Effects of large-scale habitat variability on condition of demersal exploited fish in the north-western Mediterranean

Abstract

We examine the variation in condition of ten exploited fish species as an indicator of large-scale habitat quality in the north-western Mediterranean. To determine fish condition, we used morphometric (Fulton's K) and physiological (hepatosomatic, HSI; digestivesomatic, DSI; and gonadosomatic, GSI) indices. Generalized linear models (GLMs) were applied to assess the influence of depth, latitude, length, year, sex and reproductive status on condition. The GLMs incorporating all independent variables accounted for between 25 and 74% of the variance in Fulton's K condition factor, 15-77% of the variance in HSI, 14-35% of the variance in DSI, and 77% of the variance in GSI. The mean condition factor of the species studied showed considerable interannual fluctuations along with an increasing trend over the time series (1994-2001). Length is an important explanatory variable in explaining condition of fish species. Condition typically increased with fish length and explained 72% of the deviance in GSI of M. barbatus. For some species, there were differences in condition due to sex and reproductive status, with adults being usually better in condition than juveniles and, among adults, females better than males. The influence of latitude and depth on condition of a given species varied according to the index used to assess condition. When there was a significant effect, fish with the highest condition factor, HSI and/or DSI inhabited the shallowest and northernmost parts of the area of study. Considering that condition influences growth, reproduction and survival of fish, our results support the importance of shallow habitats to productivity of demersal stocks in the Mediterranean Sea.