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COSEWIC assessment and update status report on the Atlantic Cod in Canada

Physiology

From a physiological perspective, the environmental variable of greatest import to Atlantic cod is probably water temperature.  It has been suggested that cod will actively avoid waters deemed to be low in temperature.  For example, avoidance of cold water is the primary reason given for the autumnal migration of cod out of the Southern Gulf of St. Lawrence to the northeast waters off Cape Breton (Campana et al. 1999).  And there is a sound empirical basis for believing that temperature selection by cod should be density-dependent, with the optimal temperature for growth declining as food ration declines (Swain and Kramer 1995).

Although cod are generally found in waters ranging in average annual temperature from 2 to 11o C (Brander 1994), it is clear that cod in some areas off Newfoundland are able to withstand temperatures as low as -1.5o C (Goddard et al. 1999).  This temperature is below that (-0.5 to - 0.8o C) at which ice crystals form in the blood.  Cod are able to withstand such cold waters, and to prevent the formation of ice crystals in the blood, by producing plasma antifreeze proteins or glycoproteins  (AFGPs) that improve freeze resistance.  Interestingly, there appears to be an effect of size and/or age on the ability of cod to withstand sub-zero-degree waters.  For example, Goddard and Fletcher (1994) reported that juvenile cod (10-40 cm) produce approximately twice as much AFGP as adult cod.

Some of the best evidence that Atlantic cod are adapted to local environments at scales considerably smaller than those corresponding to the NAFO divisions is physiological in nature.  In a common-garden experiment in which all individuals were reared under the same environmental conditions, Goddard et al. (1999) reported that juvenile cod from the northernmost part of Division 3K (Northern Peninsula, Newfoundland) develop antifreeze protein levels approximately 50% higher than cod located further south in Notre Dame, Trinity, and Conception Bays.  The authors attributed these physiological differences in antifreeze production to population differences in water temperatures experienced during winter.

 

Movements/dispersal

Dispersal in Atlantic cod appears to be limited to the egg and larval phases of life, during which surface and near-surface water currents and turbulence are the primary determinants of horizontal and vertical displacement in the water column.  For some cod populations, eggs and larvae are capable of dispersing very long distances.  For example, based on the movement of satellite-tracked drifter buoys, Helbig et al. (1992) concluded that eggs spawned off southeastern Labrador (NAFO Division 2J) disperse as far south as Grand Bank.  By contrast, eggs spawned by cod in inshore, coastal waters, especially at the heads of large bays, may experience dispersal distances of a few kilometres or less.

Long-term movements by cod take the form of seasonal migrations.  These migrations can be attributed to geographical and seasonal differences in water temperature, food supply, and possibly spawning grounds.  At one extreme, some inshore populations are suspected to have extremely short migrations, possibly limited to tens of kilometers, or less, in distance.  By contrast, cod in other populations are known to traverse hundreds of kilometers during their seasonal migrations.

Good examples of long-distance seasonal migrations are those undertaken by cod in the Southern Gulf of St. Lawrence and on the Northeast Newfoundland Shelf.  The former overwinter off northeast Cape Breton, migrating into the Southern Gulf in April, where they spend the summer months feeding and spawning, before returning to the deep, relatively warm waters off Cape Breton in November.  Many Northeast Newfoundland Shelf cod migrate from the relatively warm offshore waters to inshore coastal waters in spring to feed primarily on capelin (Mallotus villosus) before returning offshore in autumn.

Movements by Atlantic cod can be inferred from mark-recapture experiments, genetic analyses, and otolith micro-chemistry.  Between 1954 and 1993, a total of 205,422 cod were tagged in Newfoundland waters and released; 36,344 of these fish were recaptured by fishers (Taggart et al. 1995).  Although exceedingly rare (5 of 36,344 recaptures), some cod tagged in Newfoundland waters have been recaptured in the Northeast Atlantic, although no such recaptures have been reported since the 1960s (Taggart et al. 1995).  Based on this exhaustive set of tagging studies, coupled with those conducted more recently (Hunt et al. 1999; Brattey et al. 2001b), one can conclude that, with one exception, cod tend to be recaptured in the NAFO Management Area (as defined by the divisions given in Figure 4) in which they were initially tagged.  The one area in which movement appears to be relatively extensive is that encompassing NAFO Divisions 3Ps, 3N, 3O, and 3L along the southeastern coast of Newfoundland and including Grand Bank.  However, notwithstanding the rather extensive movements that relatively few cod undertake, genetic and otolith micro-chemical analyses are consistent with the hypothesis that these cod exist as separate populations in the Northwest Atlantic (Bentzen et al. 1996; Ruzzante et al. 1998; Campana et al. 1999; Beacham et al. 2002).

