In order to accurately assess population trends in any group of animals, two things must happen: First, an accurate initial census (baseline) must be taken that counts (or estimates within an acceptable margin of error) the total number of individuals in a population. Second, the method by which the animals are counted after the baseline census must be consistent from location to location and from year to year. Unfortunately, in the case of horseshoe crabs, there is no accurate baseline data from previous years; in addition, governmental and environmental groups have employed different (i.e., inconsistent) censusing methods over the years. The end result is that, as of the year 2000, no one can say with certainty how many horseshoe crabs inhabit the Atlantic coast, or whether their numbers have gone up or down significantly in the past 10 or 20 years.
Assessing Annual Recruitment
Assessing Spawning Stock Biomass
Of the estimated 200,000 to 250,000 horseshoe crabs bled by the biomedical industry each year, perhaps as many as 10 to 15 percent of the animals do not survive the bleeding procedure. This is a source of mortality not included in the statistics from the commercial fishing of horseshoe crabs (Rudloe, 1983; Thompson, 1998). Mortality due to the bleeding procedure may be lower (0 to 4 percent), depending on the individual biomedical facility (Swan, pers. comm., 1998). However, the mortality associated with collecting, shipping, and handling the animals remains unknown. Currently, the biomedical industry is estimated to account for the mortality of 20,000 to 37,500 horseshoe crabs per year (10 to 15 percent of the animals collected).
Fishing mortality is the number of horseshoe crabs that are removed from the population by human activities; this may include direct fishing mortality (i.e., intentional legal harvest) and non-harvest mortality (i.e., poaching and bycatch). The 1996 fishing mortality accounted for at least 2 million individuals throughout the Atlantic Coast, with at least 1.7 million individuals being taken between New Jersey and Virginia. These statistics are based on landings (catch) data provided by the individual states and the NMFS (1998). Reported commercial landings data show a substantial increase in harvest during the 1990s, which could be a result of both an increase in fishing effort and an increase in reporting.
The shrimp trawl fishery in the South Atlantic Bight may contribute to horseshoe crab mortality via bycatch (Thompson, 1998), but the amount of bycatch harvest remains unreported. Since the use of turtle excluder devices became mandatory in the shrimp trawl fishery, the amount of horseshoe crab bycatch has become very small (Cupka, pers. comm., 1998).
Description of Habitat
Older juveniles will migrate out of the intertidal sand flats to deeper bay waters, where they will remain until they have developed into adults and are ready to reproduce.
Identification and Distribution of Essential Habitat
Of all these habitats, the beaches are the most critical (Shuster, 1994). Optimal spawning beaches may be a limiting reproductive factor for the horseshoe crab population. Based on geomorphology of the beaches, Botton, et al. (1992) estimated that only 10 percent of the New Jersey shore adjacent to Delaware Bay provides optimal horseshoe crab spawning habitat. The densest concentrations of horseshoe crabs in New Jersey occur on small, sandy beaches surrounded by salt marshes or bulkheaded areas (Loveland et al., 1996).
Prime spawning habitat is widely distributed throughout Maryland's Chesapeake and coastal bays and includes some tributaries. Horseshoe crabs are restricted to areas where the salinity exceeds seven parts per thousand (Maryland Department of Natural Resources, 1998). In the Chesapeake Bay, spawning habitat generally extends to the mouth of the Chester River but can occur farther north during years of above-normal salinity levels. Prime spawning beaches within the Delaware Bay consist of sand beaches between the Maurice River and the Cape May Canal in New Jersey and between Bowers Beach and Lewes in Delaware (Shuster, 1994).
Loss and Degradation
Shoreline erosion, combined with shoreline development, results in the loss of potentially suitable spawning beaches. Beach migration is a coastwide phenomenon, where beaches move landward associated with erosional events such as storms, wind, tidal action etc. However, hard structures (e.g., bulkheads, seawalls, revetments) associated with beach "development" interfere with the natural beach migration and cause habitat loss. Beaches along the New Jersey shore of the Delaware Bay have generally eroded at varying rates ranging from one to twelve feet per year for the last 100 years (U.S. Army Corps of Engineers, 1997). Erosion rates of one to twenty-six feet per year (averaging three to five feet per year) and the existence of hard structures limiting beach migration have resulted in a decline in Delaware beaches (U.S. Army Corps of Engineers, 1991). McCormick and McCormick (1998) report that the annual rate of erosion in the Chesapeake Bay averages one foot per year.
Eroded shoreline areas with high concentrations of silt or peat are less suitable for horseshoe crab reproduction because the anaerobic conditions reduce egg survivability. Erosion affects spawning by influencing the beach characteristics that are most important in site selection, such as beach topography, sediment texture, and geochemistry (Botton et al., 1988).
