Diel differences in surface trawl fish catches off Oregon and Washington

Abstract

Abstract Surface rope trawls have recently been used to investigate pelagic fish communities off the west coast of North America. We examined surface rope trawl catches from locations off the Washington and Oregon coast to determine whether they differed day and night. We paired day and night samples from the same location taken within 7 days of each other. Fifty-one sample pairs fit our criteria with 33 of these controlled for tidal state. Samples were taken at 11 different stations during the months of May, June, and September from 1998 to 2002. We used the sign test to compare median densities for nine of the most abundant and commonly occurring fish species: two juvenile salmonids—coho salmon ( Oncorhynchus kisutch ) and yearling Chinook salmon ( O. tshawytscha ); four small pelagics—northern anchovy ( Engraulis mordax ), Pacific herring ( Clupea pallasi ), Pacific sardine ( Sardinops sagax ), and whitebait smelt ( Allosmerus elongatus ); and three larger predatory fishes—Pacific hake ( Merluccius productus ), chub mackerel ( Scomber japonicus ), and jack mackerel ( Trachurus symmetricus ). Both species of salmon were caught at higher densities during the day. The small pelagic and predatory species were caught in higher densities at night. The differences were significant for both salmonids, Pacific hake, chub mackerel, and all small pelagic species. No consistent pattern of correlation existed between the catch differences and differences in temperature or salinity between day and night samples. We believe diel patterns of fish vertical distribution and behavior coupled with the actual depths the net samples are primarily responsible for catch differences.