 

Nutrition and interspecific interactions

Atlantic cod have an extraordinarily catholic diet (Scott and Scott 1988).  As larvae, they feed primarily on zooplankton (copepods and amphipods).  As cod grow, they tend to feed on larger and larger prey.  Immediately after the larval stage, small crustaceans, mysid shrimp, and euphausiids feature prominently in the cod diet.  Once their gape is large enough, cod begin feeding on fish, including other cod (Scott and Scott 1988; Bogstad et al. 1994).  Fish that have been recorded in cod stomachs have included the following:  capelin, sand lance (Ammodytes americanus), herring (Clupea harengus), redfish (Sebastes sp.), Arctic cod (Boreogadus saidus), cunners (Tautogolabrus adspersus), alewives (Alosa pseudoharengus), haddock (Melanogrammus aeglefinus), winter flounder (Pseudopleuronectes americanus), mackerel (Scomber scombrus), shannies (Lumpenus maculatus, Stichaeus punctatus and Ulvaria subbifurcata), silversides (Menidia menidia), and sculpins (Cottus sp.).  In addition to fish, adult cod will also consume squid, mussels, clams, whelks, tunicates, comb jellies, brittle stars, sand dollars, sea cucumbers, and polychaetes.

Although studies are few, it is clear, given the wide variety of prey consumed by cod, that to varying degrees cod compete with other species for their food. 

There is no firm evidence that food availability is a limiting factor affecting the recovery of this species in Canadian waters, particularly given the historically low levels of abundance at which the species exists throughout much of its range.

However, given the considerable uncertainty that exists in contemporary estimates of the abundance of capelin off Newfoundland and Labrador (DFO 2000, 2001), it is difficult to assess the degree to which capelin may or may not be limiting the recovery of cod in the Newfoundland & Labrador population.  Nonetheless, it has been hypothesized that one of the primary sources of food for adult cod in the Newfoundland & Labrador population--capelin--may be limiting in northern areas (Rose and O'Driscoll 2002), thus affecting recovery.


Figure 4.  Map showing the NAFO (Northwest Atlantic Fishery Organization) divisions used to identify stocks of Atlantic cod managed by NAFO and the Canadian Department of Fisheries and Oceans

Figure 4.  Map showing the NAFO (Northwest Atlantic Fishery Organization) divisions used to identify stocks of Atlantic cod managed by NAFO and the Canadian Department of Fisheries and Oceans.

 

Behaviour/adaptability

Atlantic cod are generalists.  Given that cod almost certainly exist as more than one evolutionarily significant unit in Canada, reflected to some degree by the populations proposed here, it would be reasonable to predict that cod populations respond differently to anthropogenic influences, the most obvious (and best-studied) being fishing.  Such differential population responses may be reflected by differential responses to population collapse and fishery closures.  For example, despite their close proximity, and reasonably higher interchange of individuals (Brattey et al. 2001a,b), the St. Pierre Bank population in the Gulf/Maritimes population recovered relatively rapidly while the adjacent Newfoundland and Labrador population has shown no signs of recovery at all (see POPULATION SIZES AND TRENDS below).

Of potential relevance to the reproductive success of Atlantic cod are the potentially negative effects that offshore gas and oil exploration may have on spawning behaviour and recruitment.  Much of the present oil production and gas exploration activities occur on Grand Bank and the Scotian Shelf.  Recent studies (identified above) suggest that auditory communication among cod during spawning may be of considerable importance to mate choice and reproductive success in this species.  If so, then anthropogenic activities, ranging from fishing activities to exploratory drilling, that disrupt the aquatic medium in which sound so effectively travels may deleteriously affect reproductive behaviour and fertilization success in Atlantic cod.  Although such effects, if present, may not be detectable during periods of high cod abundance, they may contribute to an Allee Effect when populations are low.  However, until these hypotheses have been tested empirically, their merit cannot be evaluated.