Some consequences of variation in vessel density: a manipulative field experiment

Abstract

Recent studies have examined factors which influence the spatial distribution of trollers. Implicit in all these studies is the assumption that increasing vessel density must diminish catch rates of vessels. Existing data often make it impossible to test this assumption because it is difficult to control other variables which may also be influencing catch success. Here, we experimentally manipulated the density of three trollers. The vessels in our study predominantly caught chinook salmon ( Oncorhynchus tshawytscha), but also regularly landed coho salmon ( Oncorhynchus kisutch) and spiny dog-fish ( Squalus acanthias). Vessel density had a significant influence on the catch of chinook salmon and spiny dogfish, although there was a significant interaction with tidal velocity. As vessel density increased, there was a decrease in the catch rates of chinook salmon and an increase in the catch rates of spiny dogfish. The catch rates for coho salmon were unaffected by our manipulations. Different responses by these species resulted in a significant change in catch composition with changes in vessel density. Our ability to manipulate vessel density was limited compared with that which is observed during the commercial fishing season. That these manipulations generate observable effects suggests that variation in vessel density may exert a substantial influence on catch rates during the commercial fishing season.