Spiny dogfish, Squalus acanthias,
are distributed in the western North Atlantic from Labrador
to Florida and are considered to be a unit stock in this region
(Burgess 2002) (Figure
26.1). During spring and autumn, spiny dogfish occur in
coastal waters between North Carolina and Southern New England.
In summer, dogfish migrate northward to the Gulf of Maine Georges
Bank region and into Canadian waters and return southward in
autumn and winter (Jensen 1965). They tend to school by size
and, when mature, by sex. Dogfish feed on many species of fish
and crustaceans, but generally target the most abundant species
(Link et al. 2002). In the Northwest Atlantic, maximum reported
ages for males and females are 35 and 40 years, respectively
(Nammack 1982). The species bears live young, with a gestation
period of about 18 to 22 months, and produce between 2 to 15
pups with an average of 6. Size at maturity for females is around
80 cm, but can vary from 78 cm to 85 cm depending on the abundance
of females. (Sosebee 2005).
The principal commercial
fishing gears used to catch dogfish are otter trawls and sink
gillnets. Dogfish are frequently caught as bycatch and discarded
during groundfish operations, particularly in the Mid-Atlantic-Southern
New England area. Recreational and foreign fishing are of minor
importance. The fishery is managed under a Fishery Management
Plan developed jointly by the Mid Atlantic and New England Fishery
Management Councils for federal waters and a plan developed concurrently
by the Atlantic States Marine Fisheries Commission for state waters.
Total landings peaked at 24,700
mt in 1974, declined sharply to a fairly stable average of about
6,300 mt per year during 1979 1988, increased to a record high
of 28,200 mt in 1996, and subsequently declined to around 4,000
mt during 2003-2005 (Table 26.1,Figure
26.2 Data]). Between 1966 and 1977, distant water fleets,
mainly from the Soviet Union, accounted for virtually all of the
reported landings, but have since ceased to be important. United
States commercial landings during 1979 1988 averaged 4,300 mt
per year, increased to 27,200 mt in 1996, and then declined to
a low of 1,000 mt in 2004 due to trip limits and quota management.
Landings in the U.S. recreational fishery were estimated to be
about 300-400 mt from 1987-1989, but subsequently were less than
100 mt in 2005. Canadian landings increased from an average of
500 mt from 1979-1988, peaked at 1,800 mt in 1994, declined to
400 mt in 1996 and 1997, but increased to 3,600 mt in 2002. Minor
quantities have also been taken by European Union (EU) fleets
fishing in Canadian waters in recent years.
The U.S. fishery for spiny dogfish
targets large individuals (larger than 2.3 kg [5.1 lb] in weight,
and 83 cm [33 in.] in length), which are primarily mature females,
to meet processing and marketing requirements (NEFSC 1994; NEFSC
1998; Rago et al. 1998). Median length of landed female dogfish
averaged about 94 cm during 1982 to 1988 but declined to about
84 cm between 1989 and 1999 as a consequence of the directed fishery
26.3 Data]). Average weights exhibited a similar pattern.
Since 2000 the average lengths and weights have subsequently increased.
Spiny dogfish are caught
as bycatch in a large number of fisheries in the Northeast. Estimated
annual commercial discards during 1989-2005 range from 7,400 to
47,300 mt (NEFSC 2006). Dogfish are hardy and not all die when
discarded. Dead discards peaked in the early 1990s at nearly 20,000
mt (in 1990 and 1992), but subsequently declined to below 4,000
mt in 1998. Since 2001, dead discards have varied between 4,000
and 5,000 mt (Figure
26.4 Data]). The recreational fishery also discards dogfish
and estimates of dead dogfish have ranged between 100 and 300
mt during the last two decades. The current commercial fishery,
which is regulated by trip limits and an overall quota, discards
all sizes and both sexes of dogfish (NEFSC 2006).
NEFSC spring survey relative
biomass estimates (three-year average) of spiny dogfish increased
from the mid 1970s to 1993 but have since gradually declined (Figure
26.5 Data]). However, mature biomass (individuals ≥ 80
cm, mostly females) declined much more rapidly; estimates peaked
at about 250,000 mt in 1990, declined to less than 100,000 mt
in 1999 before increasing to above 100,000 mt in 2006. Recruitment
estimates of pups (≤ 35 cm) were record low from 1997 through
2003 and also in 2006 (Figure
26.6 Data]). Length frequency data from various surveys indicate
that the average size of females ≥ 80 cm has declined from
about 94 cm in the 1980s and early 1990s to 84 cm in the last
few years (Figure
26.7 Data]). Average length of pups in the NEFSC spring survey
has declined from 30 cm in 1980-1996 to about 27 cm since 1997
26.8 Data]). These changes are consistent with the declining
trend in body size of spawning females.
