CONTENTS Life History and Distribution Landings Surveys Fishing Mortality Estimates Status Determination for 2005 References
Northeast Fisheries Science Center Reference Document 06-20
Deborah R. Hart
Sea Scallop Stock Assessment Update for 2005
National Marine Fisheries Service, 166 Water St., Woods Hole MA 02543
Web version posted September 21, 2006Citation: Hart DR. 2006. Sea Scallop Stock Assessment Update for 2005. U.S. Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 06-20; 14 p.
Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.
This report is an updated assessment of U.S. sea scallops, using data through the end of the 2005 calendar year. The methodology used here is identical to that used in the last fully peer-reviewed stock assessment (NEFSC 2004), but is updated to include two more calendar years of landings and fishery-independent survey data (2004-2005).
The Atlantic sea scallop, Placopecten magellanicus, occurs in continental shelf waters of the Northwest Atlantic between Cape Hatteras and Newfoundland. It supports one of the most valuable fisheries in the United States, with an ex-vessel value in 2005 of over $430 million, and is the most valuable wild scallop fishery in the world. Major commercial concentrations of sea scallops in U.S. waters occur in the Mid-Atlantic Bight (Virginia to Long Island), on Georges Bank and surrounding areas (including the Great South Channel and Nantucket Shoals), and near-shore areas in the Gulf of Maine.
The U.S. federal sea scallop fishery is managed by the New England Fishery Management Council, under Amendment 10 to the sea scallop management plan. The bulk of landings come from more than 300 vessels with limited access permits, but a growing percentage are being taken by vessels with open access general category permits. Limited access vessels are controlled by annual day at sea limits, crew size limits, and trip limits to special access areas. General category vessels are limited to 400 lbs of meats per day or trip, whichever is more restrictive. Gear restrictions (4” rings with a 10” twine top on dredges) apply to all permits.
Fishery closures have strongly influenced sea scallop population dynamics and fisheries in recent years. Three large areas on Georges Bank and Nantucket Shoals were closed to groundfish and scallop fishing in December 1994. Since then, scallop biomass in these areas has increased by about a factor of 25 (Hart and Rago 2006). Portions of these areas were reopened to limited scallop fishing from June-November 1999, June 2000-January 2001, and since November 2004, with seasonal closures during February through June 15. In the Mid-Atlantic, two areas were closed to scallop fishing for three years in April 1998, and a new rotational area (the “Elephant Trunk” closed area) was closed in July 2004. Substantial increases in biomass occurred in one of the two original rotational closures, from which considerable landings were derived after this area was reopened in May 2001. Considerable increases in biomass have also been observed in the Elephant Trunk area prior to its planned 2007 reopening.
Life History and Distribution
Sea scallops occur in the Northwest Atlantic Ocean from North Carolina to Newfoundland along the continental shelf, typically on sand and gravel bottoms (Hart and Chute 2004). In Georges Bank and the Mid-Atlantic, most are harvested at depths between 30 and 100 m, while the bulk of the landings from the Gulf of Maine are from near-shore relatively shallow waters (< 40 m). Sea scallops filter-feed on phytoplankton, microzooplankton, and detritus particles. Sexes are separate with external fertilization, and larvae are planktonic for 4-7 weeks before settling to the bottom. Scallops recruit to the NEFSC survey at about 2 years old (40-70 mm), and to the commercial fishery currently at around 4-5 years old, though historically most 3-year-olds were vulnerable to the commercial fishery.
According to Amendment 10 of the Atlantic Sea Scallop Fishery Management Plan (NEFMC 2003), all scallops in the US Exclusive Economic Zone (EEZ) belong to a single stock. The US sea scallop stock can be subdivided into Georges Bank, Mid-Atlantic, Southern New England, and Gulf of Maine regional components based on survey data, fishery patterns, and other information (NEFSC 2004). The stock is likely composed of smaller regional meta-populations with some movement of larvae from Georges Bank into Southern New England and from Southern New England to the Mid-Atlantic. The main regional components are Georges Bank (including the Great South Channel and Nantucket Shoals) and the Mid-Atlantic Region (Figure 1). However, relatively small but imprecisely known amounts of sea scallop biomass occur in areas outside regularly surveyed NEFSC shellfish strata. Landings from other regions have been comparatively minor. As in NEFSC (2004), abundance and fishing mortality estimates for Georges Bank and the Mid-Atlantic are estimated separately in this assessment and then combined to characterize the condition of the stock as a whole.
Growth in sea scallops is modeled using the von Bertalanffy growth equation SH = L [1-exp(-K(t-t0))], where SH is shell height (in mm) and t is age (in years). The parameters L and K, based on Serchuk et al. (1979), are taken as L = 152.46, K = 0.3374 (Georges Bank), L = 151.84, K = 0.2997 (Mid-Atlantic). Since sea scallop assessments are not age-based, the value of t0 is irrelevant for this assessment. Shell height to meat weight equations ln(MW) = a + bln(SH) are as given in NEFSC (2004):
a = 11.6038, b = 3.1221 (Georges Bank), a = 12.2484, b = 3.2641 (Mid-Atlantic).
Total US landings of sea scallops averaged 26,639 mt meats during 2003-2005, nearly quadruple the amount typical during the mid-1990s (Table 1, Figure 2). The landings of 29,321 mt meats in 2004 was an all-time record. The recent increase in landings occurred primarily in the Mid-Atlantic area, where they were well above historical levels. Georges Bank landings remained around their long-term average from 1999-2004, but increased to a near-record 9711 mt meats in 2005, primarily due to reopening of portions of the closed area. The recent increases in landings were mainly due to increased recruitment in the Mid-Atlantic and improved management that has caused scallops to be landed at a much larger size. A majority of the landed meats from the mid-1980s through 1998 were in the smaller market categories (>30 meats per pound). Landings in more recent years have trended to much larger sizes; the mean weight of a landed scallop meat in 2005 was about twice that of a meat in the 1990s (Figure 3).
