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In Japanese rivers, there is a serious conflict between the great cormorant ( Phalacrocorax carbo hanedae ) and fisheries. The basis of this conflict is that the cormorants feed on ayu ( Plecoglossus altivelis ), a commercially important fish that is stocked primarily for recreational fishing. To understand how cormorants alter their foraging habitats in relation to the stocked fish and fishing activities, we examined the relationship between cormorant abundance and ayu biomass during the cormorant breeding season (from April to July) using two approaches that differ in spatial scale. First, we compared cormorant numbers in different river sections that were defined based on ayu stocking. The cormorant numbers in the sections stocked with ayu increased during the ayu release period, whereas the cormorant numbers in other sections showed no clear seasonal patterns. Second, we tested whether cormorant numbers were correlated with the biomass of ayu caught with cast nets. Positive correlations were observed between the biomass of ayu and the number of cormorants that were within 900 m, 1 km, or 2 km of fish sampling points; however, such correlations were not observed within 100–800 m of the sampling points. The biomass of ayu caught with cast nets increased steadily from April to June despite predation by cormorants; however, this biomass decreased sharply in July when the fishing season opened. This study indicates that although cormorants altered their feeding areas in accordance with the mass stocking of ayu in a Japanese river, sufficient numbers of ayu were still maintained for anglers. |
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