Dietary Analysis of the longnose skate, Raja rhina (Jordan and Gilbert, 1880), in central California waters
Heather Robinson email: hrobinson@mlml.calstate.edu

Skates make up almost one-fourth of the species of cartilaginous fishes living today. Together, the three families contain more than 230 species worldwide, making them the most diverse group of elasmobranchs. Skates occur in all oceans of the world yet are most common in temperate and polar waters. They are benthic fishes occurring in inshore bays to 3,000 m depth. Although skates are widespread and abundant, with new species being described every year, minimal research has been conducted to understand life history traits of these fishes. The lack of this information often makes stock assessments and fishery management decisions difficult.

Raja rhina (Jordan and Gilbert 1880), the longnose skate, is one of five members of the family Rajidae living in California waters (Ebert 2003), and is easily distinguished from the other four by its extremely long, acutely pointed snout.

Thesis Abstract - The longnose skate, Raja rhina, is one of the most important incidental species landed in central and northern California demersal fisheries. However, life history information is extremely limited for this species and its diet and feeding habits are unknown. Feeding ecology studies can provide researchers with important insights towards understanding potential fishery impacts on marine systems. The primary objective of this study was to analyze the feeding ecology of R. rhina off the coast of central California. Specimens of R. rhina were collected between September 2002 and August 2003 from fisheries-independent trawl surveys conducted by the National Marine Fisheries Service, Santa Cruz Lab (NMFS-SCL). A total of 618 R. rhina stomach samples was processed, and all prey items identified to lowest possible taxa. For every stomach, the percentage of each prey item by number (%N) and weight (%W) were calculated and averaged to obtain a mean value. These measures were combined with the overall percent frequency occurrence (%FO) to determine the Index of Relative Importance (IRI), represented as %IRI. Results indicate that the five most important prey items were unidentified teleosts (31.6% IRI), unidentified shrimps (19.6%IRI), unidentified euphausiids (10.9% IRI), Crangonidae (7.4% IRI), and Neocrangon resima (6.0% IRI). Raja rhina diet was further analyzed through comparison of the following intraspecific variables: gender (male/female), depth (shelf/slope), and size class (<600 mm/>600 mm), using Morisita’s Index of Overlap. Thus, there was a high degree of overlap between gender (95.9%) and shallow depths (96.4%), while size classes overlapped very little (34.8%).

Future Work - The diet of R. rhina will be further analyzed using multivariate statistics to give significance to the Morisita's Index of Overlap results.

Funding Sources - I have recieved most of my funding for this project from the Pacific Shark Research Center. Additional funds have been acquired from the American Elasmobranch Society.