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The Life History and Habitat Requirements of Atlantic Cod: The Story Behind a Plate of Fish & Chips
By Anthony R. Wilbur, CZM
The Atlantic cod (Gadus morhua)—a legendary fish of great economic, social, and ecological value to Massachusetts—is memorialized by a bronze statue on prominent display at the State House, honoring this staple of New England's economy for 300 years. For centuries, this prolific species drew settlers to colonize areas of North America for easy access to productive fishing grounds. Despite a long history of exploitation and the extensive study of Atlantic cod, much remains to be discovered of its habits and habitats.
The biology and ecology of fishes is an exciting area of research, not only because there are more fish species than all other vertebrates combined, but because this diversity is magnified by their range of life history and behavioral traits. Marine teleosts (bony fish) go through four general life history stages (egg, larvae, juvenile, and adult), with species showing specialized adaptations. For fish species that undergo complex life history development, larval, juvenile, and adult phases differ in almost all characteristics, including morphology, physiology, behavior, and resource requirements.
Our knowledge of most fishes is far from complete. The ecological relationships between fishes and habitat are documented in tropical systems (e.g., coral reef and mangrove marshes), temperate seagrass beds, and kelp forests. In New England, monitoring programs, such as the National Oceanic and Atmospheric Administration (NOAA) Fisheries stock assessment trawl survey, focus on large-scale patterns of fish distribution and abundance, rather than habitat associations. The role of habitat in fishery productivity has, however, gained attention in both management and scientific communities, with the identification of Essential Fish Habitat by the federal fishery management councils and the NOAA Fisheries Service (See Essential Fish Habitat and Fishery Management).
Habitat is not easily defined or identified, but it is widely accepted that environmental features influence the distribution, abundance, and productivity of fishes. Since more is known about Atlantic cod than most marine fishes in New England, this article summarizes the life history and habitat requirements of cod in the Northwest Atlantic Ocean (encompassing the Gulf of Maine and Georges Bank/Southward stocks), and underscores the value of marine habitat to the diversity of fishes in Massachusetts waters.
Life History and Range
Atlantic cod are part of the family of fishes known as Gadidae (the codfish family), which contains 55 species, including haddock (Melanogrammus aeglefinus) and pollock (Pollachius virens). Cod are widely distributed in the North Atlantic Ocean, extending from Greenland to North Carolina. Populations are a fraction of historic levels, with the largest concentrations remaining in Canadian waters, Georges Bank, and western Gulf of Maine.
Throughout their life history, cod inhabit a number of habitats, ranging from surface waters to the seafloor at 250 fathoms (~1,500 feet). Cod can live for 20 years and grow to an excess of 50 inches and 100 pounds. The largest cod ever recorded was caught off the Massachusetts coast at 211 pounds! Today, cod of this size are rarely, if ever, caught.
Cod are omnivorous, feeding on a variety of fishes, crabs, and clams, and the incidental plant. (Extremely strange fare has been found in some cod stomachs, including a boot!) Typically, younger cod forage on crustaceans and then eat more fish as they grow.
Cod have annual migration patterns that depend on geographic location, life history stage (e.g., spawning movements), and season, but cod typically move in relation to water temperature, swimming inshore in autumn and retreating to deeper, offshore waters as spring approaches. Cod in the Gulf of Maine follow this pattern, but cod populations off Cape Cod and southern New England not only retreat to offshore waters in the fall, they also migrate south along the mid-Atlantic coast (historically, as far south as Chesapeake Bay) and return to New England in the spring. Adult and juvenile cod tend to congregate and migrate long distances in schools, with larger and older cod leading the way. There is also evidence that certain cod populations have limited home ranges, residing in the same area for prolonged periods.
Although fishermen know where to catch adult cod, these fish have a complex life cycle and many aspects of their life history, including habitat requirements, remain to be explained. For a cod, life begins near the surface of the ocean as an egg and continues in the water column until the young juvenile settles to the seafloor, where it matures and grows. Survivorship of these early life history stages is especially important to the population status of harvestable adults.