Marked changes in the
size and sex composition have occurred as a result of the directed
commercial fishery that occurred in the 1990’s. The abundance
of large females declined sharply whereas the size frequencies
of the lightly harvested males were almost unaffected (Figure
26.9 Data]). As a result, the ratio of mature males (≥
60 cm) to mature females (≥ 80 cm) increased from about 2:1
before 1993 to about 7:1 since 2000 (Figure
26.10 Data]). The long-term implications of this altered sex
ratio are unknown. The decline in recruitment shown in (Figure
26.6 Data]) is evident in (Figure
26.9 Data]) for both males and females as is a progressive
increase in the length of the smallest dogfish captured (from
20 to ca 60 cm since 1995).
Assessments of spiny dogfish are based on
NEFSC spring survey indices expanded to swept area biomass estimates.
Uncertainty of the biomass estimates incorporates the sampling
variability of the individual surveys, uncertainty in the area
swept per tow, and inter-annual variation over a 3-year moving
average. Stochastic biomass estimates for mature females decreased
from about 250,000 mt in 1990-1991 to less than 60,000 mt from
26.11 Data]). In 2006 the spawning stock biomass increased
to 106,000 mt but this increase is driven entirely by the survey
results in 2006.
Fishing mortality estimates on the exploitable
female stock peaked in 1994 at almost 0.5, remained above 0.2
through 1999 and declined to 0.1 in recent years, except for
a value of 0.5 in 2004 (Figure
26.12 Data]). Fishing mortality rate on the fully recruited
dogfish followed a similar trend in the 1990s when total removals
were high. During recent years this measure of fishing mortality
has been higher owing to the shift toward larger size dogfish
in the landings.
Biological Reference Points
Reference points for spiny dogfish are based
on the female spawning stock biomass (i.e., ≥ 80 cm) and
the rate of fishing mortality applied to the fully vulnerable
stock. The biomass target is based on the relationship between
indices of recruitment (≤ 36 cm) and spawning stock biomass
(females ≥ 80 cm). A Ricker stock-recruitment relationship
was used to estimate the relative biomass at which recruitment
is maximized. The relative biomass can be rescaled to swept
area biomass using a conversion factor based on the nominal
average area swept per tow.
Biological reference points for fishing
mortality are based on joint effects of size at entry into the
fishery and the rate of fishing mortality applied to the fully-recruited
size class. A life history model is used to estimate the size
specific fishing mortality rate corresponding to a lifetime
female production of 1.0—the rate at which each female
is expected to replace itself in the next generation. Size-specific
estimates of fishing mortality are based upon the relationship
between the composite length-frequency distribution of landings
and discards and the length frequency of the NEFSC spring trawl
survey. The size-specific pattern of selectivity varies annually
resulting such that the fishing mortality applied to the fully
recruited size classes will also vary annually.
Reference points established
in the MAFMC/NEFMC Spiny Dogfish Fishery Management Plan (1999)
include: Btarget = 180,000 mt; Bthreshold
= 100,000 mt, (both expressed in terms of adult (≥ 80 cm)
female biomass), and FMSY = 0.11 and Ftarget
=0.08. These threshold and target fishing mortality rates represent
fully recruited Fs, and were calculated assuming a knife edge
fishery selectivity pattern with a minimum size of 70 cm (Figure
26.13 Data]). FMSY corresponds to a lifetime female
pup production of 1.0; Ftarget corresponds to lifetime
female pup production of 1.5. The Btarget reference
point in the FMP was subsequently disapproved by NMFS because
it did not correspond to the biomass associated with maximum recruitment
(~200,000 mt) in a Ricker stock-recruitment function.
The biomass target in the ASMFC
FMP (SSBmax = 167,000 mt) was derived by applying a
Ricker model to the swept area estimates of SSB and recruits in
the NEFSC spring trawl surveys. The ASMFC reference point differs
from that used by the MAFMC and NEFMC because it is based on a
slightly larger estimate of the average area swept by the NEFSC
trawl survey. All of the biomass targets, however, are based on
the same relative biomass index equal to an average of 31 kg/tow
in the Ricker model analysis; the relative biomass threshold is
50 % of the value or 16.5 kg/tow.
