species to one now dominated by small-bodied forage species. Overfishing removed a functional group in the late 1980s, large piscivorous fish (primarily cod and redfish), which has not recovered 14 years after the cessation of heavy fishing. This has left only marine mammals as top predators during the mid-1990s, and marine mammals and small Greenland halibut during the early 2000s. Predation by marine mammals on fish increased from the mid-1980s to the early 2000s while predation by large fish on fish decreased. Capelin and shrimp, the main prey in each period, showed an increase in biomass over the three periods. A switch in the main predators of capelin from cod to marine mammals occurred, while Greenland halibut progressively replaced cod as shrimp predators. Overfishing influenced community structure directly through preferential removal of larger-bodied fishes and indirectly through predation release because larger-bodied fishes exerted top-down control upon other community species or competed with other species for the same prey. Our modelling estimates showed that a change in predation structure or flows at the top of the trophic system led to changes in predation at all lower trophic levels in the northern Gulf of St. Lawrence. These changes represent a case of fishery-induced regime shift.
Diet of Atlantic cod in the southern Gulf of St Lawrence as an index of ecosystem change, 1959-2000
Journal of Fish Biology, 2002
Major temporal changes in the importance of euphausiids and Atlantic herring Clupea harengus in the diet of Atlantic cod Gadus morhua, 10-75 cm L F , occurred in shallow waters (<100 m depths) of the southern Gulf of St Lawrence between 1959 and 2000. Euphausiids represented 6-70% of prey mass for Atlantic cod 31-60 cm L F from 1959 to 1987 but only trace amounts were detected in stomachs collected from 1990 to 2000. Mysids and gammarid amphipods (for Atlantic cod c45 cm L F), and Atlantic herring (for Atlantic cod >45 cm L F) largely replaced euphausiids in the Atlantic cod diets from 1990 to 2000. This diet change suggested there has been a major perturbation of the food web of the southern Gulf of St Lawrence. The importance of fishes (mostly Atlantic herring) in the diet of Atlantic cod >45 cm L F increased significantly between the periods 1959-1980 and 1987-2000. Atlantic herring comprised 0-4% (mean 1•3%) of prey mass of Atlantic cod 46-60 cm L F from 1959 to 1980 and increased to 2-42% (mean 19•6%) of the diet from 1987 to 2000. Atlantic herring comprised 0-25% (mean 9•4%) of the prey mass of Atlantic cod 61-75 cm L F from 1959 to 1980 and increased to 42-81% (mean 54•2%) of prey mass from 1987 to 2000. This increased consumption of Atlantic herring was consistent with observed changes in abundance of Atlantic herring in the ecosystem between the late 1970s and 2000. The large changes in consumption of euphausiids and Atlantic herring represent highly significant changes and would need to be included in the development and interpretation of ecosystem-based management models for this ecosystem.
Fisheries Oceanography, 2012
Annual bottom-trawl surveys (1990-2010) were used to examine associations between 12 environmental conditions, spatial distribution, and size-specific abundance of Greenland halibut Reinhardtius hippoglossoides in the estuary and Gulf of St. Lawrence (EGSL), and 14 to test the influence of hypoxic conditions on habitat selection. Size classes representing 15 juvenile, immature and adult fish were used for the analyses. The highest concentrations of 16 fish were found in the St. Lawrence estuary at both high and low levels of stock abundance. 17 The areas occupied by 50%, 75%, and 95% of juvenile fish expanded with higher 18 population abundance. However, contrary to our predictions, densities in marginal habitats 19 did not increase at a higher rate than in optimal habitats. Fish longer than 32 cm were 20 distributed over a broader area than juvenile fish. Their abundance explained a limited 21 proportion of the variability in spatial distribution. The spatial dynamics of Greenland 22 halibut in the EGSL is best described by a proportional density model where the rate of 23 increase in local density is associated with population abundance. Habitats selected by 24 Greenland halibut were characterized by low dissolved oxygen (DO) levels. The strong 25 association between high fish densities and low DO concentrations indicates a high 26 tolerance of Greenland halibut to hypoxia. It also indicates that negative effects, if present, 27 could be compensated by other factors like food availability and/or refuge from predation. 28 The results of this study also clearly define the St. Lawrence estuary as the major nursery 29 area for the EGSL population. 30