Eggs and Larvae
Cod invest a great deal of energy in reproduction and spawn many times throughout their life, releasing millions of eggs. Spawning behavior, which evolved to give offspring an increased chance of survival (reproductive success), involves the congregation of female and male cod and links the reproductive cycle to seasonal changes in temperature and light to coincide with peak production of prey species (phytoplankton and zooplankton) that are needed for early life history development. Cod spawn in waters that limit the dispersal of eggs, such as gyres and nearshore waters. Known spawning habitat exists on the northeast peak of Georges Bank, coastal Gulf of Maine, and the eastern portion of Nantucket Shoals. In Massachusetts, northwestern Massachusetts Bay (e.g., Nahant Bay, Salem Sound, and Ipswich Bay) is a particularly important spawning area.
Eggs are fertilized in the water column and receive no parental care, floating in surface waters until they hatch. They are buoyant, spherical, and transparent and hatch in eight to 60 days (depending on water temperature). Egg density is highest in late winter and early spring, but eggs are found almost year round.
When hatching, larvae break away from the egg casing, and develop at a rate dependent on water temperature (with faster development and bigger larvae generally being produced in warmer water). The period of highest mortality in cod life history occurs between hatching and larval first feeding because of extensive predation by zooplankton and starvation. To counteract this high mortality, huge numbers of eggs are released during spawning.
The first stage of larval development is the yolk sac phase. The yolk sac contains nutrition used by the larval cod while it adapts to the pelagic environment (i.e., water column). The larvae absorb the yolk and continue to grow, while they begin to look for prey (at this stage, plankton). Larval cod are initially free-floating, drifting with currents. As larvae grow, they begin to look more like juveniles and develop the ability to swim and capture prey. Larval fishes are typically not strong swimmers but can actively seek habitat conditions that improve survival and growth.
Favorable water temperature, salinity, and food availability, as well as egg quality, are critical to larval cod survivorship, growth, and subsequent metamorphosis to the juvenile stage. Studies suggest that egg quality is related to the condition and age of spawning females, with poor feeding opportunities and first-time spawners producing lower viability in eggs. Fast growth has been correlated to the presence of high plankton concentrations, and regions that congregate plankton (e.g., areas where different ocean currents collide) are valuable forage habitats. Other environmental features influence egg and larval development, such as dissolved oxygen, nutrients, turbidity, water movement, and meteorological events, but these influences are not thoroughly explained.
Body shape, size, and pigmentation dramatically change during larval-juvenile metamorphosis, which occurs in the water column where juveniles remain for approximately one month. Pelagic juveniles may “test” the seafloor by bouncing along the bottom to locate suitable settlement habitat—but just how juvenile cod find these important habitats is not certain. At the point of settlement to the seafloor, cod become demersal (meaning they live near the seafloor) and look like miniature adults.
Settlement is another period of high mortality. Newly settled cod are susceptible to predation from a number of piscivores—that is fish eaters (e.g., spiny dogfish [Squalus acanthias], winter skate [Leucoraja ocellata], silver hake [Merluccius bilinearis], and adult cod). Complex seafloor habitats that provide refuge increase survival, making habitats such as cobble, hard bottom with attached epifauna (i.e., animals living attached to the bottom such as sponges and amphipod tubes), and eelgrass beds beneficial to early life history development. Nearshore waters of Massachusetts Bay and offshore shoal areas (e.g., Jeffreys Ledge and Georges Bank) contain these important nursery habitats. Juvenile cod are, however, not limited to complex seafloor habitats, as evidenced by studies in Salem Sound and Gloucester that collected cod on less-complex mud bottoms (although survivorship may be lower without adequate cover). In addition to protection from predation, juveniles require refuge from currents and ample supply of prey. Cod use a range of complex and simple seafloor environments before they reach the adult phase. Juvenile life history is not fully understood, and more studies that describe the life history and habitat requirements of cod from the period of settlement to two years of age are needed.
