Shift in fish assemblage structure due to loss of seagrass Zostera marina habitats in Sweden

The areal extent of Zostera marina in the archipelago of the Swedish Skagerrak has decreased by 60% over two decades. To investigate the effects of Z. marina loss on the local fish assemblages, the fish fauna was compared between existing seagrass beds and sites where seagrass had vanished. A field study was carried out at four shallow locations in the outer archipelago of the coast in June 2004. Within each location two sites were sampled, one with an existing Z. marina bed and another where Z. marina had disappeared. Fish were sampled semi-quantitatively with a beach seine. Samples were taken during both day and night and captured fish were examined to species, enumerated and measured in the field, and released thereafter. The number of fish species was found to be significantly higher in Z. marina habitats compared to areas where seagrass was missing, and density and biomass of fish were generally lower in areas dominated by bare sediment compared to those in the seagrass habitats. Several species and groups of fishes (i.e., gadoids, labrids, syngnathids) were absent or occurred in low densities at sites where Z. marina was missing. For example, juvenile 0-group cod density was reduced by 96% at sites where Z. marina had disappeared. Such a reduction in recruitment of cod is in the same order of magnitude as the combined effect of seal predation and mortality due to by-catches in the eel fyke-net fishery estimated for the archipelago of the Swedish Skagerrak. Hence, the results clearly indicate a shift in the fish assemblage, including a loss of taxa at the family level as a result of degradation in habitat-forming vegetation.