The Newfoundland Shelf supported one of the world’s greatest fisheries until the main commercial species collapsed more than two decades ago. We calculated three ecological indices for individual populations and five for community from the data obtained in the research surveys conducted by Spain in NAFO Regulatory Area Divisions 3NO between 2002 and 2013. We use data for 24 species to study the dynamics of major demersal fish assemblages (38–300, 301–600, and 601–1460 m depth) and evaluated how they have responded to different levels of exploitation. Trends and changes for individual populations (abundance and biomass, intrinsic population rate of growth, and mean length) and for all the community (ABC curves, indices of faunal diversity, proportion of non-commercial species, mean length in community and size spectra) were used to test ecological trends. Indices showed no homogeneous status and responded to different exploitation patterns, management, and environmental regimes in each assemblage. Our results show an improvement in the shallower and deeper assemblages and that fishing effort does not explain differences among each assemblage.
Compensatory responses are important because they increase spawning stock biomass (SSB) growth rates in depleted fish populations. We analyze recruits per spawner (R/SSB) and spawners per recruit (SSB/R) data from six well-studied depleted, or depleted and recovering, Northwest Atlantic groundfish stocks for evidence of compensatory responses. All stocks had periods of high R/SSB at low SSB; however, R/SSB showed considerable variability, and significant compensatory responses were only found in two stocks. For compensation in SSB/R, relationships were significant for three of the stocks. We examined the potential impact of compensation in R/SSB and SSB/R on SSB growth rate and concluded that SSB/R could have a greater potential impact on enhancing SSB growth rate in depleted stocks. Overall, cod (Gadus morhua) populations showed greater compensation than flatfish, mainly due to the lower potential in flatfish to increase SSB/R. This suggests that cod are more resilient to overfishing than are flatfish. Estimates of population growth rate at low stock size, which ignore compensation in SSB/R, will underestimate maximum SSB growth rates. Compensation in both R/SSB and SSB/R should both be considered when evaluating stock productivity, sustainable harvest levels, and biological reference points.
There are three different species of redfish (Sebastes spp.) in the waters of the Flemish Cap (Division 3M, NAFO Regulatory Area): S. fasciatus, S. mentella, and S. norvegicus. Historically, S. fasciatus and S. mentella have been managed together as a single stock because of similar biology and difficulty in species identification. Here we use multivariate autoregressive state-space models to examine the abundance trajectories of the three species and to determine whether they can be treated as a single stock for management purposes or whether they should be treaty separately. We also included covariates to evaluate relationships with climate, commercial catch, and the abundance of predators and (or) competitors and prey. We did two separate analyses: (i) a single-period analysis over the full time series and (ii) a blocked, two-period analysis over different regulatory periods. In both analyses, the best-fit model included separate trajectories for each species at each depth but one overall stock growth rate; both also included commercial catches as a covariate. These analyses suggest that a single assessment for the Sebastes complex is acceptable.
In this study, a multispecies gadget model (GadCap) simulating the interactions between the Flemish Cap cod Gadus morhua, redfish Sebastes sp. and shrimp Pandalus borealis has been incorporated as the operating model in a Management Strategy Evaluation (MSE) framework (a4a-FLR), to test the performance of multiple combinations of HCRs for the three stocks when recruitment uncertainty and assessment error are accounted for. The results indicate that due to the strong trophic interactions, it is not possible to achieve the precautionary exploitation of all the stocks at the same time. Maintaining shrimp biomass above the limit reference point (Blim) would require unsustainable fishing pressure on cod and redfish to reduce predation mortality. In contrast, maintaining cod biomass above Blim would involve high predation on and high risk of collapse of the shrimp and redfish stocks. The implementation of alternative two-stage HCRs would reduce predation, resulting in higher productivity and lower probability of collapse for cod and redfish. The results of this study support the need of accounting for species interactions when designing management strategies for a group of interdependent commercial stocks.
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