Is By-catch Mortality Preventing the Rebuilding of Cod (Gadus morhua) and American Plaice (Hippoglossoides platessoides) Stocks on the Grand Bank?

Abstract

Cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) on the Grand Bank of Newfoundland supported substantial commercial fisheries for extensive periods in the past. Both stocks declined and have been under fisheries moratoria since 1994, but continue to be harvested as by-catch in other directed fisheries on the Grand Bank. For each stock, a precautionary spawning stock biomass limit reference point (B_lim) has been established by the Scientific Council of the Northwest Atlantic Fisheries Organization. In this study, stochastic simulations were used to evaluate the probability distributions of the time horizons required to rebuild each of the two stocks to above B_lim. A range of by-catch mortality levels, recruitment models, and body growth and maturation rates were applied in the simulations. Uncertainty in the estimates of the numbers at age in the last assessed year and uncertainty in future recruitment were also incorporated within the simulations. If by-catches are not substantially reduced, the modelling results indicate there is virtually zero probability of attaining B_lim by 2020 for either the cod or American plaice stocks on the Grand Bank.

Key words: American plaice, by-catch, cod, collapse, precautionary approach, rebuilding, recruitment, stochastic simulation, spawning stock biomass.

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