M.S. Thesis 

The roughtail skate, Bathyraja trachura (Gilbert 1892) is a medium sized skate, maximum total length 910cm, that lives on soft bottom habitat from 200-2,550m, but is more abundant in waters deeper than 600m (Ebert 2003; Davis et al. 2007). These depths on the upper continental slope have recently received attention globally as large commercial fisheries move into these deeper waters from the depleted shelf habitats (Haedrich 2007). A recent paper by Tolimeri and Levin (2006) determined that the six most abundant fishes on the upper continental slope caught in the NMFS/NOAA Fisheries Resource Analysis and Monitoring Division (FRAM) groundfish cruises were: Pacific grenadiers, Coryphaenoides acrolepis; giant grenadiers, Albatrossia pectoralis; deepsea soles, Embassichthys bathybius; longspine thornyheads, Sebastolobus altivelis; California slickheads, Alepocephalus tenebrosus; and B. trachura. Unfortunately, diet and trophic level information for these deepwater fishes is often limited by small sample sizes (Buckley et al. 1999) due to the great effort required to collect an adequate sample size. Regurgitation and swim bladder expansion upon ascent often hinder the recovery of viable samples (Drazen et al. 2001). An alternative to the traditional gut content analysis to determined trophic level may be necessary in deep-sea fishes due to sampling constraints.

As science moves more towards ecosystem-based management it is important to focus on the interaction and trophic dynamics of entire communities instead of individual species (Botsford et al. 1997). This study attempts to meet these needs by employing the relatively new technique of stable isotope analysis to analyze diets of deep-sea fishes and move towards ecosystem (multi-species) based management of upper slope communities. The objectives of this study is to provide insight into the trophic levels of a community of deep sea benthic fishes using stable isotope analysis (SIA) of nitrogen and carbon as a potentially viable alternative to gut content analyses. If feasible in the deep sea, employing this method in other communities could revolutionize the way we understand food web and trophic dynamics within these ecosystems.

The objectives of this study are to: (1) characterize the overall diet of Bathyraja trachura based on gut content analysis; (2) determine if the diet varies due to differences in sex, latitude, ontogenetic state; (3) determine the trophic level of B. trachura based on the gut content analysis; (4) determine the trophic level of the six most dominant fishes of the upper slope as determined by Tolimeri and Levin (2006) via nitrogen and carbon stable isotope analysis of the white muscle of adult fishes; (5) determine if an ontogenetic shift in diet occurs in B. trachura as determined by nitrogen and carbon stable isotope analysis of vertebral cartilage; and (6) compare the trophic levels (TL) of these fishes determined by gut content analysis and stable isotope analysis.

Newly hatched B. trachura (Disc Length approx. 5cm)	Photograph of B. abyssicola (left) and B. trachura (right), both of which are found in deepwater off the west coast of the United States. Photo courtesy of Dr. David Ebert

Other Research Projects:

Boyle, M.D., B.H. Nagareda, and J.M. Shenker. In prep. Feeding Habits and Gastrointestinal Parasite Abundance in Five Species of Batfishes (Ogcocephalidae) from Florida and the Gulf of Mexico.

Posters and Presentations:

ALASKA MARINE SCIENCE SYMPOSIUM JANUARY 2008
WESTERN GROUNDFISH CONFERENCE FEBRUARY 2008

Funding:

NOAA NMFS to the National Shark Research Consortium and Pacific Shark Research Center at Moss Landing Marine Laboratories.

Literature Cited:

Botsford, L.W., J.C. Castilla, and C.H. Peterson. 1997. The management of fisheries and marine ecosystems. Science. 277(5325):509-515.

Buckley, T.W., G.E. Tyler, D.M. Smith, and P.A. Livingston. 1999. Food habits of some commercially important groundfish off the coasts of California, Oregon, Washington, and British Columbia. NOAA Technical Memo NMFS-AFSC-102. Aug 1999.

Davis, C., G.M. Cailliet, and D.A. Ebert. 2007. Age and growth of the roughtail skate Bathyraja trachura (Gilbert 1892) from the eastern North Pacific. Environ Bio Fish. 80:325-336.

Drazen, J.C., T.W. Buckely, and G.R. Hoff. 2001. The feeding habits of slope dwelling macrourid fishes in the eastern North Pacific. Deep-Sea Research (48)909-935.

Ebert, D.A. 2003. Sharks, Rays, and Chimaeras of California. University of California Press, Berkeley, California, 284 pp.
Haedrich, R.L. 2007. Deep Trouble. Natural History, Oct 2007:28-33.

Tolimeri, N. and P.S. Levin. 2006. Assemblage structure of Eastern Pacific groundfishes on the US continental slope in relation to physical and environmental variables. Trans of Am Fisheries Soc. 135:317-332.