Silver hake, also known as whiting,
Merluccius bilinearis, range primarily from Newfoundland
to South Carolina. Silver hake are fast swimmers with sharp teeth,
and are important fish predators that also feed heavily on crustaceans
and squid (Lock and Packer 2004). Although they do not swim in definitive
schools, silver hake tend to aggregate in large numbers (Collette
and Klein-MacPhee eds. 2002). In U.S. waters, two stocks have been
identified based on differences of head and fin lengths (Almeida
1987), otolith morphometrics (Bolles and Begg 2000), otolith growth
differences, and seasonal distribution patterns (Lock and Packer
2004). The northern silver hake stock inhabits Gulf of Maine - Northern
Georges Bank waters, and the southern silver hake stock inhabits
Southern Georges Bank - Middle Atlantic Bight waters (Figure
4.1).
As nocturnal, semi-pelagic predators,
silver hake move up in the water column to feed at night, primarily
between dusk and midnight, and return to rest on the bottom during
the day, preferring sandy, muddy or pebbly substrate (Collette and
Klein-MacPhee eds. 2002).
There is some difference in diet between the two stocks. Northern
silver hake primarily feed on euphausiids, Atlantic herring, silver
hake and other fish, while southern silver hake primarily feed
on crangonid shrimp, squids, cephalopods, and sand lance. Diet
varies depending on size, sex, season, migration, spawning, and
age (Lock and Packer 2004). Small silver hake prey on euphausiids,
while larger, especially older females prey primarily on fish.
Silver hake tend to prey more heavily on fish during the spring
and autumn, whereas during the summer months, their diets often
include a mixture of fish, crustaceans, and mollusks (Lock and
Packer 2004).
Silver hake migrate in response to seasonal changes in water
temperatures, moving toward shallow, warmer waters in the spring.
They spawn in these shallow waters during late spring and early
summer and then return to deeper waters in the autumn (Brodziak
et al. 2001). The older, larger silver hake especially prefer
deeper waters. During the summer, portions of both stocks can
be found on Georges Bank, whereas during the winter fish in the
northern stock move to deep basins in the Gulf of Maine, while
fish in the southern stock move to outer continental shelf and
slope waters. Silver hake are widely distributed, and have been
observed at temperature ranges of 2-17° C (36-63° F) and
depth ranges of 11-500 m (36-1,640 ft). However, they are most
commonly found between 7-10º C (45-50º F) (Lock and
Parker 2004).
Female silver hake are serial spawners, producing and releasing
up to three batches of eggs in a single spawning season (Collette
and Klein-MacPhee eds. 2002). Major spawning areas include the
coastal region of the Gulf of Maine from Cape Cod to Grand Manan
Island, southern and southeastern Georges Bank, and the southern
New England area south of Martha's Vineyard. Peak spawning occurs
earlier in the south (May to June) than in the north (July to
August). Over one-half of age-2 fish (20 to 30 cm, 8 to 12 in.)
and virtually all age-3 fish (25 to 35 cm, 10 to 14 in.) are sexually
mature. Silver hake grow to a maximum length of over 70 cm (28
in.) and ages up to 14 years have been observed in U.S. waters,
although few fish older than age 6 have been observed in recent
years (Brodziak et al. 2001).
Due to their abundance and availability, silver hake have supported
important U.S. and Canadian fisheries as well as distant-water
fleets. Following the entrance of distant-water fleets in 1962,
nominal catches from both stocks increased rapidly, peaking at
more than 350,000 mt in 1965, but declined to 55,000 mt by 1970.
Landings increased to 137,000 mt in 1973 and then declined sharply
with increased restrictions on distant-water fleet effort and
implementation of the Magnuson Fishery Conservation and Management
Act (MFCMA) in 1977. Prior to MFCMA, distant-water fleets accounted
for about 49% and 87% of the total landings of the northern and
southern stocks, respectively. Fishing activity by distant-water
fleets ceased after 1977 for the northern stock, but distant-water
fleet exploitation of the southern stock continued until 1987,
primarily as a bycatch in the squid fishery. U.S. landings during
1987-1996 were relatively stable, averaging 16,000 mt per year,
but have gradually declined to a historic low of 6,800 mt in 2005
(Figure
4.2
[Fig 4.2 Data]).
The otter trawl remains the principal gear used in the U.S. fishery,
and recreational catches have been low since 1985. Silver hake
are managed under the New England Fishery Management Council's
Northeast Multispecies Fishery Management Plan ("nonregulated
multispecies" category). In 2000, the New England Fishery
Management Council implemented Amendment 12 to this FMP, and placed
silver hake into the “small mesh multispecies” management
unit, along with red hake and offshore hake. This amendment established
retention limits based on net mesh size, adopted overfishing definitions
for northern and southern stocks, identified essential fish habitat
for all life stages, and set requirements for fishing gear (NEFMC
2000).
NORTHERN
STOCK SILVER HAKE
The
Fishery
Commercial landings of silver
hake from the northern stock were significantly lower than those
from the southern stock during the mid and late 1960s and throughout
the 1970s (Figure
4.2
[Fig 4.2 Data]). In 1975, commercial landings
peaked at 40,000 mt but have since progressively declined. After
1976, landings declined due to the departure of the distant water
fleets. Commercial landings attained a historical low of 240 mt
in 2005 (Figure
4.3
[Fig 4.3 Data], Table
4.1).