Jetties, however, may benefit horseshoe crab spawning activities by reducing the amount of wave action sustained by a particular beach (Maryland Department of Natural Resources, unpublished data, 1998). Beach nourishment projects, such as dune restoration and low-impact dredging, that protect developed areas and their associated infrastructure may provide habitat for horseshoe crab spawning. However, if beach nourishment projects do not keep pace with erosion in developed areas, potential horseshoe crab spawning beaches may be reduced. Ultimately, the long-term and short-term benefits and potential adverse impacts from beach nourishment projects on horseshoe crabs must be assessed.
Channel dredging and overboard spoil disposal are common throughout the Atlantic coast, but currently have unknown effects on the environment and horseshoe crab habitat. Changes in salinity as a result of dredging projects could alter horseshoe crab distribution. Additionally, dredging associated with whelk and other fisheries may damage the horseshoe crabs benthic realm; however, the significance of this impact also remains unknown.
Global warming and the subsequent rise in sea level could adversely affect horseshoe crab spawning activities. Sea level is predicted to rise above current levels by approximately one-half to one meter by the year 2100 (Oerlemans 1989; Titus et al., 1991). Land subsidence along the Atlantic Coast adds to the effect of sea level rise, resulting in an increase greater than the global average (Hull and Titus, 1986).
Pollution has the potential to adversely impact the horseshoe crab population or its habitat. Currently, no data exist suggesting unusual sensitivity by horseshoe crabs to urban or agricultural contaminants (e.g., pesticides and herbicides) (Botton, 1995). However, mosquito control agencies in New Jersey and Delaware have recently expanded their use of the mosquito larvicide methoprene, an insect growth regulator (IGR) that mimics juvenile growth hormones. IGR insecticides have been found to adversely affect crustaceans when the animals attempt to molt, according to laboratory experiments at levels of exposure higher than field applications (Kas'yanov and Costlow, 1984). However, due to the low concentrations of IGRs applied in the field, the low potential for bioaccumulation, its short half life, and a low probability of direct exposure to horseshoe crabs, it is unlikely that IGRs would have any measurable adverse impact on horseshoe crabs (Meredith, pers. comm., 1998). Additional information needs to be collected to determine if there is any impact on horseshoe crabs from actual or simulated operational use under normal field conditions of mosquito larvicides applied in coastal marshes.
Horseshoe crabs are relatively tolerant of petroleum hydrocarbons, but their tolerance decreases with increasing temperature. Nelson (pers. comm., 1997) reports that high-density #6 oil resulted in adult horseshoe crab mortality in New Hampshire. However, this mortality was probably due to mechanical impairment of the horseshoe crabs book gills by the oil and not from systemic toxicity caused by absorption of the product. Exposure to oil and chlorinated hydrocarbons resulted in delayed molting and elevated oxygen consumption in horseshoe crab eggs and juveniles (Laughlin and Neff, 1977). Maghini (1996) found trace metal and organochlorine concentrations to be relatively low in shorebird, horseshoe crab, and substrate samples from Delaware beaches and concluded that existing concentrations were of low toxicological concern. Red tide events may result in significant mortality, particularly to juveniles inhabiting intertidal areas and tidal flats (Rudloe, pers. comm., 1998).
In the Delaware Bay, Burger (1997) identified low levels of mercury (27 to 93 parts per billion) in horseshoe crab eggs between 1993 and 1995 and low cadmium levels in 1993 and 1995 (17 ppb and 24 ppb, respectively). However, relatively higher levels of cadmium were found in 1994 (310 ppb). Lead (558 to 87 ppb), chromium (5,059 to 250 ppb), and manganese (18,371 to 7,118 ppb) levels in eggs generally decreased from 1993 to 1995 in the Delaware Bay, while selenium levels (1,965 to 3,472 ppb) increased in those years (Burger, 1997). Burger (1997) concluded that the additional stress on horseshoe crab eggs from the presence of heavy metals could lower reproductive success.
Because the Delaware estuary is a major petrochemical center on the East Coast (Sharp, 1988) with an associated high level of tanker traffic, oil spills can and do occur. A large spill during the horseshoe crab spawning season could threaten populations in the Delaware Bay. In addition, mercury, lead, zinc, and cadmium may be of concern in some coastal estuaries and rivers, such as the Cohansey (New Jersey) and Saint Jones (Delaware) Rivers (Sharp, 1988). Delaware Division of Fish and Wildlife's 16-foot trawl survey data indicate that the area off the Saint Jones River is a major nursery area for horseshoe crabs.
