Steeper biomass spectra of demersal fish communities after trawler exclusion in Sicily

The effects of trawling on Mediterranean demersal fish communities were assessed by comparing them with the normalized biomass spectra among three gulfs on the north Sicilian coast, one of which had been subject to 15 years of trawler exclusion. Comparisons were conducted across seasons and among depth strata. Biomass-spectra slopes were significantly steeper in the gulf that was closed to trawling than in the unprotected gulfs. This was attributed to the exclusion of trawlers, which lacked catch-size selectivity, and to the continued fishing by artisanal gears that are more size selective. The biomass of all size classes was higher in the protected gulf than in unprotected areas, and increases were greatest in smaller size classes. Community mean individual fish mass was similar among all areas with a wider range of body masses present in the trawl exclusion area, compensating for the greater abundance of small fish. Size-spectra slopes were generally shallower and midpoint heights lower with increasing depth and were greater in autumn than spring; the effect of these seasonal and depth factors was as great as that of protection. Depth patterns are explainable by bathymetric trends in within-and among-species fish size. Seasonal differences were attributed to variation in spawning behaviour and fishery recruitment, with seasonal differences being greater in unprotected locations as a result of recruitment over fishing.