Transcript Why the horseshoe crab fishery?

Bycatch associated with the Horseshoe Crab (Limulus polyphemus) Trawl Survey
Larissa Graham, Brian Murphy, David Hata
[email protected]
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.
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.
Project Objectives
Data Collection
1. Identify and quantify taxa caught by horseshoe crab trawl gear within the survey area.
2. Describe species assemblages within the survey area, specifically identifying assemblages that include horseshoe
crab.
3. Identify the biotic and/or abiotic factors that determine species assemblages (i.e. local abundance of horseshoe
crab, bottom type, temperature, salinity).
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.
Skate and rays (55%)
Figure 4. Percent of
total biomass of both
surveys combined.
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/nontrough). 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.
Inverts (4%)
Horseshoe crab trawl gear has potential to
catch many non-target species.
Figure 1.
Catch that is
not valuable
to fishers is
discarded.
• The majority of the catch was composed of skates
and rays (Figure 4).
http://www.njscuba.net/artifacts/ship_fishing.html
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.
Why the horseshoe crab fishery?
Horseshoe crabs are harvested using bottom trawl nets
(Figure 2). This gear has potential to catch many nontarget organisms (Figure 3). Currently, few data have been
collected on bycatch in the horseshoe crab trawl fishery,
leaving ecosystem-level effects unknown.
• Many species were present in more than 50% of tows
during both years.
Horseshoe crab (33%)
Sharks (2%)
Finfishes (6%)
Figure 5. Sites trawled
during 2005 and 2006
surveys.
Bycatch composition differs between northern sites and southern sites.
At the northern sites, horseshoe crab were associated with:
= Fall 2005 sites
= Fall 2006 sites
• little and winter skate (Leucoraja spp.)
• summer flounder (Paralichthys dentatus)
• windowpane flounder (Scophthalmus aquosus).
Windowpane flounder
Little/winter skate
Summer flounder
At the southern sites, horseshoe crab were predominately found with:
Data Analysis
• clearnose skate (Raja eglanteria)
Descriptive analysis: For
each tow, total number of
individuals and biomass
were calculated for each
taxa.
• knobbed whelk (Busycon carica)
• channel whelk (Busycotypus canaliculatus)
Figure 6. All bycatch was
identified, measured, and weighed.
• spider crab (Libinia spp.).
Clearnose skate
Figure 2. A crew member sorts
through a typical catch off Ocean
City, Maryland.
Figure 3. Bycatch in the horseshoe
crab trawl fishery.
1. Harrington, J. M., R. A. Myers, et al. (2005). "Wasted fishery resources: discarded by-catch in the USA." Fish and Fisheries 6(4): 350-361.
2. Crowder, L. B. and S. A. Murawski (1998). "Fisheries bycatch: implications for management." Fisheries Management 23(6): 8-17.
3. Davis, M. W. (2002). "Key principles for understanding fish bycatch discard mortality." Canadian Journal of Fisheries and Aquatic Sciences
59: 1834-1843.
N
Channel whelk
Spider crab
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.
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.
Funding by:
National Marine Fisheries Service
Horseshoe crab Research and Monitoring
NMFS Grant # NA05NMF4751135