INTRODUCTION
The Atlantic halibut, Hippoglossus hippoglossus, is the largest of all flatfish (Figure 1). It is found on both sides of the North Atlantic Ocean and in parts of the Arctic Ocean. A directed fishery for Atlantic halibut in U.S. waters began in the early 19th century and peaked from 1845 to 1900 (A.B. Howe, Massachusetts Division of Marine Fisheries, personal communication). By the 1940’s it had collapsed and for many years there was no directed Atlantic halibut fishery in U.S. waters. Consequently, no management plan was developed for the species.
Currently, a small-scale fishery for “chicken” halibut (3.6-6.8 kg) exists off the coast of Maine. The September 1997 ‘Status of Fisheries of the United States’ (National Marine Fisheries Service 1997) reports that the U.S. Atlantic halibut population is currently in an overfished condition, and the New England Fishery Management Council intends to place Atlantic halibut within the Northeast Multispecies Fishery Management Plan (NEFMC 1996). This Essential Fish Habitat Source Document provides information on the life history and habitat characteristics of Atlantic halibut.
LIFE HISTORY
A synopsis of the life history of Atlantic halibut is presented here. More detailed information is provided in reviews by Haug (1990), Trumble et al. (1993), and Collette and Klein-MacPhee (in prep.). EGGS The halibut egg is among the largest of planktonic fish eggs (Russell 1976). Fertilized halibut eggs in the western Atlantic have a diameter of 3-4 mm (Fahay 1983; Scott and Scott 1988; Miller et al. 1991).
In Norway, eggs range from 2.86-2.98 mm (Trumble et al. 1993) to 3.06-3.49 mm (Haug et al. 1984). The eggs are bathypelagic, floating not at the surface, but rather, suspended in the water column at depths ranging from 54 m (Scott and Scott 1988) to 200 m (Blaxter et al. 1983). In the eastern Atlantic, eggs rise for 2-4 days after deposition to a depth of neutral buoyancy (Haug 1990; Trumble et al. 1993). Laboratory studies indicate that eggs are neutrally buoyant at salinities of 35- 37 ppt (Blaxter et al. 1983; Trumble et al. 1993); however, this is considerably higher than salinities found on the continental shelves of the North Atlantic. Thus, eggs are negatively buoyant due to their high organic matter content (Riis-Vestergaard 1982) and sink towards the bottom where development is thought to proceed (Blaxter et al. 1983). In northern Norway, eggs were found at intermediate depths, temperatures of 4.5-7o C, and salinities of 33.8-35.0 ppt (Haug et al. 1984). The incubation period is strongly temperature-dependent, lasting from 13-20 days at 4.7-7o C (Miller et al. 1991; Collette and Klein-MacPhee, in prep.).
LARVAE
Information on larvae is scarce since they have been difficult to catch in sufficient numbers (Haug 1990; Trumble et al. 1993). The 6 to 7 mm long larvae (Lonning et al. 1982; Blaxter et al. 1983) hatch at an early stage of development, with no pigment, functional eyes or mouth, and possess a very large yolk sac (Blaxter et al. 1982; Lonning et al. 1982; Haug 1990). Little information on the distribution of the pelagic stages is known, but larvae are thought to remain close to the water surface until metamorphosis (Nickerson 1978). Browns Bank may be a significant rearing area for young Atlantic halibut (Neilson et al. 1993). The larval development period is long. Exogenous feeding commences 28-35 days after hatching, and the yolk sac is completely absorbed 50 days after hatching at 5.3o C (Blaxter et al. 1983), at which point the larvae are 11.5-13.0 mm in length (Pittman et al. 1987). Metamorphosis begins with the migration of the left eye about 80 days after hatching, at a length of about 20 mm at 6o C (Pittman et al. 1987). Settlement occurs at 34-40 mm, prior to completion of eye movement and metamorphosis is complete by approximately 50 mm (Haug 1990). However, Nickerson (1978) reports that the left eye completes its migration one year after hatching, at a length of 10 cm, at which point settlement to the bottom occurs.
