Biological Bases for Mixed-Species Fisheries: Species Co-distribution in Relation to Environmental and Biotic Variables

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Abstract
Factors influencing the degree of temporal and spatial co-occurrence of seven important demersal fish species on Georges Bank (Northwest Atlantic) were analyzed. Species/age co-occurrence data from a 24-yr time series of research vessel bottom trawl surveys were evaluated in relation to temperature and depth preferences, spatial distribution, and species/age abundance trends. Co-occurrence of various species/age groups varied greatly by season and was significantly correlated with overlap in temperature/depth preference indices and annual species abundance. Although species abundance and proportion of survey tows having particular species varied widely over the time series, trends in spatial dispersion indices of particular species were not generally related to abundance. Multiple regression models incorporating temperature and depth preference overlap and species abundance as independent variables accounted for a high proportion of total variance in the transformed proportion of survey tows containing co-occurring species (mean R2 = 0.69 for age 1 + fishes, mean R2 = 0.44 for age 0). Results indicate that although the frequency of technological (bycatch) interaction among pairs of species may be highly variable depending on season, area, and year, co-occurrence among species is nonetheless relatively predictable. Minimization of the potential adverse impacts of mixed-species harvesting (i.e. failure to achieve management goals for all species simultaneously) may be accomplished by exploiting stocks individually during periods of maximum spatial separation and by using information on the determinants of species co-distribution to more accurately forecast bycatch levels.