The overfishing threshold was
updated in NEFSC (2006) using the current (2005) size selectivity
of the fishery. Detailed analyses of the size composition of discards
and landings in NEFSC (2006) revealed that discards occurred over
all size classes. The revised estimate of FMSY of 0.39
reflects a shift in size selectivity to larger individuals in
the landings and discards. The estimate of FMSY is
highly sensitive to changes in size at entry into the fishery
when the size exceeds about 82 cm (Figure
26.13 Data]). Estimates of FMSY vary from 0.3 to
0.6 as size-at-entry increases from 82 to 87 cm. Estimates of
FMSY in Figure
26.13 Data] are based on a simplifying assumption of so-called
knife-edge selection to the fishery. The complexity of reference
point estimation increases when actual selection patterns are
estimated from the catch and survey size frequencies. In particular,
the selectivity pattern can distribute fishing mortality over
a broad range of size classes. The general convention of expressing
FMSY as the rate applied to fully recruited size classes
can lead to seemingly high estimates. Marked changes in the discarding
patterns of the mix of fisheries that encounter spiny dogfish
are expected to affect the population‘s rate of stock rebuilding
in the coming years. Nonetheless, the basic principle remains—spiny
dogfish are a slow growing, low productivity species with limited
capacity to withstand fishing mortality concentrated on mature
Based on the existing biomass
threshold (NEFSC 2003), the spiny dogfish stock is not currently
overfished. The current estimated stock size of mature females
(> 80 cm) is 106,000 mt (72,000-140,000; 80% confidence interval)
26.11 Data]), and this value exceeds Bthreshold
(100,000 mt mature females, P=0.724). The biomass target in the
spiny dogfish FMP (180,000 mt) was subsequently disapproved by
NMFS; currently there is no approved biomass target in place.
The estimate for 2005 of F on fully recruited females is 0.128
(0.09-0.17; 80% confidence interval) (Figure
26.12 Data]. This fishing mortality rate exceeds the existing
overfishing threshold (Fthreshold=0.11) and the existing
rebuilding target (Frebuild=0.03). The overfishing
threshold was updated in the current assessment (Fthreshold=0.39).
Based on the updated estimate, overfishing is not occurring. Despite
the much lower level of landings since 2001, fishing mortality
rates on fully recruited females have remained above the rebuilding
Spawning stock biomass
of spiny dogfish declined rapidly in response to a directed fishery
during the 1990s. Management measures, initially implemented in
2001, have been effective in reducing landings and reducing fishing
mortality. Overfishing is not occurring. The directed fishery
targeted mature females but had little effect on the males. Current
sex ratio of mature males to mature females rose from about 2:1
in 1990 to over 7:1 in 2006. Recruitment has been very low since
1997. The NEFSC spring survey biomass index increased markedly
in 2006 and the stock is not considered overfished. Rebuilding
however, is expected to take more than a decade even if fishing
mortality remains low. Recent poor recruitment poses a substantial
risk to the long-term spawning stock of spiny dogfish. Reductions
in fishing mortality are expected to increase stock biomass for
several more years until the reduced recruitment acts to lower
SSB. Conclusions regarding the overfished and overfishing status
of spiny dogfish are strongly dependent on the NEFSC spring survey
estimates in 2006. Concerns have been raised about the influence
of these data (NEFSC 2006); future surveys will be closely monitored
to determine if the 2006 results signal a true increase in abundance.
Burgess, G.H. 2002. Spiny Dogfishes.
Family Squalidae. In: Bigelow and Schroeder’s fishes of the
Gulf of Maine 3rd ed., p. 48-57. Collette, B.B. and G. Klein-MacPhee
(eds). Smithsonian Institution Press.
Jensen, A.C. 1965. Life history
of the spiny dogfish. Fish. Bull. 65: 527-554.
Link, J.S., L.P. Garrison, and
F.P. Almeida. 2002. Ecological interaction between elasmobranches
and groundfish species on the northeastern U.S. Continental Shelf.
I. Evaluating predation. N. Am. J. Fish. Man. 22: 550-562.
Nammack, M.F. 1982. Life history
and management of spiny dogfish, Squalus acanthias, off
the northeastern United States. College of William and Mary, Williamsburg,
VA. Master’s thesis, 63 pp.
NEFSC [Northeast Fisheries Science
Center]. 1994. Report of the 18th Northeast Regional Stock Assessment
Workshop (18th SAW), Stock Assessment Review Committee (SARC)
consensus summary of assessments. Northeast Fish. Sci. Cent. Ref.
Doc. 94-22. 199 p.
Rago, P. J., K. A. Sosebee, J.
K. T. Brodziak, S. A. Murawski, and E. D. Anderson. 1998. Implications
of recent increases in catches on the dynamics of Northwest Atlantic
spiny dogfish (Squalus acanthias). Fish. Res. 39: 165-181.
Sosebee, K.A. 2005. Are
density-dependent effects on elasmobranch maturity possible? J.
Northw. Atl. Fish. Sci. 35: 115-124.