Substantial quantities of juvenile silver hake are discarded in
the large mesh and small mesh otter trawl fisheries and in the northern
shrimp fishery. Discard estimates during 1989-1992 range from 1,700
mt to 7,200 mt (17 million to 76 million fish) per year. High juvenile
discards can diminish future yields and spawning potential (Brodziak
et al. 2001).
Research Vessel Survey Indices
The NEFSC autumn bottom-trawl
survey biomass index declined during the mid-1960s, reaching a minimum
in 1967-68 (Figure
4.4
[Fig 4.4 Data]). With the appearance of the strong
1973 and 1974 year classes, biomass indices increased during the
mid-1970s, but declined during the late 1970s. Biomass indices were
higher in the 1980s and early 1990s, peaking in 1998 at 22.0 kg/tow.
Subsequently, biomass indices have markedly declined to a low of
1.95 kg/tow in 2005. Age structure of the northern stock was more
uniform through the early 1980s, with fish up to age 8. However,
the current age structure shows few fish older than age 4 (Figure
4.5
[Fig 4.5 Data]).
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Biological Reference Points
The
northern silver hake stock overfishing definition (NEFMC 2003)
uses a relative exploitation index (total landings divided by
NEFSC autumn survey biomass index) as a proxy for fishing mortality.
The northern stock is considered overfished when the 3-year average
biomass is less than ½ the BMSY proxy (6.63
kg/tow). The 3-year average biomass has been above the ½
BMSY proxy (> 3.31 kg/tow) since 1971, although
the 2005 survey biomass index is below this value (Figure
4.6
[Fig 4.6 Data]). Overfishing occurs when the
3-year average exploitation index is below 2.57, the FMSY
proxy (the average exploitation index during 1973-1982), and is
used as both a target and threshold value for fishing mortality
for the northern stock (NEFSC 2006). Exploitation indices have
been below the FMSY proxy since 1978 (Figure
4.7
[Fig 4.7 Data]).
Summary
Northern silver hake landings have decreased
substantially since 1977, and are presently at a historical low.
The autumn survey biomass index has fluctuated over the years, but
has continuously declined since 1998, and is at a low of 1.95 kg/tow
in 2005. In 2005, the 3-year average exploitation index for 2003-2005
was below the FMSY proxy and the 3-year average biomass
index remained above the ½ BMSY proxy, indicating
that the stock is not overfished and overfishing is not occurring.
Domestic landings from the southern
silver hake stock have varied between 5,000-30,000 mt, reaching
a peak of about 27,000 mt in 1964. However, between 1960 and 1980,
distant-water fleet landings of southern silver hake were very high,
peaking at about 280,000 mt in 1965 and around 100,000 mt in 1974.
Distant-water fleet landings tapered off in the mid-1980s, and total
landings have since continued to gradually decrease. In 2005, total
landings were near a historic low at 7,000 mt (Figure
4.8[Fig
4.8 Data]).
As in the northern stock, significant quantities of juvenile southern
silver hake are discarded in both large mesh and small mesh otter
trawl fisheries. Annual discard estimates during 1989-1992 range
from 1,300 mt to 10,000 mt (10 million to 81 million fish) per year.
High discarding of juveniles may severely limit opportunities to
rebuild the southern silver hake stock (Brodziak et al. 2001).
Research Vessel Survey
Indices
The NEFSC autumn bottom trawl
survey biomass index declined sharply from a high of 5.52 kg/tow
in 1965 to 0.86 kg/tow in 1974. The survey biomass index recovered
slightly during the early 1980s as distant-water fleet fishing
effort diminished. However, between 1985 and 2000, the biomass
index decreased by more than 50%. Since 2001, the biomass index
has fluctuated between 1.1 and 2.4 kg/tow (Figure
4.9[Fig
4.9 Data]). The NEFSC autumn survey shows a
progressive truncation of the southern silver hake age composition
throughout the time series, with no fish older than age 4 after
2002 (Figure
4.10 [Fig
4.10 Data]).
Figure 4.8
Figure 4.9
Figure
4.10
Figure 4.11
Figure 4.12
Biological Reference
Points
The southern silver hake stock is considered
to be overfished when the three-year moving average of the NMFS
autumn survey weight per tow index is less than half of the
BMSY proxy (1.78 kg/tow) (NEFMC 2003). The three-year
average of the survey biomass during 2003-2005 was 1.41 kg/tow,
which was above 1/2 BMSY (0.89 kg/tow) but below
the BMSY target of 1.78 kg/tow (Figure
4.11[Fig
4.11 Data]).
Overfishing is considered to be occurring in the silver hake
stock when the exploitation index (landings divided by the three-year
moving average of the delta-distributed fall survey biomass
index) exceeds the FMSY threshold proxy of 34.39
(NEFMC 2002). The 2003-2005 average exploitation index of 4.67
is below the FMSY threshold proxy as well as the
FMSY target proxy of 20.63 (Figure
4.12
[Fig 4.12 Data]).
Summary
The southern silver hake stock is not overfished
and overfishing is not occurring. However, the 2005 biomass index
is less than half of the indices observed in the early 1960s,
and except for 2001, the biomass index has remained below the
BMSY proxy since 1989 (Figure 4.9)
(Figure
4.9[Fig
4.9 Data]). Another source of concern is that the age
structure of the southern silver hake stock is severely truncated
in recent years, with few fish older than age 4 (Figure
4.10[Fig
4.10 Data]).
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