Atlantic Cod: A Bio-Ecology
Introduction
Atlantic cod (Gadus morhua) has been called fish for centuries in the many languages of fishing cultures across the North Atlantic. There was no need to say more, it was just fish. Cod was fish, and fish was cod. In Norway, Scotland, the Faroes, Iceland and Newfoundland, if fish was for dinner there was no doubt that it was cod. It could be nothing else. As with deities, it was considered inappropriate to refer directly to something held in such esteem. And rightly so, because for coastal communities around the North Atlantic, life or death depended on the cod. If fish failed to appear during their annual migrations, starvation, or severe economic depression was a likely outcome. All other species were referred to with common names, without reverence. In New England, despite the rapid advancement of industry and commerce that soon subsumed the cod fishery in the nineteenth century, the 'sacred cod' still hangs in the Massachusetts State House, bearing tribute since 1798 to a fish that gave life, and some say freedom, to a fledgling nation (Figure I.1).
Figure I.1 The 'Sacred cod', in the State House of the Commonwealth of Massachusetts. On 17th March 1784, Mr John Rowe of Boston arose from his seat in the Hall of Representatives at the Old State House, and offered the following motion: 'That leave might be given to hang up the representation of a cod fish in the room where the House sit[s], as a memorial of the importance of the Cod-Fishery to the welfare of the Commonwealth.'. A symbolic cod was placed in the hall, and was later moved to the new State House building in 1798. A wooden 'Sacred cod' has remained there ever since - the current carving is the third since 1784. From Celebrate Boston and Ecology and Evolutionary Ethology of Fishes websites.
The association of cod with the North Atlantic is more than a human construction. Gadoids, the classification group that includes the Atlantic cod, are one of the few families of fishes endemic to the North Atlantic, many others having made their way to these waters from the Pacific or southern waters (Chapter 1). Gadoids began, as have other key groups, as rather small and inconspicuous species, but once gaining a foothold in the expanding waters of the North Atlantic, over the past five million years have evolved to become dominant components of these ecosystems. The gadoid family came to occupy most all of the continental shelf regions of the North Atlantic, and even expanded to the North Pacific (see Chapter 1). The Atlantic cod became the most dominant of all. The main stocks - this is a human construction - are given in Table I.1, along with their commonly used names, with their geographic range shown in Figure I.2.
Table I.1 Atlantic cod stocks, their management, statistical areas, geographic areas, and commonly used names (most stocks are referred to by their geographic location).
Management Statistical area(s) Name of area Common names ICES NAFO 1 inshore West Greenland inshore ICES NAFO 1 A-E offshore West Greenland offshore ICES ICES 14b and NAFO 1F East and South Greenland ICES ICES 1 and 2 Barents Sea-Norway Northeast Arctic cod, Arcto-Norwegian cod, Barents Sea cod ICES ICES 1, 2, 4a Norwegian coastal Norwegian Coastal cod ICES ICES 3a Kattegat ICES ICES 3b, c Western Baltic ICES ICES 3d Eastern Baltic ICES ICES 4, 7d, 3a North Sea, Eastern English Channel, Skagerrak ICES ICES 5a Iceland Icelandic cod ICES ICES 5b Faroe Plateau Faroe Island cod ICES ICES 6a West Scotland ICES ICES 6b Rockall ICES ICES 7a Irish Sea ICES ICES 7e-k Eastern English Channel and Southern Celtic Sea NAFO NAFO 3M Flemish Cap NAFO NAFO 3NO Southern Grand Bank Grand Bank cod Canada NAFO 2JH Labrador Canada NAFO 2J3KL Northeast Newfoundland-Labrador Northern cod Canada NAFO 3Ps Southern Newfoundland 3Ps cod Canada NAFO 4RS (3Pn-4RS) Northern Gulf of St. Lawrence Canada NAFO 4T (4TvN) Southern Gulf of St. Lawrence Canada NAFO 4Vs Northern Scotian Shelf Canada NAFO 4W Southern Scotian Shelf Canada NAFO 4X Bay of Fundy USA NAFO 5Y Gulf of Maine USA-Canada NAFO 5Z New England offshore-Georges Bank Georges Bank cod
Figure I.2 The statistical regions assigned to the Atlantic cod stocks (red are the NAFO zones and blue those of ICES). The 500?m bathymetric contour which encompasses most but not all of the cod range appears as grey line.
A highly adaptable physiology has enabled cod to inhabit a wide variety of habitats from coastal bays to large offshore banks across its range (Chapter 2). Cod exist over a wide range of sea temperatures, salinities, and feeding opportunities, with large and small populations adapting to local conditions. There is literally a cod for every continental shelf habitat.
Cod are highly fecund, as are most gadoids. Large females can produce tens of millions of very small eggs, and being a broadcast spawner, offer no parental care (Chapter 3). Cod rely on numbers and egg release in the right place at the right time. Thousands of years of behavioural conditioning and selection have resulted in spawning locations that result in sustainable if imperfect and variable survival. Only a tiny fraction of released eggs will ever grow to adulthood, most dying, or being consumed by predators. The act of spawning is far from a mundane occupation. The cod is above all a social species, with complex behaviours occurring both before and during courtship and spawning, involving soundings, spatially-specific and pelagic behaviours whose functions are only partially understood.
Cod begin life as a drifting egg, completely at the mercy of the prevailing near-surface currents (Chapter 4). If their release timing and location leads to drift to waters with favourable environmental and feeding conditions after their on-board food supply (the yolk sac) is depleted, there is a small chance they may survive through the larval stage and settle to near bottom in a liveable location. If not, they cannot survive. It is perilous journey - many factors must work in their favour for survival. Predicting survival of young cod to adulthood or to a fishery, one of the holy grails of cod science, has proven to be an elusive goal.
The biggest cod stocks, those in the Barents Sea (Northeast Arctic cod), Icelandic waters and off the Northeast coast of Newfoundland and Labrador (Northern cod), are all highly migratory. Other stocks are less so and many are sedentary (Chapter 5). Nonetheless, most stocks move to some extent over the North Atlantic seasons, even if within small coastal regions. Many groups exhibit what can be very exact homing behaviour between spawning and feeding areas, repeating patterns that largely determine stock structures. Fisheries have been dependent on this regularity for hundreds of years (Chapter 7).
Cod can and will consume most anything (Chapter 6). No doubt, their success as a species partly depends on being a 'generalist' predator - feeding on a wide range of prey across their range - and often focussing on prey that are both abundant and available. Nonetheless, they appear to have developed a preference for and dependence on certain types of prey with high energy content. The strongest association of cod with a singular prey occurs in the most northerly groups, the Northeast Arctic, Icelandic and Northern cod off Newfoundland and Labrador, that depend heavily on capelin (Mallotus villosus) and appear to have developed a preference for this prey. Cod appear to know what is good for them, and will pursue...
