SILVER HAKE

SILVERHAKE

LIFE HISTORY

EGGS

The eggs are pelagic, 0.88-0.95 mm in diameter, drift with the prevailing currents, and hatch in about two days at 20^oC.

LARVAE

Newly hatch larvae are pelagic and are about 2.6-3.5 mm long. Pelagic life lasts about 2 months and at about 17-20 mm long the larvae descend to the bottom as juveniles.

JUVENILES AND ADULTS

Studies of age and growth of silver hake in U.S. waters began with Schaefer (1960). Nichy (1969) aged juvenile fish and conducted an age validation study. In an investigation of the effect of stock size on growth of various age classes in the northern Georges Bank-Gulf of Maine stock, Ross and Almeida (1986) found that mean weight-at-age (2-5 years) was significantly inversely correlated with three measures of stock density during 1962-1979. Almeida (1978) and Penntila et al. (1989) used von Bertalanffy growth functions to describe growth of the northern and southern stocks. Almeida (1978) found faster growth and greater size in the northern stock compared to the southern stock.

Helser (1996) made a detailed analysis of the growth of silver hake in four areas of the U.S. continental shelf during 1975-1980, 1982-1987, 1988-1992. He found significant differences in growth between the Middle Atlantic and southern Georges Bank in all three periods, between northern Georges Bank and the Gulf of Maine during 1975-1980 and 1982-1987, and between northern and southern Georges Bank during 1982-1987. Growth in the Gulf of Maine, on average, was slow, but the fish attained the large asymptotic length (k = 0.33, L_¥ = 47 cm) compared to the Middle Atlantic Bight (k = 0.51, L_¥ = 43 cm); growth on Georges Bank was intermediate.

Females grow faster and live longer than males. Reported maximum lengths and ages are 34 cm and 6 years for males and 60-64 cm and 12 years for females (Bigelow and Schroeder 1953; Nichy 1969). Interestingly, maximum lengths of silver hake collected during the NMFS Northeast Fisheries Science Center (NEFSC) bottom trawl survey are 78 cm in spring and 76 cm in autumn. Table 1 lists mean lengths at age to indicate relative size-at-age for Gulf of Maine, Middle Atlantic, northern Georges Bank, and southern Georges Bank for 1988-1992.

Helser and Almeida (1997) performed a detailed analysis of the possible effects of stock abundance, growth, spring water temperatures, and depths on the proportion of silver hake that are mature at age 2 and 3. Population abundance accounted for most of the variation in sexual maturity and growth in the southern stock accounted for some additional variation. Both catch-weighted water temperature and catch-weighted bottom depth had no statistically significant effect on the proportion mature at age 2 or 3 for the northern stock, or at age 2 for the southern stock. A slight effect was found for the proportion mature at age 3 for the southern stock.

SILVERHAKE

SUMMARY TABLE

 

Mean length (cm) at age (years) in silver hake, Merluccius bilinearis,

 for Gulf of Maine (GM), Middle Atlantic (MA), northern Georges Bank (NGB),

and southern Georges Bank (SGB) from samples collected during 1988-1992; based on Helser (1996).

Observations of silver hake, Merluccius bilinearis, age and length at maturity.

  

 

Published observations about silver hake, Merluccius bilinearis:

 bottom water temperature, depth distributions, and preferences.

 

 

Summary of habitat parameters for silver hake, Merluccius bilinearis.

 

Published reports of the nearshore and estuarine occurrences of silver hake,

 Merluccius bilinearis, eggs and larvae from the area south of Cape Cod.

  

Average number of silver hake, Merluccius bilinearis, in Narragansett Bay,

per tow by season. From the Rhode Island Division of Fish and Wildlife bottom trawl surveys of Narragansett Bay, 1990 - 1996.

  

 

Summary of the distribution and abundance of silver hake, Merluccius bilinearis,

 in North Atlantic and Mid-Atlantic estuaries based on Jury et al. (1994) and Stone et al. (1994).

 

 

SILVERHAKE HABITAT CHARACTERISTICS

EGGS

Data from the NEFSC Marine Resources Monitoring, Assessment and Prediction (MARMAP) ichthyoplankton survey was used to determine the relationships between bottom depth and water temperature in the upper 15 m and silver hake egg abundances (Figure 2). Eggs occurred in relatively deeper waters than the stations sampled. The bottom depths where eggs were captured ranged from 10-1250 m, but most eggs were collected between 50-150 m. A shift to shallower shelf waters during late spring is evident (Figure 2); the average depth of egg occurrences declined from about 100 m in spring to about 60 m in summer.

