Bycatch associated with the Horseshoe Crab (Limulus polyphemus) Trawl Survey Larissa Graham, Brian Murphy, David Hata firstname.lastname@example.org Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University Bycatch: A fishery’s impact on non-target species It is estimated that among the 27 major fisheries in the USA, 1.06 million tons of fish are discarded for every 3.7 million tons landed1. Of these major fisheries, shrimp and bottom trawl fisheries are responsible for about 72% of total bycatch, or non-targeted individuals, by gear type1. Project Objectives Data Collection Sampling occurred from eastern Long Island, New York to the Delmarva Peninsula, Virginia (Figure 5). Sites were randomly-selected based on distance from shore (inshore/offshore) and bottom topography (trough/non-trough). A total of 156 sites were sampled for bycatch composition during the fall of 2005 (n = 73) and 2006 (n = 83). Sampling was conducted aboard a chartered commercial stern trawler, using a benthic trawl net. The net was modified to maximize the capture of horseshoe crabs. After towing the net for fifteen minutes, all taxa were emptied onto the deck, identified, counted, and weighed (Figure 6). Depth, bottom temperature and salinity were also recorded at each site. • Identify and quantify taxa caught by horseshoe crab trawl gear within the survey area. • Describe species assemblages within the survey area, specifically identifying assemblages that include horseshoe crab. • Identify the biotic and/or abiotic factors that determine species assemblages (i.e. local abundance of horseshoe crab, bottom type, temperature, salinity). Some bycatch are kept and sold at fish markets, but most are thrown back and often die as a result of stress or trauma2,3 (Figure 1). These losses may affect total population size, age structure, and sex ratios, as well as the overall food web, habitat, and other species within the ecosystem2. Preliminary Results Percent of total biomass A total distance of 193.7 km was sampled (88.7 km in 2005, 105 km in 2006). Eighty taxa were caught during the two surveys (60 taxa in 2005, 72 taxa in 2006), including 47 species of finfish, from 33 families. Figure 4. Percent of total biomass of both surveys combined. Skate and rays (55%) Inverts (4%) • Horseshoe crab trawl gear has potential to catch many non-target species. • The majority of the catch was composed of skates and rays (Figure 4). • Many species were present in more than 50% of tows during both years. Figure 1. Catch that is not valuable to fishers is discarded. Horseshoe crab (33%) Sharks (2%) Finfishes (6%) http://www.njscuba.net/artifacts/ship_fishing.html Figure 5. Sites trawled during 2005 and 2006 surveys. N Many trawl fisheries require bycatch reduction devices (BRDs), which allow certain species to escape. However, before BRDs can be developed, managers must identify which species are being caught. • Bycatch composition differs between northern sites and southern sites. • At the northern sites, horseshoe crab were associated with: • little and winter skate (Leucoraja spp.) • summer flounder (Paralichthys dentatus) • windowpane flounder (Scophthalmus aquosus). • At the southern sites, horseshoe crab were predominately found with: • clearnose skate (Raja eglanteria) • knobbed whelk (Busycon carica) • channel whelk (Busycotypus canaliculatus) • spider crab (Libinia spp.). • Identifying species that are caught as bycatch in the horseshoe crab trawl fishery is important. Many of the species listed above are not managed species and the impacts of catching and discarding individuals are unknown. = Fall 2005 sites = Fall 2006 sites Why the horseshoe crab fishery? Horseshoe crabs are harvested using bottom trawl nets (Figure 2). This gear has potential to catch many non-target organisms (Figure 3). Currently, few data have been collected on bycatch in the horseshoe crab trawl fishery, leaving ecosystem-level effects unknown. Windowpane flounder Little/winter skate Summer flounder Data Analysis Descriptive analysis: For each tow, total number of individuals and biomass were calculated for each taxa. Figure 6. All bycatch was identified, measured, and weighed. Multivariate statistical analyses: Cluster analysis was used to group sites. Relationships will be identified between biotic (species assemblages) and abiotic (bottom type, bottom temperature, salinity, or depth) factors. Clearnose skate Channel whelk Spider crab Figure 2. A crew member sorts through a typical catch off Ocean City, Maryland. Figure 3. Bycatch in the horseshoe crab trawl fishery. Funding by: National Marine Fisheries Service Horseshoe crab Research and Monitoring NMFS Grant # NA05NMF4751135 1. Harrington, J. M., R. A. Myers, et al. (2005). "Wasted fishery resources: discarded by-catch in the USA." Fish and Fisheries6(4): 350-361. 2. Crowder, L. B. and S. A. Murawski (1998). "Fisheries bycatch: implications for management." Fisheries Management23(6): 8-17. 3. Davis, M. W. (2002). "Key principles for understanding fish bycatch discard mortality." Canadian Journal of Fisheries and Aquatic Sciences59: 1834-1843.