JUVENILES
In the western Atlantic, juveniles are known to exist in distinct nursery grounds (Haug 1990; Miller et al. 1991). Metamorphosis into the adult stage begins at a length of approximately 24 mm and, depending on temperature, after approximately 90 days of development. Transformation is complete by 4-10 cm, and may take up to one year (Miller et al. 1991). ADULTS Atlantic halibut show considerable sexual dimorphism in size at length, with females attaining a substantially larger size than males (McCracken 1958; Bowering 1986). Sizes as large as 3 m in length and 300 kg in weight, and ages of 50 years have been documented (Trumble et al. 1993). During the height of the halibut fishery in the 19th century, the average size of femalesPage 2 was 100 to 150 pounds and males rarely exceeded 50 pounds (Goode 1884).
More recent studies report smaller sizes: Bowering (1986) reported captures of males up to 189 cm and females up to 229 cm in length off Newfoundland, and Miller et al. (1991) reported females up to 220 cm and 35 years of age. Most halibut caught in recent years weighed less than 100 kg (Nickerson 1978). In the northeast Atlantic, adults are thought to leave spawning areas and disperse randomly, apparently in search of food (Haug 1990), to shallow and deep waters as well as inshore and offshore areas (Godø and Haug 1988). Similar observations have been made in North American waters (McCracken 1958; Bowering 1986). Stobo et al. (1988) hypothesized that larger, sexually mature halibut (i.e., adults) exhibit limited dispersal and an annual return migration to spawning grounds.
REPRODUCTION
The age and size at maturity of Atlantic halibut vary considerably; females mature at a much larger size and older age than males (Table 1). Atlantic halibut are annual, group-synchronous spawners (Neilson et al. 1993). Females are batch spawners, able to ovulate several batches of eggs in a single reproductive season (Methven et al. 1992). Depending on body size, females can produce from 0.5-7 million eggs in a single season (Haug and Gulliksen 1988). Spawning in the western Atlantic is believed to occur on the slopes of the continental shelf and on the offshore banks (McCracken 1958; Nickerson 1978; Neilson et al. 1993), at depths of at least 183 m (Scott and Scott 1988), over rough or rocky bottom (Collins 1887).
In Norwegian coastal waters, halibut spawning has been reported over soft clay or mud bottom, in deepwater (300- 700 m) locations at temperatures ranging from 5-7o C and salinities of 34.5-34.9 ppt (Haug 1990). Spawning occurs during late winter and early spring (McCracken 1958; Scott and Scott 1988; Miller et al. 1991; Methven et al. 1992; Trumble et al. 1993), with peak spawning having been reported during November to December (Neilson et al. 1993). Kohler (1964) reported that spawning occurred during winter to early spring on the Scotian Shelf, during February to April in the Gulf of St. Lawrence, and during winter to late spring off Newfoundland (Kohler 1964). In northern Norway, spawning has been reported during December to March, with peak spawning at the end of January/beginning of February (Haug 1990). However, historical descriptions of spawning have reported ripe halibut as late as August (Goode 1884).
FOOD HABITS
The diet of Atlantic halibut changes with increasing size. Fish up to 30 cm in length feed almost exclusively on invertebrates, mainly annelids and crustaceans (crabs, shrimps); those 30-80 cm in length feed on both invertebrates (mainly crustaceans, some mollusks) and fish; and those greater than 80 cm in length feed almost exclusively on fish (Kohler 1967). In the Gulf of Maine, the most important prey of adult halibut during 1977-1980 were squid (Illex), crabs (Cancer), and fish (silver hake, northern sand lance, ocean pout, and alewife) (Collette and Klein-MacPhee, in prep.). Maurer and Bowman (1975) report that 91% of the stomach contents of juvenile and adult halibut (by weight) were fish (> 50% were longhorn sculpin and its eggs, but also cod and other gadids), and 8% were crustaceans.
Nickerson (1978) reports that the fish prey of halibut includes cod, cusk, haddock, ocean perch, sculpins, silver hake, herring, capelin, skates, flounder and mackerel. The Northeast Fisheries Science Center (NEFSC) bottom trawl survey data on food habits [see Reid et al. (1999) for details] show a similar ontogenetic shift in the diet of Atlantic halibut (Figure 2). The 1973-1980 data clearly illustrate that, while crustaceans dominate the diet of smaller halibut, fish increase in importance with size to dominate the diets of larger halibut (Figure 2a).