The percent frequencies of stations sampled show the expected annual cycle in water temperatures (Figure 2) and the occurrences of eggs show a similar cycle. During the months of coldest temperatures (March and April) silver hake eggs occurred in the coldest temperatures of the year (5-12^oC). As the water warmed in the spring, the occurrences of eggs shifted to warmer waters until summer, when eggs were found mostly between 13 and 20^oC. During autumn the waters began to cool and egg occurrences shifted to cooler waters (10-15^oC). Peak abundance of eggs occurred from June to September and during these months most eggs were found in temperatures 11-17^oC. The large bar in June at 21^oC reflects an unusually large catch of eggs (18,908 eggs/10 m²).

The results presented for the NEFSC MARMAP survey agree with published observations of depth and temperature occurrences of silver hake eggs (Bigelow and Schroeder 1953; Fahay 1974; Waldron 1988).

LARVAE

The relationships of bottom depth and water temperature in the upper 15 m of water to the abundance of silver hake larvae from the NEFSC MARMAP survey are shown in Figure 3. Larvae occurred in relatively deeper water than the overall depths of stations sampled although generally shallower than eggs. The bottom depths where larvae were captured ranged from 10 to 1250 m, but most occurred between 50-130 m. The depths of larvae occurrence remained stable throughout most of the year but a shift to slightly deeper waters occurred during May and June.

The occurrences of larvae in relation to water temperature in the top 15 m showed a shift in preferred temperatures that followed the annual warming and cooling cycle (Figure 3). During the months of coldest temperatures (February and March) silver hake larvae were found in the coldest water of the year (5-12^oC). As the water warmed in the spring, the occurrences of larvae shifted to warmer waters until summer when they were found mostly between 10 and 16^oC. During autumn, the waters began to cool, but larvae remained in warmer waters (10-16^oC). Peak abundance of larvae occurred from July to October and during these months most were found in temperatures 11-16^oC.

JUVENILES

Based on the NEFSC bottom trawl survey, the relationships of bottom temperature and depth to catches of juvenile silver hake in spring and autumn are shown in Figure 4. In spring, silver hake were captured at temperatures of 3-17^oC and bottom depths of 5-350 m. Their preferred temperatures and depths were 6-9^oC and 150-275 m. In autumn, they occurred in temperatures and depths of 5-22^oC and 5-350 m and their preferred ranges were 10-17^oC and 25-75 m.

The relationships of bottom temperature and depth to catches of juvenile silver hake in spring and autumn from the Massachusetts trawl survey are shown in Figure 5. Juveniles were caught at all depths and temperatures during both seasons. No clear temperature preference was detected in spring, but in autumn, 7-11^oC was preferred. During both seasons, the shallowest depths (< 20 m) had the smallest catches of juveniles.

Analyses of the Hudson-Raritan estuary trawl study data on the relationships of bottom temperature, dissolved oxygen, salinity, and depth to catches of juvenile silver hake in spring and autumn are shown in Figure 6. Silver hake occurred at temperatures from 1-17^oC and preferred temperatures from 9-14^oC. The preferred levels of dissolved oxygen were 7-9 mg/l. They occurred at all depths sampled except 10 m and showed preference for depths of 10-20 m. Juveniles avoided salinities less than 20 ppt and showed preferences for salinities above 28 ppt.

In the Rhode Island trawl surveys, juvenile hakes were relatively abundant in Narragansett Bay at depths near 30 m during winter, spring and summer and at 10 m in autumn (Figure 7). They occurred at most bottom temperatures but preferred 9^oC temperatures in summer.

 

ADULTS

Silver hake adults and juveniles are found over wide temperature and depth ranges in U.S. continental shelf waters (Table 3). The relationships of bottom temperature and depth to catches of adult silver hake in spring and autumn from the NEFSC bottom trawl survey are shown in Figure 4. In spring, adult silver hake were captured at temperatures of 3-19^oC and bottom depths of 5–400 m. Their preferred temperature and depths in spring were 7-13^oC and 125–325 m. In autumn, they occurred in temperatures and depths of 4-22^oC and 5–500 m and their preferred ranges were 7-11^oC and 150–225 m.