Sea scallop surveys using a lined 8’ dredge have been conducted by NEFSC since 1979, but the survey of Georges Bank was incomplete prior to 1982. Thus, survey data used for this assessment are for 1982-2005 for Georges Bank, and 1979-2005 in the Mid-Atlantic. Since 2004, rock chains have been used in four strata in the Great South Channel. In rocky areas, the rock chains increase the efficiency of the gear by about a factor of 1.56 (NEFSC 2004, Appendix 2). In order to be consistent with previous years, the catches in these four strata were reduced by a factor of 1.56 in 2004-2005. Further details regarding the surveys can be found in NEFSC (2004).
Survey biomass in both resource areas remained low through the mid-1990s (Table 2, Figure 4). The closure of three large areas on Georges Bank and Nantucket Shoals, combined with drastically reduced fishing effort (due to shifts of effort to the Mid-Atlantic and later to effort reduction measures) caused a rapid increase in biomass from 1994-2000, with biomass in this area remaining roughly stable since then. Mid-Atlantic biomass remained low until 1998, when the closure of two areas combined with effort reduction measures and very strong recruitment induced a rapid increase in biomass. The overall biomass index began increasing in the mid-1990s, and stood at 7.8 kg/tow in 2005, well above the biomass target of 5.6 kg/tow.
Fishing mortality estimates
Following NEFSC (2004), fishing mortality was estimated using the “rescaled catch-biomass” method. In summary, fishing mortality trends for Georges Bank and the Mid-Atlantic were estimated by the ratio of landings to survey biomass. These trends were scaled so that they averaged the long-term average fishing mortality estimated in each year by the “two-bin” method:
where Rt was the mean population number of scallops per standard survey tow in the first bin (new recruits) during survey year t, and Pt was the mean population number of scallops per standard survey tow in the second bin (plus group). Natural mortality M was estimated as 0.1 as in NEFSC 2004. The estimates from the two regions were combined using a number-weighed average. Further details on these calculations can be found in NEFSC (2004) and Hart and Rago (2006).
Georges Bank fishing mortality peaked at about 1.7 in 1991, but declined drastically starting in 1994 (Table 3 and Figure 4). In recent years (2000-2005), fishing mortality has been around 0.1; the 2005 fishing mortality was slightly higher than the recent average (0.15) primarily due to reopenings of portions of the closed areas. Mid-Atlantic fishing mortality peaked at about 1.6 in 1992. Fishing mortality declined greatly between 1996 and 1999, and since then has modestly varied without trend. Fishing mortality in 2005 was the lowest in the time series (0.3); the recent decrease is primarily due to the rotational closure of the Elephant Trunk area. Fishing mortality for the overall resource peaked at 1.55 in 1991 and then declined considerably between 1991 and 1998. Since 1998, overall fishing mortality has varied between 0.18 and 0.34; it was 0.22 in 2005, slightly under the overfishing threshold of 0.24, but just over the fishing mortality target of 0.2.
Status determination for 2005
The overall NEFSC sea scallop survey index stood at 7.8 kg/tow for 2005, above the biomass target of 5.6 kg/tow (NEFMC 2003). Sea scallops were therefore not overfished. The point estimate for fishing mortality of the overall sea scallop resource was 0.22, below the overfishing threshold of 0.24. Thus, overfishing of sea scallops was not occurring. However, there are important caveats to this conclusion. First, the confidence interval for fishing mortality contains the overfishing threshold, so it cannot be concluded with statistical certainty that overfishing was not occurring. Perhaps more importantly, the fishing mortality estimate in 2005 is a spatial average over heavily fished areas and areas that are either closed (e.g., the Elephant Trunk Closed Area and the Nantucket Lightship Closed Area) or where fishing mortality was low (e.g., Georges Bank Closed Areas I and II). Because over half the scallop biomass is contained in the closed areas, fishing mortality in the remainder of the resource must be over the fishing mortality threshold, and localized overfishing of some areas must be continuing. There is a possibility that unless fishing effort elsewhere is reduced, overfishing of the overall resource may reoccur when the Elephant Trunk area is reopened and fishing mortality there is ramped up. Finally, there has been considerable growth in general category fishing effort in the last several years which also threatens to induce overfishing unless management action is taken to contain effort in this sector.
Hart DR, Chute AS. 2004. Essential fish habitat source document: sea scallop, Placopecten magellanicus, life history and habitat characteristics (2nd edition). Woods Hole MA: NOAA Tech Memo NMFS-NE-189, 21 p.
Hart DR, Rago PJ. 2006. Long-term dynamics of U.S. sea scallop (Placopecten magellanicus) populations. N Am J Fish Manage 26:490-501.
NEFMC [New England Fishery Management Council]. 2003. Final Amendment 10 to the Atlantic sea scallop fishery management plan with a supplemental environmental impact statement, regulatory impact review, and regulatory flexibility analysis. Newburyport MA: NEFMC.
NEFSC [Northeast Fisheries Science Center]. 2004. 39th Northeast Regional Stock Assessment Workshop (39th SAW) assessment summary report & assessment report, Woods Hole MA: NEFSC Ref Doc 04-10; 211 p.
Serchuk FM, Wood PW Jr, Posgay JA, Brown BE. 1979. Assessment and status of sea scallop (Placopecten magellanicus) populations off the northeast coast of the United States. In: Proc Natl Shellfish Assoc 69:161-191.