Juvenile red rockfish, Sebastes sp., associations with sponges in the Gulf of Alaska

Marine Fisheries Review, Summer, 2003 by J. Lincoln Freese, Bruce L. Wing

The juvenile red rockfish that were observed during this study probably can be classified in category 4, fortuitous sponge-dwellers. Personal observations made by the authors during other investigations (unpubl. observ.) in the GOA indicate that juvenile red rockfish of the size encountered in 2001 can occupy a wide variety of habitat types, including other species of sponge, gorgonian coral colonies, and interstices between cobbles and boulders. The fact that the juvenile rockfish observed in this study were associated only with Aphrocallistes sponges can likely be attributed to the paucity of other available habitat types along the transect.

The lack of a clear relationship between sponge size and number of juvenile red rockfish harbored was surprising, but can most likely be attributed to the fact that the fish preferred those sponges with many finger-like protuberances in which to seek cover. Sponges in general are highly polymorphic, and there was no clear relationship between sponge size and number of protuberances present on the body of the sponge. Likewise, the fact that only 4% of the rockfish were associated with Rhabdocalyptus sponges can likely be attributed to the fact that this species is cylindrical in shape with a smooth texture, and affords minimal cover.

The locations of nursery grounds for many Sebastes spp. in the northeastern Pacific are generally not well known. Based on the fact that juvenile rockfish are lacking in offshore trawl catches but are often caught at nearshore trawl sites, Carlson and Haight (1976) hypothesized that nearshore hard-bottom coastal areas and adjacent fiords serve as nursery grounds for Pacific ocean perch, the most abundant rockfish in Alaskan waters. Their hypothesis was supported by observations made from a submersible (Carlson and Straty, 1981), during which thousands of juvenile red rockfish (6-8 cm length), believed by the authors to be Pacific ocean perch, were observed over just such habitat. The observations made during our investigation suggest that juvenile red rockfish may not exclusively use nearshore sites as nursery areas. The location at which we observed juvenile red rockfish of a size similar to those observed by Carlson and Straty (1981) was about 50 km offshore in the GOA; furthermore, the substrate at our study site was mostly a sand and silt mixture rather than rocky.

In conclusion, our observations show that Aphrocallistes sponges provide habitat for juvenile red rockfish. The association is fortuitous, in that many juvenile red rockfish have been noted in a wide variety of other habitat types in the GOA (personal observ.), including other types of emergent epifauna, as well as in cover provided by nonliving substrates such as boulders and cobbles. The fish observed in this study probably benefited from their association with the sponges through predator avoidance. The sponges provided most of the vertical relief on the seafloor at this site, with the exception of the boulders to which they were attached. Disruption of this type of sponge community by bottom trawling would be expected to have a negative impact on juvenile red rockfish survival in areas where other types of cover are not available.