Description of the Fishery
Current Fishery Regulations
The Commercial Fishery
The Bait Fishery
The NMFS depends on central dealers for much of its landings data, but in many cases, horseshoe crabs are harvested and used directly by eel, catfish, or whelk fishers, or arrangements are made for harvesters to sell directly to such fisheries without going through dealers. These private sales are not reported, resulting in an underestimation by the NMFS of the total catch. Based on the NMFS data, commercial harvest for the northeastern Atlantic coast has ranged between 10,000 pounds in 1969 to over five million pounds in 1996 (NMFS, 1998). Since 1988, commercial landings have averaged 1,436,808 pounds annually.
The total average horseshoe crab catch for the Atlantic Coast, assuming an adult horseshoe crab weighs four pounds, has increased from 476,515 animals in 1993 to 1,288,408 in 1996 (NMFS, 1998). This increase is similar to increases reported by Michels (unpublished data, 1997) for the Delaware Bay harvest, which ranged from 330,333 horseshoe crabs in 1993 to 896,540 in 1996. However, Michels (unpublished data, 1997) did not include the Maryland harvest, which can be substantial.
The Horseshoe Crab Technical Committee's Stock Assessment Subcommittee (SAS) and Peer Review Panel (PRP) concluded that commercial landings data show a substantial increase in reported harvest during the 1990s (see below for statistics by state). This growth is primarily a result of the increase in demand for American eel and whelk; horseshoe crabs are used as bait in these fisheries (Michels, unpublished data, 1997; NMFS, 1998; Thompson, 1998). This increase could also be, in part, a function of increased harvest reporting efficiency and the institution of mandatory reporting.
NMFS data compiled by Delaware Division of Fish and Wildlife (1997) identified that among the northeastern and mid-Atlantic states, Maryland, New Jersey, and Delaware harvest the majority of horseshoe crabs (36, 31, and 14 percent of the total annual catch, respectively). Estimates in Delaware, Maryland, New Jersey, New York, and Rhode Island indicate a rapid increase in the growth of the horseshoe crab fishery (Michels, unpublished data, 1997; NMFS, 1998; Thompson, 1998). Massachusetts, North Carolina, and Virginia indicate declines in current harvest compared with harvest in the late 1970s and early 1980s (NMFS, 1998), and little to no harvesting of horseshoe crabs was reported in Maine, New Hampshire, or Connecticut (Botton and Ropes, 1987b). The Chesapeake Bay in Maryland and Virginia likely has a substantial harvest, but without quantitative studies, the catch remains under-reported.
New Jersey Landings
In 1997, the majority (85 percent) of horseshoe crabs in Delaware were landed by hand-harvest, while dredge harvest made up approximately 15 percent (Delaware Division of Fish and Wildlife, 1997). Between 1991 and 1996, the majority of the horseshoe crabs were landed by hand-harvest (63 percent) compared to dredging (37 percent) (Delaware Division of Fish and Wildlife, 1997). This increase in harvest mirrored the increase in the number of hand-collection permits issued (Delaware Division of Fish and Wildlife, 1997).
New York Landings
Rhode Island Landings
In 1989, the FDA reported that 130,000 horseshoe crabs were used in the biomedical industry. The current estimate of medical usage is between 200,000 and 250,000 horseshoe crabs per year on the Atlantic Coast (Swan, pers. comm., 1998; McCormick, pers. comm., 1998). The FDA mandates conservation by requiring the return of horseshoe crabs to the environment. Most labs return bled crabs to their habitat within 72 hours of capture, but may or may not release crabs at the collection site (Botton, 1995). Approximately 10 percent of the crabs do not survive the bleeding procedure, which comprises a source of mortality that is not included in the commercial catch statistics (Rudloe, 1983). Based on a tagging and controlled mortality study, Thompson (1998) reported similar post-processing mortality of horseshoe crabs (10 to 15 percent). Mortality due to the bleeding procedure may be lower (0 to 4 percent), depending on the biomedical facility (Swan, pers. comm., 1998), but the mortality associated with collection, shipping, and handling remains unknown. This mortality is minimal compared to that from the commercial bait fishery.
In South Carolina, live horseshoe crabs may be taken only for use in LAL production, and the animals are returned to their natural habitat after the procedure. Landings in South Carolina by hand-harvest and by trawl have increased since the late 1980s. The annual reported harvest in South Carolina has increased over 300 percent since reporting requirements were established in 1991 (Thompson, 1998). Presumably, this increase in harvest was driven by the biomedical industry's demand for more horseshoe crab products.
Horseshoe crabs are also used to make chitin filament for suturing (Hall, 1992). Since the mid-1950s, medical researchers have known that chitin-coated suture material reduces healing time by 35-50 percent. Currently, horseshoe crabs are harvested on a limited basis to manufacture chitin-coated suture material and chitin wound dressings (Hall, 1992).
