Profile of Shark Bycatch in the U.S. Gulf of Mexico Menhaden Fishery

Abstract

We describe the 1994 and 1995 releasable bycatch (bycatch) of sharks (Carcharhinidae and Sphyrnidae) in the U.S. Gulf of Mexico fishery for gulf menhaden Brevoortia patronus. “Releasable bycatch” is defined as catch of nontarget species observed in the purse seine from the time it is brought alongside the carrier ship until all the catch is pumped into the hold and the net is cleaned. Shark bycatch in the gulf menhaden fishery was positively skewed, 726 sharks being observed in 30% of the fishing sets sampled. Ten species of sharks were identified, blacktip sharks Carcharhinus limbatus being the most common. Approximately 20% of sharks were not identified to species. At the time of release, 75% of the sharks in the bycatch were dead, 12% were disoriented, and 8% were healthy. An annual bycatch of approximately 30,000 sharks for the fishery was estimated for the 1994 and 1995 fishing seasons. By means of a logit model, the likelihood of observing shark bycatch was explained by spatial and temporal differences within the fishery and the size of the menhaden catches. In 1995, we observed significant temporal differences in bycatch in the fishing grounds between 90° and 93°W, where much of the fishery's effort occurred. The odds of observing sharks was highest in April and May. In addition, the odds of observing a set with shark bycatch were 2.4 times as great for fishing sets with a catch greater than 16 metric tons than for sets with a smaller catch. Stomach analyses of sharks suggest their occurrence in the fishery is probably the result of a predator–prey relationship between sharks and gulf menhaden. Although detailed age and maturity data were not collected, comparisons of the length distributions to those found in the literature indicated that the fishery may be affecting a portion of the primary and secondary nursery grounds of sharks in the northern Gulf of Mexico.