Compensation in exploited marine fish communities on the Scotian Shelf, Canada

Abstract

We evaluated the temporal dynamics of the aggregate groundfish community, further decomposed into functional groups, individual species abundances, and health indicators, from adjacent management units on the Scotian Shelf. On the colder, eastern half of the shelf, several species collapsed in the early 1990s, while, on the western half, no such collapses were evident despite similar exploitation regimes. The decline in the eastern aggregate biomass was influenced by a decline in average body size, which was interpreted as a past integrator of temperature and size-selective fishing. Biomass of 3 out of 4 functional groups declined in the east; 3 out of 4 groups increased in the west. Some species from the east appear to be slowly compensating, but not enough to counter the decline in aggregate biomass. Species inhabiting the warmer western region show strong evidence of compensation. In effect, warmer waters allow potential compensating species to increase at a faster rate. Quantitative measures of functional group temporal stability revealed no differences between areas; therefore, neither area can be considered stable. Physiological condition declined in many species in both regions. Species that increased in the west, or had slower rates of decline, had higher levels of condition than eastern populations. With the exception of cod in the western region, species growth rates declined in both areas. Although the west appears more stable, it is following a similar, but protracted trajectory, to that found in the east. The protracted response in the west may be due to higher demographic rates in warmer waters. To foster resilience in the western area, we should address the rapid pace of new fisheries, effects of size-selective mortality, and the diminishing number of natural refugia. © Inter-Research 2007.