If harvesting is not carefully managed, the risk of adversely affecting the horseshoe crab population becomes a certainty. Several factors that contribute to this risk includeHorseshoe crabs mature slowly, requiring nine to eleven years to attain sexual maturity (Shuster and Botton, 1985). Some bait harvesters prefer gravid females (those carrying eggs). Horseshoe crabs congregate inshore seasonally to spawn, which makes them especially vulnerable to exploitation.
Population data indicate that after harvesting ceases, horseshoe crabs do not rebound for approximately one decade, which corresponds to the time required for horseshoe crabs to reach sexual maturity (Shuster, 1994).
A decline in the number of horseshoe crabs will impact other species, particularly shorebirds and sea turtles. Shorebirds primarily feed on horseshoe crab eggs exposed on the surface, but sufficient surface eggs are available only if horseshoe crabs are spawning at high densities. Therefore, adequate spawning densities must be maintained to ensure availability of horseshoe crab eggs for shorebirds. Sea turtles feed on adult horseshoe crabs, but their diet depends on relative abundance of the prey species.
Identifying and maintaining optimal sustainable yield for the commercial fishery is critical. Appropriate coast-wide management of the horseshoe crab population would ensure the long-term viability of the population for continued harvest and would provide necessary quantities of adults and eggs for fish and wildlife resources.
Horseshoe crabs are the primary bait for the American eel and conch fisheries in many mid-Atlantic States. In Maryland, the estimated value of the horseshoe crab fishery in 1996 for 10 horseshoe crab harvesters was $398,596 (Maryland Department of Natural Resources, 1998). Also in 1996, one Maryland seafood dealer, supplying horseshoe crabs to 20 American eel and 25 conch harvesters, estimated that the value of horseshoe crabs for these fisheries was $151,200. Horseshoe crab prices vary and are reported to be between $0.65 to $0.75 per animal (Maryland Department of Natural Resources, 1998).
In 1997, American eel and conch harvesters in Delaware used an average of 4,714 and 20,502 horseshoe crabs per season per harvester, respectively. In New Jersey, American eel and conch harvesters used an average of 4,005 and 22,654 horseshoe crabs per season per harvester, respectively (Munson, 1998). Many conch and American eel harvesters in New Jersey and Delaware harvest their own bait, supplying 18 to 65 percent of their bait needs (Munson, 1998). While only nine percent of the fishing income (of respondents in the Delaware Bay Watermen's study) is attributable to the direct sale of horseshoe crabs, an average of 58 percent of the eel and conch fishing income depends on using horseshoe crabs as bait (Munson, 1998). American eel harvesters in the Delaware Bay area report that approximately 21 percent of their total fishing income is attributable to eeling, while conch harvesters report that an average of 53 percent of their total fishing income depends on the conch fishery (Munson, 1998). In 1996, the commercial harvest of horseshoe crabs was estimated to be a $1.5 million industry.
Horseshoe crabs are vital to medical research and the pharmaceutical products industry. The worldwide market for LAL is currently estimated to be approximately $50 million per year. This estimate is based on bleeding 250,000 horseshoe crabs per year, generating approximately $200 in revenue per crab for the biomedical industry. The biomedical industry either directly collects horseshoe crabs on spawning beaches or purchases horseshoe crabs for as much as $3.00 per crab. The biomedical industry pays approximately $375,000 per year for horseshoe crabs based on an estimate of 250,000 horseshoe crabs harvested at an average price of $1.50 per crab.
Eco-tourism is critical to the economies of many states, including New Jersey and Delaware, and it depends on the abundance and health of the ecosystems within the region. In 1988, over 90,000 "birders" spent $5.5 million in Cape May, New Jersey (Kerlinger and Weidner, 1991) to watch the interaction between spawning horseshoe crabs and migrating shorebirds. In 1996, approximately 606,000 people in New Jersey and Delaware took trips away from their residence (travelling more than one mile) for the primary purpose of watching wildlife. Of these people, 409,000 individuals specifically stated that they were watching shorebirds (U.S. Bureau of Census and USFWS, 1998).
In 1996, New Jersey and Delaware wildlife watchers spent between nine and 12 days per year (on average) away from home (travelling more than one mile) watching wildlife (U.S. Bureau of Census and USFWS, 1998). In New Jersey and Delaware, total expenditures, including food, lodging, transportation, and equipment in 1996 for the primary purpose of wildlife watching was $639,992,000 (USFWS, 1998). The type of wildlife watched was not identified in this survey. The 1996 regional economic impact resulting from expenditures by wildlife watchers in New Jersey and Delaware was the creation of 15,127 jobs and the generation of a total household income of $399 million (USFWS, 1998).