Faunal Composition and Diversity of Elasmobranchs in the Gulf of California

The Gulf of California supports a rich elasmobranch fauna, with more than 85 documented species.   However, environmental alteration (e.g., damming of the Colorado River, marine pollution, coastal development) and years of unregulated fishery exploitation have resulted in substantial changes to the marine fauna of this region, including considerable declines in populations of several elasmobranch species. Apparent changes in elasmobranch faunal composition will be treated qualitatively (based on a synthesis of available information) because detailed historic records are not available.  An updated list of all documented elasmobranch species will be provided, and single occurrences and recent absences will be discussed.  Patterns of taxonomic diversity will be compared to those of other comparable subtropical regions.  In addition, trends in species composition and diversity of demersal, nearshore elasmobranchs will be quantitatively assessed based on the results of comprehensive, year-round fishery surveys at two fishing sites in Sonora (El Choyudo, 28º 19.12’ N, 111º 27.18’ W, and Bahía Kino, 28º 49.11’ N, 111º 56.35’ W). These surveys, conducted during 1999, resulted in the examination of 141,605 elasmobranchs among 37 species (20 sharks, 17 batoids). Fishing gear in both regions consisted of gillnets typically fished in shallow (< 50 m), nearshore waters. Fishermen were opportunistic and retained the entire elasmobranch catch, with the exception of round stingrays (Urotrygonidae), which were discarded at sea.  Landings are therefore generally representative of local species composition. Temporal and spatial trends in catch composition and diversity will be combined with biological (e.g., size, sex, reproductive condition) and physical (e.g., depth, temperature) information to infer patterns of seasonal abundance, including immigration and emigration of reproductively active adults to pupping and mating grounds, and determine resident and transient species.