Catch Efficiency of Longlines for Shortraker and Rougheye Rockfish in Alaska
Populations of rockfish (Sebastes spp.) can be difficult to assess with bottom-trawl survey gear because the species often inhabit untrawlable rocky habitats. Conversely, longline gear can be set in most bottom habitats and is used to assess abundance of many benthic fish species.
Several factors besides abundance, however, can affect catch rate and consequently diffuse the relationship between catch rate and density, making catch rates a less reliable tool for assessing abundance trends. Because the relationship between actual fish densities and catch rates can be affected by so many factors, it is important to characterize the relationship when using catch rates as an index.
To evaluate shortraker and rougheye rockfish catchability on longline gear, ABL scientists compared longline catch rates to observations of density from a manned submersible at 19 sites in Southeast Alaska based on field studies originally conducted in 1994 and 1997.
Densities of shortraker rockfish ranged from 0 to 12,616 fish per km2. Densities of rougheye rockfish ranged from 0 to 24,222 fish per km2. Linear regressions between density and longline catch rates were not significant for either species, but a positive trend was evident for rougheye rockfish.
The estimate of rougheye rockfish catchability from this study was about 18% of the stock assessment estimated catchability. Simply obtaining a comparable value (the same order of magnitude) lends credence to both estimates.
When longline gear was 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. Rockfish behavior and soak time may account for the lack of strong correlations between catch rates and densities, although other factors such as environmental conditions and sample size may also have affected our results.
Future research should be aimed at describing the catching process for these rockfish in Alaska to further investigate the reliability of longline survey rates as an index of abundance.
By Cara Rodgveller, Mike Sigler, Dana Hanselman, and Dan Ito