Adult cod, with their characteristic whisker-like barbel on the chin, are the most familiar life stage—tugging fishing lines, filling fishermen's nets, and often ending up on a plate. The relationship between adult cod distribution and habitat is largely based on trawl data (both from research and commercial trawls), and the largest catches are made near the seafloor on rocky slopes, ledges, and hard bottom (cobble, gravel, and sand with broken shells). Cod migrate for a variety of reasons, such as to spawn and locate prey, and during these annual movements they inhabit a variety of other habitats, ranging from deep-water mud basins and boulder reefs to the upper water column. Studies show a preference for structured seafloor landscapes, but the use of habitats varies and a full understanding of the ecological function of particular seafloor habitats is not thoroughly described.
Extensive commercial exploitation has changed population characteristics of Atlantic cod. Not only are populations at or near historic lows (although there are promising data demonstrating that Gulf of Maine cod populations are increasing), but cod currently mature at 1.7 to 2.3 years compared to 5.4 to 6.3 years in 1959. Because larger fish produce more eggs, today's population of smaller spawning adults may produce fewer eggs (lower fecundity) than in the past. Compounding this situation, the selective removal of harvestable fish (i.e., fish of sufficient size to be captured by conventional fishing gear) also removes their genes from the population, resulting in depressed genetic diversity for cod in future generations.
Despite Atlantic cod being one of the most intensively studied fish species in Massachusetts waters, many questions remain about their life history and habitat requirements. Although overfishing has substantially depressed populations, the effect of habitat degradation warrants attention. Atlantic cod populations have supported fishing communities for more than 300 years, but the current population may not sustain this important local industry. While there are promising data indicating that management measures are helping Atlantic cod populations, a multifaceted approach that includes understanding the life history, habitat requirements, and ecology of fishes is required to manage fisheries and habitats.
Most of the cod life history and habitat characteristics information is summarized in:
Fahay, M.P., P.L. Berrien, D.L. Johnson, and W.W. Morse. 1999. Essential Fish Habitat Source Document: Atlantic cod, Gadus morhua, life history and habitat characteristics. NOAA Technical Memorandum NMFS-NE-124.
Able, K.W. and M.P. Fahay. 1998. The First Year in the Life of Estuarine Fishes in the Middle Atlantic Bight. Rutgers University Press, New Brunswick, NJ.
Bigelow, H.B and W.C. Schroeder. 1953. Fishes of the Gulf of Maine. First Ed., Fishery Bulletin of the Fish and Wildlife Service. Washington, DC.
Clark, S.H., editor. 1998. Status of Fishery Resources off the Northeastern United States for 1998. NOAA Technical Memorandum NMFS-NE-115. Woods Hole, MA.
Collette, B.B. and G. Klein-MacPhee, Eds. 2002. Bigelow and Schroeder's Fishes of the Gulf of Maine, Third Ed. Smithsonian Institution Press, Washington, DC.
Colvocoresses, J.A. and J.A. Musick. 1984. Species associations and community composition of middle Atlantic bight continental shelf demersal fishes. Fishery Bulletin 82(2):295-313.
Kurlansky, M. 1998. Cod: The Biography of the Fish That Changed the World. Viking Penguin, New York, NY.
Lindholm, J.B., P.J. Auster, and L.S. Kaufman. 1999. Habitat-mediated survivorship of juvenile (0-year) Atlantic cod Gadus morhua. Marine Ecology Progress Series 180:247-255.
Lough, R.G., P.C. Valentine, D.C. Potter, P.J. Auditore, G.R. Bolz, J.D. Neilson, and R.I. Perry. 1989. Ecology and distribution of juvenile cod and haddock in relation to sediment type and bottom currents on eastern Georges Bank. Marine Ecology Progress Series 56:1-12.