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Alaska Whitefish The cold clean waters off Alaska support huge stocks of whitefish, many of which are in high demand in seafood markets worldwide. The term “whitefish” is synonymous with “groundfish” or “bottomfish”, and refers to several species of white-fleshed fishes that live on or near the bottom of the ocean. The scientific term for this lifestyle is “demersal”, which distinguishes them from fishes that swim throughout the water column, called “pelagic”. While there are dozens of species of whitefish, this Buyer’s Guide produced by the Alaska Seafood Marketing Institute, deals with the nine species of greatest interest to the seafood trade:

Click here to see photos of Alaska Whitefish species (requires Adobe Acrobat).

All nine of these Alaska whitefish species are sustainably harvested from the Bering Sea and Gulf of Alaska. Here are some important statistics about Alaska:

• Alaska has 47,300 miles of coastline, more than all of the other 49 states put together
• Alaska has over 795,000 square miles of continental shelf, 70% of the U.S. total
• the 200-mile Exclusive Economic Zone (EEZ) off Alaska is 28% of the U.S. total
• more than half of all seafood harvested from American waters originates in Alaska

The Bering Sea and the Gulf of Alaska are huge bodies of water (see the chart at right depicting the 200-nautical-mile limit of the EEZ). All of this region lies within the United Nations’ Food & Agriculture Organization’s (FAO) statistical catch area 67.

These two bodies of waters are among the most productive on the planet because of the upwelling of nutrient-rich water from deep regions along the shelf edge toward the surface. The combination of nutrients and sunlight supports very large populations of phytoplankton (microscopic drifting plants such as diatoms), which are eaten by zooplankton (small drifting animals such as shrimp and krill).

The phytoplankton and zooplankton are in turn eaten by small fishes which are eaten by bigger fishes, marine mammals, and seabirds. For decades this rich ecosystem has supported abundant, sustainable fisheries, and it has been the subject of hundreds of studies by scientists from universities and government agencies.

Alaska whitefish spend all of their lives out in the ocean. They never enter fresh water. Out in the deep, clean, cold waters of the Gulf of Alaska and the Bering Sea, Alaska whitefish are a natural part of the ecosystem. At different stages in their lives they feed on plankton, shrimp and other crustaceans, other fishes, and other marine organisms. In turn they themselves are food for bigger fishes, birds, and mammals.

Cod, pollock, halibut, and yellowfin sole are known for their high fecundity: individual females produce tens of thousands to over a million eggs per spawning. All species spawn many times during their lives. These attributes serve to stabilize the populations by enabling these fishes to withstand natural fluctuations in ocean conditions and to recover from environmental changes.

Alaska Pollock
Alaska pollock are the most abundant fish species in the Bering Sea, comprising 60% of the total biomass. They are members of the family Gadidae, which includes both Pacific cod and Atlantic cod. In the Gulf of Alaska, pollock are the second-most abundant fish species, comprising 20% of the biomass. Fisheries scientists recognize four stocks of pollock: Gulf of Alaska, eastern Bering Sea, Aleutian Islands, and Aleutian Basin. The three Bering-Aleutian stocks appear to be interrelated, but they are considered separate from the Gulf stock. Pollock in the Aleutian Basin are distributed across the U.S. EEZ, the Russian EEZ, and international waters.

Most adult pollock are found in waters 70-300 meters (230-1,000 feet) deep. They spawn between late February and early May. The eggs are pelagic, drifting in the currents for 15-25 days until hatching. The newly hatched larvae drift in the upper 40 m (130 ft) and feed on plankton for 60 days until they metamorphose into pelagic juveniles. As they age and grow they move deeper in the water and join the adult stock in about four years. Their diet changes at different life stages: juveniles feed on invertebrate eggs and small planktonic crustaceans, while adults feed mainly on copepods, krill, and other fishes, mainly juvenile pollock. Pollock are an important food source for other fishes, marine mammals, and birds.

Pacific Cod
Pacific cod are also a member of the family Gadidae and are related to both Alaska pollock and Atlantic cod. Pacific cod are considered a trans-oceanic demersal fish, and are found at depths to 500 m (1,650 ft). They spawn from January through May. Their eggs are demersal and adhesive, clinging to rocks, coral, and other features on the seabed. The eggs hatch in 15-20 days, and the larval cod drift near the surface of the water.