Based on the Massachusetts trawl survey, the relationships of bottom temperature and depth to catches of adult silver hake in spring and autumn are shown in Figure 5. They occurred at all temperatures in spring and no clear temperature preference could be detected. In autumn, they occurred in temperatures from 5-17^oC but preferred temperature from 7-11^oC. They were caught at depths > 5 m in spring and > 15 m in autumn. During both seasons, the lowest numbers of adults were caught in the shallower depths (< 20 m).

The Hudson-Raritan estuary trawl surveys caught a total of just one adult silver hake in January 1995. In Narragansett Bay, adult hakes were relatively abundant at depths > 20 m in all seasons (Figure 8) and showed a preference for bottom temperatures between 7 and 16^oC.

In Long Island Sound, silver hake were found at all temperatures sampled, except at 23^oC in autumn, and they showed no particular preference for temperature in either spring or autumn. Hakes were captured at most depths sampled and showed no preference for particular depths in spring or autumn. Silver hake were captured throughout the range of salinities observed in the sound. No clear salinity preference was evident in spring, but in autumn, hakes were captured at higher than average salinities.

Scott (1982) reports that silver hake on the Scotian Shelf prefer salinities between 33 and 34 ppt, but were captured in waters with salinity from 31 to 34 ppt. In a large-scale analysis of research bottom trawl surveys on the Scotian Shelf, Scott (1982) found silver hake occurred on all bottom types from gravel to fine silt and clay, but were mainly associated with silts and clays. The areal trends in bottom type on the Scotian Shelf shows that coarse sand and gravel are associated with shallow water and finer grain deposits with deeper water and that the proportion of gravel increases from east to west. Thus, the areal trends in grain size and bottom water temperatures, as well as prey distributions, confound our ability to determine which factors contribute to distribution patterns of silver hake on the Scotian Shelf.

On the extremely fine-scale, Auster et al. (1997) found that silver hake (1.5-5 cm) were more abundant on silt-sand bottoms containing amphipod tubes at 55 m deep in the Middle Atlantic Bight. Silver hake (12.6-27.6 cm) were found on flat sand, sand-wave crests, shell and biogenic depressions, but were most often found on flat sand at one site in the Middle Atlantic Bight (Auster et al. 1991).

SILVERHAKE GEOGRAPHICAL DISTRIBUTION

The overall distribution of silver hake is from the Gulf of St. Lawrence to Cape Hatteras (Figure 9). The areas of highest abundance are the southern edge of the Grand Bank, the Scotian Shelf, the Gulf of Maine, Georges Bank, and the Middle Atlantic Bight off Long Island.

EGGS

Silver hake eggs were found throughout the area surveyed during NEFSC MARMAP ichthyoplankton surveys (Figure 10). They were most abundant in the deeper parts of Georges Bank (> 60 m) and the shelf off southern New England. Eggs were captured in all months of the year. From January to March, eggs occurred in small numbers in the deep waters of the Middle Atlantic Bight. By April, the occurrence of eggs extended eastward along the southern edge of Georges Bank and the total number of eggs increased slightly. During May and June the catches of eggs extended onto the shelf and into nearshore waters in the Middle Atlantic Bight and southern New England areas. Some eggs were captured in the western part of the Gulf of Maine. By July and August the center of abundance had shifted east onto Georges Bank with southern New England and the Gulf of Maine continuing to show some catches of eggs. In September and October the occurrences of eggs began to decline with centers of abundance still on Georges Bank and into southern New England. Few eggs were captured in November or December but most occurred again in deeper waters of the Middle Atlantic Bight.

LARVAE

The individual survey distributions of silver hake larvae from Cape Hatteras, North Carolina to Nova Scotia, based on 50 ichthyoplankton surveys in the NEFSC MARMAP series (1977-1984), reveal considerable year-to-year differences in the temporal and spatial patterns of abundance. Centers of abundance in July - September occurred on Georges Bank and in southern New England in 1977, 1978, 1982, and 1983. In 1981 few larvae were found anywhere in the survey area. Only in 1979 and 1980 were significant numbers of larvae encountered in the Gulf of Maine (Morse et al. 1987).