Halibut 21-30 cm in length fed exclusively on crustaceans, especially decapods. Those 31-80 cm in length fed on crustaceans (45%, mostly decapods), fish (33%, including gadids and clupeids), and mollusks (6.5%, all cephalopods). The occurrence of fish and mollusks (cephalopods) in the diet of 81-120 cm halibut increased to 50% and 17% respectively, while the occurrence of crustaceans decreased to 25%. The 1981-1990 data show a similar trend (Figure 2b). The diet of 31-80 cm halibut was dominated by crustaceans (66%, mostly decapods); fish and mollusks comprised 25% and 4% respectively. The diet of 81-134 cm long halibut was almost exclusively comprised of fish (80%), but also included decapods (20%, all Majidae).
MIGRATION
Juveniles start to emigrate from nursery areas when the fish are 3-4 years old (Haug and Sundby 1987). They then undergo a period during which most movement occurs; juveniles (< 75 cm) undergo greater migrations than adults (Stobo et al. 1988). Although most tagging study recaptures have been made within the same main region where the juvenile fish were tagged, very long distance migrations have been documented from Labrador to the western coast of Greenland (Godø and Haug 1988), the Gulf of St. Lawrence to Iceland (McCracken and Martin 1955), the Scotian Shelf to the Grand Bank (Jensen and Wise 1961; Kohler 1964; Stobo et al. 1988),Page 3 and the western coast of Greenland to the Grand Bank (Godø and Haug 1988). Extensive migrations have also been documented from northern Norway to the White Sea, Iceland and Greenland, from the Faroe Islands to the North Sea and Iceland, and from Iceland to the Faroe Islands, Greenland and Newfoundland (Haug 1990).
HABITAT CHARACTERISTICS
Detailed information on the habitat characteristics of Atlantic halibut follows and is summarized in Table 2.
EGGS AND LARVAE
The eggs of Atlantic halibut are spawned at temperatures of 4-7o C (Miller et al. 1991), depths as deep as 700 m (Blaxter et al. 1983), salinities of # 35 ppt (Blaxter et al. 1983; Haug et al. 1986), and on harder substrates of sand, gravel, and clay (Collette and KleinMacPhee, in prep.). The larvae are pelagic, floating within 50 m of the surface (Nickerson 1978), are buoyant at salinities of 34.8-36.4 ppt, and prefer salinities in the 30-34 ppt range (Blaxter et al. 1983).
JUVENILES AND ADULTS
Juvenile Atlantic halibut are quite localized, being found in apparently well-defined nursery grounds and in coastal areas 20-60 m deep with sandy bottoms (Haug 1990). Stobo et al. (1988) hypothesize that the area around Sable Island Gully on the Scotian Shelf may serve as a nursery area for juveniles before they begin their dispersive phase. Juveniles are able to survive sub-zero temperatures, but prefer temperatures > 2o C (Goff et al. 1989). Adults are found over sand, gravel or clay substrates (Collette and Klein-MacPhee, in prep.), at temperatures ranging from -0.5 to 13.6o C (Mahon 1997). However, most are caught within 3-9o C, and generally prefer temperatures > 4o C (McCracken 1958; Bowering 1986). They are typically found at depths of 100-700 m (720-900 m is their depth limit) (Bowering 1986, Miller et al. 1991), and most commercial catches are made at depths of 200-300 m (Scott and Scott 1988). Most of the Atlantic halibut taken during the NEFSC trawl surveys (see Geographical Distribution below) were at temperatures of 4-13 oC and depths of 25-200 m.
GEOGRAPHICAL DISTRIBUTION
Atlantic halibut in the northwest Atlantic were distributed from north of Labrador south to Long Island during 1975-1994 (Figure 3). The areas of highest abundance of the species seem to be along the southern edge of the Grand Bank and on the Scotian Shelf from Browns Bank to Banquereau Bank. This corresponds to their accepted center of abundance (Trumble et al. 1993). In U.S. waters, halibut are found on the northeast part of Georges Bank, Nantucket Shoals, Stellwagen Bank, and off the coast of Maine and Massachusetts. Although Atlantic halibut have been taken as far south as Virginia, these are few and considered stragglers from the main population (Smith et al. 1975).
