Northeast Fisheries Science Center Reference Document 06-20
Sea Scallop Stock Assessment Update for 2005
by Deborah
R. Hart
National
Marine Fisheries Service, 166 Water St.,
Woods Hole MA 02543
Print
publication date September 2006;
web version posted September 21, 2006
Citation: Hart DR. 2006.
Sea Scallop Stock Assessment Update for 2005.
U.S. Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 06-20;
14 p.
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Introduction
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).
Landings
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).
Surveys
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.
References
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.
