Sampling efficiency of longlines for shortraker and rougheye rockfish using observations from a manned submersible

Abstract

Populations of demersal rockfish of the genus Sebastes are challenging to assess because they inhabit rocky areas that are difficult to sample with trawl gear. In contrast, longline gear can sample rocky areas, but several factors besides fish density can affect the relationship between catch rates and density. In this study, longline catch rates of shortraker rockfish Sebastes borealis and rougheye rockfish S. aleutianus were compared with observations of density from a manned submersible to evaluate the species' catchability on longline gear. On separate occasions, rockfish behavior in the presence of longline gear was observed from the submersible. Densities averaged 3.0 shortraker and rougheye rockfish (combined) per 330 m2 of bottom (the effectively sampled area of a 100-m transect). Longline catch rates averaged 2.7 shortraker and rougheye rockfish per skate of 45 hooks. Longline catch rates were not statistically affected by submersible observations. There was a positive trend between density and longline catch rates, but the relationship was not significant. As observed from the submersible, the proportion of fish free-swimming near the longline increased through the duration of the set, indicating that rockfish were attracted to the line faster than they were caught. The catching process for shortraker and rougheye rockfish lasts longer than for more mobile species such as sablefish Anoplopoma fimbria. © American Fisheries Society 2011.