In Canadian waters, historical distributions of Atlantic halibut ranged along the entire coast of Labrador and Newfoundland, the Gulf of St. Lawrence, the eastern shores of Nova Scotia, and the Bay of Fundy. In U.S. waters, halibut were abundant on Georges Bank, Nantucket Shoals, and between Gloucester and Cape Cod, Massachusetts, and were occasionally found as far south as New Jersey (Goode 1884, 1887).
EGGS AND LARVAE
No Atlantic halibut eggs were captured during the 1977-1991 NEFSC offshore ichthyoplankton surveys. They are negatively buoyant and thought to develop on or near the sea bed (Riis-Vestergaard 1982; Blaxter et al. 1983) and thus are not sampled in the ichthyoplankton surveys. Larvae were captured at only two of 1,672 stations sampled during the NEFSC ichthyoplankton surveys [see Reid et al. (1999) for details], on the northeast part of Georges Bank, and near Petit Manan Island off the eastern coast of Maine (Figure 4). This is not surprising since very few larvae have ever been captured in the wild (Haug 1990) and since spawning is believed to no longer occur in the Gulf of Maine (Collette and Klein-MacPhee, in prep.).
JUVENILES AND ADULTS NEFSC
Bottom Trawl Survey
In the western Atlantic, juveniles are typically found on the southwestern Scotian Shelf, but rarely off Newfoundland, supporting the view that the former is an important rearing or nursery area (Neilson et al. 1993). Catches of juvenile and adult Atlantic halibut from the 1963-1997 NEFSC bottom trawl surveys [see Reid et al. (1999) for details] are presented in Figure 5.
Halibut were caught in low numbers from throughout the Gulf of Maine area, as far south as Nantucket Shoals; a single halibut was caught southwest of Cape Cod. The highest concentrations were found in Canadian waters, on Browns Bank and off southwestern Nova Scotia. In U.S. waters, lower concentrations were found on the northern slope of Georges Bank, Nantucket Shoals, Stellwagen Bank, and off the coast of Maine. There does not appear to be aPage 4 significant seasonal effect on distribution and abundance (Figure 5).
There was a definite seasonal effect on the temperature inhabited by Atlantic halibut (Figure 6). In spring, > 70% of halibut were caught at 4-6o C, while in autumn, > 65% were caught at 9-13o C. Similarly, Scott and Scott (1988) found that commercial catches were most common at 3-9o C. Halibut were caught at depths ranging from 25-200 m, with the majority caught between 50-100 m (Figure 6). In spring, > 65% were found at 75-100 m, whereas in autumn, > 70% were caught at 50-75 m. Miller et al. (1991) states that Atlantic halibut in the western North Atlantic have been found over depths ranging from 37-1000 m.
Massachusetts Inshore Trawl Survey
Only 18 Atlantic halibut (all juveniles, 19-75 cm in length) have been taken in Massachusetts inshore waters between 1978 and 1997.
STATUS OF THE STOCKS
Historical landings of Atlantic halibut in the Gulf of Maine/Georges Bank area are presented in Figure 7. In 1900, landings had already declined 95% from 1879 levels (A.B. Howe, Massachusetts Division of Marine Fisheries, personal communication), and catches have since declined even further.
Prior to 1930, landings were variable, but often exceeded 600 metric tons (mt) annually, and catches exceeding 800 mt were common. Since then, landings have exceeded 400 mt only twice, and have generally been well below 200 mt. Landings averaged 756 mt per year from 1893 to 1930 (516 mt if the two especially high years are omitted), compared to only 164 mt annually from 1931 to the present. Since 1953 U.S. landings have been 100 mt or below, and have hit historical lows in recent years. Canadian landings in area 5 were more than twice the U.S. landings in the 1960’s, but have since also declined considerably. Currently, the area of highest exploitation of the species in the northwestern Atlantic is the Scotian Shelf area (Neilson et al. 1993).
NEFSC survey indices have fluctuated considerably since the 1960’s (Figure 7), and overall, have declined considerably. Mean weight per tow during spring surveys has remained at an historic low since 1988. During both spring and autumn surveys, mean number per tow has been considerably higher than mean weight per tow, indicating a decrease in the size of halibut. In fact, based on size, almost all halibut caught in the NEFSC surveys from 1988-1998 were juveniles (Figure 8).
The September 1997 ‘Status of Fisheries of the United States’ (National Marine Fisheries Service 1997) reports that the U.S. Atlantic halibut population is currently in an overfished condition.
May 2005