The cumulative distribution and abundance of silver hake larvae from all NEFSC MARMAP surveys is shown in Figure 11. The overall pattern of distribution is similar to the distribution of eggs, wherein the center of abundance is on Georges Bank and a secondary center is in the southern New England area. Few larvae were captured from January through March, none in April, and most occurred in the Middle Atlantic Bight (Figure 11). The low catches reflect the relatively low egg production of silver hake during the early months of the year. During May and June larvae were abundant from off Virginia to the southwestern part of Georges Bank in depths from 60-130 m. The peak months of larval abundance were July-September and the centers of abundance were now in southern New England and on Georges Bank. Distribution of larvae during October to December had shifted to the west from Georges Bank to southern New England and the Middle Atlantic Bight and abundance had dropped following the decrease in egg production during the late autumn and early winter. Table 5 lists the published reports of the nearshore and estuarine occurrences of silver hake eggs and larvae south of Cape Cod.

JUVENILES

Based on the NEFSC bottom trawl survey, juvenile silver hake occurred across the shelf in the Middle Atlantic Bight and southern New England areas, along the southern and northern edges of Georges Bank, and scattered throughout the Gulf of Maine in the winter (Figure 12). In the spring, they occurred throughout the survey area but tended to avoid the shallowest part of Georges Bank. Concentrations occurred in relatively shallow waters in southern New England and in most of the southern and western parts of the Gulf of Maine. Positive catches during summer surveys show a distribution similar to that in the spring. By autumn, the distribution of juveniles again covered the shelf with small catches in the southern part of the Middle Atlantic Bight and in the center of the Gulf of Maine. Centers of abundance occurred in nearshore waters off New Jersey and Long Island, south of Rhode Island, on Georges Bank, and along the coast of Massachusetts and Maine.

Silver hake juveniles were caught throughout the Massachusetts trawl survey area in spring and autumn and were concentrated off Cape Ann, in Cape Cod Bay and south of Martha’s Vineyard. Light catches were made in Vineyard Sound and Buzzards Bay (Figure 13).

Juvenile silver hake were collected throughout most of the Hudson-Raritan estuary survey area but were concentrated in areas near the ocean (Figure 14). The highest catches occurred in autumn (Oct.-Dec.) and no juveniles were captured in summer (July-Aug.).

Juvenile silver hake were captured throughout Narragansett Bay in all seasons with the highest average catches of juveniles (~3 hake/tow) occurring in summer (Table 6; Figure 15). In general, low numbers of hake occurred throughout the Bay and the highest average catches were at the ocean station 1. Seasonal length frequencies indicate that juveniles and adults occurred in similar proportions during all seasons except summer when juveniles dominated the catches.

The catches of silver hake in Long Island Sound were dominated by fish < 23 cm long in both seasons and occurred throughout the bay (Figure 16).

ADULTS

Based on the NEFSC bottom trawl survey, silver hake were distributed across the shelf in the northern Middle Atlantic Bight and southern New England areas, along the southern and northern edges of Georges Bank, and scattered throughout the Gulf of Maine in the winter (Figure 12). In the spring, they occurred throughout the survey area but tended to avoid the shallowest parts of the Middle Atlantic Bight and Georges Bank. Concentrations occurred in relatively deep waters of the Middle Atlantic, southern New England, along the southern and northern edges of Georges Bank, and in the western part of the Gulf of Maine. Positive catches during summer surveys showed a distribution similar to that in the spring. By autumn, the distribution of adults again covered the shelf except for the southwestern part of the Middle Atlantic Bight and in the center of Georges Bank. Centers of abundance occurred in south of Rhode Island, on the northern edge of Georges Bank, and most of the Gulf of Maine.

Based on the Massachusetts trawl survey, silver hake adults were caught throughout the survey area in spring and north of Cape Cod in autumn. They were concentrated south of Martha’s Vineyard and Nantucket in spring and off Cape Ann and in Cape Cod Bay in autumn. Small catches were made in Vineyard Sound and Buzzards Bay in both seasons (Figure 13).

