RELATIVE EFFECTS OF TRAP VERSUS TRAWL FISHERIES ON POPULATION-DYNAMICS OF THE NORTHERN COD (GADUS-MORHUA) STOCK

We compared the effects of the inshore trap and the offshore trawl fis heries on the population dynamics of the northern cod (Gadus morhua) s tock using data analyses and simulation modelling. We first statistica lly characterized the catch versus stock biomass relationships for the two fisheries (1977-1986). We found a significant (P < 0.0001) relati onship between the trawl catch at time t and the stock biomass at time t - 2. No temporal lag was evident in the trap catch versus stock bio mass relationship. The variability in these two relationships was simi lar. We then modelled the catch and stock biomass dynamics of the two fisheries in parallel, incorporating the observed catch versus stock b iomass relationships, but assuming equal mean catches, to examine the effects on cod population dynamics of the temporal lag and variability in the catch versus stock biomass relationships. The results suggest that, for the same amount of fish taken, a quota-based trawl fishery p resents a much greater risk of collapse to the cod stock than does an inshore trap fishery. Current management methods overestimate the ''sa fe'' catch for the trawl fishery because they do not incorporate the c onsequences of the lag in the relationship between stock biomass and t rawl catch.