Abundance-distribution relationships and conservation of exploited marine fishes

Abstract

The effects of human exploitation on macroecological patterns have received little attention, although such investigations may highlight unique spatial and temporal changes characteristic of species and assemblages subject to persistent disturbance. In unexploited systems (mainly among temperate avifauna) positive relationships between local abundance and geographic distribution are prevalent for individual species through time (intraspecific pattern) and among species during fixed time periods (interspecific pattern). We investigated intraspecific and interspecific relationships for 24 common marine fishes on the Scotian Shelf and Bay of Fundy, Canada, some of which have been commercially exploited for several decades. Based on extensive fisheries-independent trawl survey data from 1970 to 2001, 16 of the 34 stocks, comprising 13 species, exhibited significant positive intraspecific relationships. Significant relationships were associated mainly with those stocks that demonstrated significant temporal trends in both abundance and geographic distribution. The time-averaged (32 yr) interspecific relationship was positive and significant at the largest scale examined. Significant annual interspecific relationships were also detected over 26 yr. Surprisingly, the slopes of the annual relationships increased systematically and doubled through time, probably due to size-selective exploitation, shifting target species, and associated species interactions. In contrast to previous studies, our results indicate that the contributions of individual species to the interspecific relationship can change through time, and these changes dramatically alter the interspecific abundance-distribution relationship. Temporal trends in the interspecific relationship have not previously been reported, and appear to be due to the large spatial- and temporal-scale effects of exploitation.