The diet of Pacific cod also changes throughout their lives. Small fish feed mostly on invertebrates while large cod feed mostly on other fishes. In turn they are preyed upon by halibut, salmon sharks, northern fur seals, harbor porpoises, various whale species, and tufted puffins.

Sablefish
Sablefish are sometimes called “black cod”, but they are not part of the cod family. Sablefish live in deep waters, usually below 200 m (660 ft). They spawn in late winter and early spring, along the continental slope. The eggs incubate near the bottom but the larvae rise to near the surface.

Juvenile sablefish are found in shallower water close to shore, but they soon move to deeper offshore waters. Larvae and juveniles feed on planktonic crustaceans while adults are considered “opportunistic” feeders, taking benthic fishes and invertebrates, squid, and jellyfish.

Pacific Halibut
Pacific halibut are related to Atlantic halibut, and some scientists consider them to be the same species. They are the largest of all flatfishes and are among the largest fish in the sea, growing up to 2.7 m (9 ft) in length. The heaviest reported halibut was 318 kg (700 lbs), and several have been documented in the range of 227 kg (500 lbs). Female halibut are bigger than the males. Besides being bigger, female halibut generally grow faster and live longer than the males. Few males reach 36 kg (80 lbs) in weight. Age is determined from their earbones called otoliths. Seasonal layers that are visible in otoliths can be counted.

Halibut spawn from November to March at depths of 180-460 m (600-1,500 ft) along the continental shelf. A large female halibut can produce four million eggs. The eggs and larvae are free-floating but slightly heavier than surface seawater, so they drift in deep ocean currents. As the larvae grow they become lighter and rise nearer the surface. They drift from east to west in the Gulf of Alaska gyre for hundreds, sometimes thousands of miles.

Like all flatfishes halibut are compressed laterally and actually swim on their sides. Their “backs” are darkly pigmented while their “bellies” are whitish in color. Halibut larvae begin life in an upright position with one eye on each side of the head like most other fishes. Whenthey are about an inch long, the left eye moves across the snout to the right side of the head, while the pigmentation on the left side of the body fades. By the age of about six months the young halibut look like small adults and settle to the seafloor. There they begin a multi-year eastward migration back toward the spawning grounds from which they came.

Most adult halibut remain in roughly the same area year after year. But they are strong swimmers, and some halibut undertake extensive movements of thousands of miles.

Larval halibut feed on plankton while juveniles eat crustaceans and small fishes. Adult halibut eat mostly fishes of other species such as cod, pollock, sablefish, rockfish, and other flatfishes. They will even leave the sea bottom to consume pelagic fishes such as herring and sand lance. Because adult halibut are big, active, strong-swimming, and bottom-dwelling, they are less vulnerable to predation than smaller species. They are occasionally eaten by marine mammals.

Yellowfin Sole
Yellowfin sole are much smaller than halibut, reaching a length of 46 cm (18 inches). They live on the seafloor where the adults feed on bivalve molluscs,crustaceans, and other invertebrates. On the eastern Bering Sea shelf, yellowfin sole overwinter near the shelf edge and migrate toward the inner shelf in spring for feeding and spawning. A female can produce up to 3.3 million eggs. As the larvae mature into juveniles they undergo a metamorphosis similar to that of halibut, in which the left eye moves to the right side of the head, and the left side of the body loses pigmentation.

Dover Sole, Rex Sole, Rock Sole & Flathead Sole
The collection of small flounders known as “deepwater flatfish” includes Dover sole, rex sole, rock sole, and flathead sole, along with other, less abundant species. They live on the ocean bottom of the continental shelf and conduct annual migrations between spawning grounds and feeding grounds. They eat similar foods ranging from crustaceans and molluscs to a variety of other invertebrates. In turn, they are consumed by a wide range of predators including cod, halibut, skates, and large flounders such as arrowtooth. They occupy slightly different ecological niches, based on species-specific preferences for food, substrate (mud, sand, gravel, rock), depth, and other factors.