Regional variation in non-targeted bycatch composition in the U.S. Atlantic bottom longline shark fishery
Bycatch in US fisheries has become an increasingly important issue to both fisheries managers and the public as both parties have seen a wide range of marine resources being caught as bycatch in many fisheries. From 1994-2005, the Commercial Shark Fishery Observer Program (CSFOP) collected data on targeted sharks and all non-shark bycatch caught on monitored vessels of the U.S. Atlantic bottom longline fishery. Our objective was to evaluate the composition of this bycatch and how it varied among regions, which has important implications for management actions such as marine protected areas and time area closures. A two-way ANOVA test was performed using time period as a block effect and geographical subregion and taxon as factors. Three subregions (eastern Gulf of Mexico, south Atlantic, Mid-Atlantic Bight), two time periods (1994-2001 and 2002-2005), and seven broad taxonomic categories (Batoidea, Anguilliformes, Serranidae, Carangidae, Lutjanidae, Other Osteichthyes, and Other Vertebrata) were used in the analyses. There was a significant difference between regions (P = 0.0003, F = 13.36, a = 0.05) and taxa (P = 0.0002, F = 8.68, a = 0.05) but no significant interactions of these factors. Non-shark bycatch was most abundant in the eastern Gulf of Mexico. Groupers and their kin (Serranidae) was the most common bycatch group in all three regions. Rays and skates (Batoidea) were commonly taken in the Mid-Atlantic Bight and south Atlantic subregions and eels (Anguilliformes) were a frequent bycatch in the eastern Gulf of Mexico. Interactions with conservation-sensitive sea turtles, cetaceans and sea birds were relatively rare, representing less than two percent of the total bycatch.
The capture depth, time and hooked survival rate for bottom longline caught large coastal sharks
The primary gear type used to harvest large coastal sharks in the U.S. Atlantic fishery is the bottom longline. Longline characteristics vary regionally with gear typically consisting of about 5-15 miles of longline and 500-1500 hooks (Burgess and Morgan, 2003). Gear is set at sunset and allowed to soak overnight before hauling back in the morning. During 1998-2002, between 241,000 and 383,200 sharks, primarily caught with longline gear from the large coastal complex, were harvested or discarded as bycatch (Cortés, 2003). A recently completed stock assessment of large coastal sharks has estimated that overfishing of this resource may still be occurring and that a reduction in the recent level of catches is likely to be necessary for the resource to reach maximum sustainable yield (MSY) (Cortés et al., 2002). Sharks in the large coastal grouping include relatively large, slow-growing, and long-lived species such as sandbar shark (Carcharhinus plumbeus), scalloped hammerhead shark (Sphyrna lewini), and blacktip shark (C. limbatus). In addition, the dusky shark (C. obscurus), a species highly susceptible to overexploitation, is currently listed as a prohibited species (cannot be landed), as a result of the National Marine Fisheries Service's (NMFS) precautionary approach to fisheries management (NMFS, 1999).
There are no restrictions on the bottom longline fishing gear used in the large coastal shark fishery (i.e. length of set, number of hooks, soak time) (NMFS, 2003). Alternative measures such as reduced soak time, restrictions on the length of gear, and fishing depth restrictions could aid in the reducing mortality of those species still considered over-fished. In order for such management measures to be enacted, data concerning the correlation between soak time and fishing mortality for individual species, capture depth and catch-per-unit effort (CPUE) in a depth stratified manner must first be collected for the bottom longline shark fishery. For example, Boggs (1992) deployed hook timers on pelagic longline fishing gear in waters off Hawaii targeting bigeye tuna and found that eliminating shallow hook depths would reduce mortality on bycatch of billfish without reducing efficiency for the target species.
Gear modifications implemented in the bottom longline fishery, such as placing restrictions on fishing depth and soak time, could reduce bycatch in the fishery and allow fishers to return unwanted species to the water alive, while still effectively catching targeted species. We are currently deploying hook timers on bottom longline gear with the following objectives:
- To determine what length of time the fishing gear is in the water prior to a shark biting and being hooked and at what point during the fishing process this occurs.
- To obtain more accurate data pertaining to the length of time individual shark species remain alive after being hooked on bottom longline fishery gangions.
- Using depth stratified sampling, to record the accurate depths of capture of sharks and to provide a description of the depth range and catch per unit effort (CPUE) by depth for up to 25 species of sharks.