Theses and Dissertations

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Defense Date

2000

Document Type

Thesis - NSU Access Only

Degree Name

M.S. Marine Biology

Second Degree Name

M.S. Coastal Zone Management

Department

Oceanographic Center

First Advisor

Mahmood Shivji

Second Advisor

Enric Cortes

Third Advisor

Richard Spieler

Abstract

Data collected by fisheries observers aboard U.S. pelagic longline vessels were examined to quantify and describe elasmobranch bycatch off the southeastern U.S. coast (22° - 35° N, 71° - 82° W). From 1992 to 1997, 608 "sets" or gear deployments were observed which resulted in the capture of 2649 elasmobranchs (17.5 percent of the total catch). Of the 21 elasmobranch species observed, silky sharks, Carcharhinus falciformis, were numerically dominant (34.4%) followed by dusky, C. obscurus (16.3%), blue, Prionace glauca (8.9%), unidentified sharks (6.4%), tiger, Galeocerdo cuvier (5.9%), and scalloped hammerhead, Sphyrna lewini (5.7%). The catch status of the animal (alive or dead) when the gear was retrieved varied widely depending on the species, with high mortalities seen for the commonly caught silky and dusky sharks and low mortalities for rays, blue sharks, and tiger sharks. Discard percentages depended mostly on the marketability of the species, ranging from low discards (20.4 %) for shortfin mako, Isurus oxyrhinchus, to high discards for blue (99.5 %), tiger (96.7 %), and rays (100 %). Percentages of live and dead discards also varied considerably among species. Analysis of variance indicated significant catch rate differences among seasons for silky, dusky, blue, oceanic whitetip (C. longimanus), sandbar (C. plumbeus), and shortfin mako sharks. Mean fork lengths indicated the majority of the observed bycatch, regardless of species, was immature, and significant seasonal variation in fork length was found for several species including silky, dusky, blue, night (C. signatus), and oceanic whitetip sharks. While sex ratios overall were relatively even, blue and tiger shark catches were dominated by females. Only silky sharks showed significant seasonal sex ratio variation, with a higher proportion of males observed in the winter.

Bootstrap methods were used to generate yearly mean catch rates (catch per unit effort) and 95 % confidence limits. Catch rates were in turn used to generate an overall estimate of the numbers of elasmobranchs discarded dead and alive during 1992-1997. These estimates predict 38,775 (18,780 and 63,751, lower and upper 95 % confidence limits) and 31,859 (13,837 and 56,712, lower and upper 95 % confidence limits) sharks were discarded alive and dead, respectively, during this time period. Possible implications of the results of this study for shark fishery management are discussed for the most common bycatch species.

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