The Hudson-Raritan estuary trawl survey caught a total of just one adult silver hake in January 1995. Results of the Rhode Island trawl survey showed that, in general, low numbers of hake occurred throughout Narragansett Bay and the highest average catches were at the ocean station 1. Adult hakes occurred throughout the bay in small numbers in all seasons (Figure 15). Seasonal length frequencies indicate that juveniles and adults occurred in similar proportions during all seasons except summer when juveniles dominated the catches (Figure 15).

The catches of silver hake in Long Island Sound showed a high incidence of adults (> 22 cm) in spring. Based on the wide distribution of silver hake adults, it appears that they occur in all areas of the sound (Figure 16).

Jury et al. (1994) compiled summaries of the occurrence and relative abundance of eggs, larvae, juveniles, and adults in estuaries north of Cape Cod and Stone et al. (1994) compiled similar data for Cape Cod to Cape Hatteras (Table 7).

SILVERHAKE BIOLOGICAL INFORMATION

REPRODUCTION

Female silver hake are asynchronous spawners that produce and release several batches of eggs during the spawning season (Sauskan and Serebryakov 1968).

Silver hake eggs and larvae have been collected in all months on the continental shelf in U.S. waters, although the onset of spawning varies regionally (Bigelow and Schroeder 1953; Marak and Colton 1961; Sauskan and Serebryakov 1968; Fahay 1974; Morse et al. 1987; Waldron 1988; Berrien and Sibunka 1999). Silver hake spawn over a wide range of temperatures and depths. In the Middle Atlantic Bight, newly spawned eggs were collected at surface water temperatures between 13^oC and 22^oC (Fahay 1974). In the Gulf of Maine, Bigelow and Schroeder (1953) reported that spawning temperatures ranged from 5^oC to 13^oC.

Spawning begins in January along the shelf and slope in the Middle Atlantic Bight and proceeds north and east to Georges Bank into May. By June spawning has spread into the Gulf of Maine and continues to be centered on Georges Bank through summer. In October, spawning is centered in southern New England and by December spawning is observed again along the shelf and slope in the Middle Atlantic Bight. Over the U.S. continental shelf, significant numbers of eggs are produced beginning in May, numbers increase through August, and then decline rapidly during September and October (Berrien and Sibunka 1999).

The primary spawning grounds probably coincide with concentrations of ripe adults and newly spawned eggs. These grounds occur between Cape Cod, Massachusetts, and Montauk Point, New York (Fahay 1974), on the southern and southeastern slope of Georges Bank (Sauskan 1964) and the area north of Cape Cod to Cape Ann, Massachusetts (Bigelow and Schroeder 1953).

Silver hake median length at maturity (i.e., 50% mature) has declined in recent years. Table 2 lists observations of silver hake age and length at maturity.

FOOD HABITS

The diet of silver hake consists primarily of fish, crustaceans, and squids, depending on the size, age, and sex of the silver hake (Schaefer 1960; Domanevsky and Nozdrin 1963; Dexter 1969; Edwards and Bowman 1979; Bowman 1984; Waldron 1993; Helser et al. 1995). Young fish (< 20 cm) eat mainly crustaceans, such as euphausiids and shrimps. As silver hake grow they consume a larger proportion of fish and individuals > 35 cm feed almost exclusively on fish (Bowman 1984).

MIGRATION

During spring and summer, silver hake move into nearshore waters in the Gulf of Maine, to the northern edge of Georges Bank, and northward in the Middle Atlantic Bight. By autumn, they return again to the deeper basins in the Gulf and along the continental slope. The pattern for juveniles is similar to adults in general distribution and movements, except that the centers of juvenile abundance occur in shallower waters. Juveniles are abundant inshore along the Middle Atlantic and on the shoals of Georges Bank.

STOCK STRUCTURE

Silver hake occurring on the U.S. continental shelf have been divided into at least two stocks. Numerous studies using a variety of methods, including morphometrics (Conover et. al. 1961; Almeida 1987), growth patterns on otoliths (Nichy 1969), serological analyses (Konstantinov and Noskov 1969; Schenk 1981), seasonal distribution patterns (Anderson 1974; Almeida 1987), and growth (Helser 1996), indicate that fish in the Middle Atlantic Bight are distinct from fish in the Gulf of Maine. The line dividing these two stocks occurs somewhere between Nantucket Shoals and the northern edge of Georges Bank. However, the results of these studies are inconclusive and additional work is needed to confirm the existence of biologically separate stocks (e.g., Helser 1996).