Document Type

Master's Thesis

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Quantifying deep-sea food webs can be resource intensive due to the difficulties of sampling fishes from the deep sea. The diet of fishes is often quantified through stomach content analysis, through this method has many sampling constraints, and it can be difficult to obtain sufficient samples for an in-depth study. This study attempts to fill a critical data gap by determining the diet and trophic level of the deep-sea Roughtail Skate, Bathyraja trachura, using traditional stomach content analysis. This study also attempts to determine the validity and accuracy of stable isotope analysis in the continental slope fishes of the deep-sea of the eastern North Pacific, as an alternative method to determine trophic level in fishes. The Roughtail Skate is an abundant deep-sea skate in the eastern North Pacific. Little is known about the diet of this skate, which is landed as by-catch in commercial bottom trawls. Skates were collected between 2005 and 2008 from fishery-independent trawl surveys of the continental slope and outer shelf. Geometric Index of Importance (GII) values indicated that crustaceans (71.4%), fishes (17.8%), polychaetes (4.3%), and cephalopods (3.7%) were the most important prey groups in the diet. Diet differed significantly with total length, but not with sex. Larger individuals (by total length) had significantly higher trophic level values, and year and latitude explained variation in the diet for three prey categories. In this study, fishes and invertebrates collected from the continental slope (1,000 m depth) of the eastern North Pacific were analyzed using stable isotope analysis (SIA). The carbon and nitrogen stable isotope results were used to construct dual isotope plots to investigate the trophic relationships of this deep-sea community. The plots indicated a decoupling of the benthic and pelagic food webs, with the benthic food web being isotopically enriched. Stomach and isotope samples were collected from 32 Roughtail Skates (Bathyraja trachura) to determine the validity and accuracy of SIA in determining the trophic levels of the skates. A linear regression analysis indicated that nitrogen values from SIA and trophic levels calculated from stomach content analysis, when plotted against skate total length, exhibited similar variation and patterns, although only the stomach content analysis yielded significant results (stomach content: p=0.020, r2=0.168; stable isotope: p=0.077, r2=0.101).


Thesis (M.S.) Division of Science and Environmental Policy. Moss Landing Marine Laboratories