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

Habitat

Habitat requirements

Knowledge of the habitat requirements of Atlantic cod is rather poor.  Despite the paucity of data, it is reasonable to predict that habitat requirements change significantly with age in this species.  With the exception of the few cod that have been observed in situ, the following information is based on the sampling of cod at various life stages from different depths and different areas of the ocean.

During the first few weeks of life, cod exist as eggs, and then as larvae, in the upper 10 to 50 metres of the ocean.  The primary factors affecting habitat suitability for cod during these early stages of life are probably food availability and temperature (the lower the temperature, the longer the development time, and the longer the period of time during which cod are at sizes that make them highly vulnerable to predation).

The most critical habitat characteristics for Atlantic cod may be those required during the juvenile stage when cod have settled to the bottom for the first 1 to 4 years of their lives.  Several studies suggest that a heterogeneous habitat, notably in the form of vertical structures, such as eelgrass, Zostera marina, in nearshore waters, is favoured by juvenile cod (e.g., Gotceitas et al. 1995, 1997; Tupper and Boutilier 1995; Gregory and Anderson 1997).  Based on observational studies (Tupper and Boutilier 1995) and on experimental manipulations (Gotceitas et al. 1995; Linehan et al. 2001), physically heterogeneous habitat appears advantageous to juvenile cod because it reduces the risk of predation and may also allow for increased growth.

Offshore, it is logical to assume that physical structure would also reduce predator-induced mortality of juvenile cod.  For example, video recorded on a submersible during an August 2001 survey of deep, continental-slope waters off southwestern Nova Scotia revealed juvenile cod amongst the extensive growths of deep-sea corals (personal communication, Anna Metaxas, Department of Oceanography, Dalhousie University, Halifax, Nova Scotia).

As cod grow older, it appears as though their habitat requirements become increasingly diverse.  Indeed, it is not clear that older cod have particular depth or bottom-substrate requirements.  The primary factors affecting the distribution and habitat of older cod are probably temperature and food supply.  In a general sense, it appears that cod tend to avoid cold temperatures.

But what is cold for cod in one area is evidently not cold for cod in other areas.  For example, it is widely believed that cod migrate out of the southern Gulf of St. Lawrence in autumn to avoid the cold water temperatures in the Gulf during winter (Campana et al. 1999).  However, cod off eastern Newfoundland, notably those that overwinter in inshore waters, exist at temperatures below 0oC (Goddard et al. 1999).  Perhaps the most reasonable explanation for these apparent differences in water temperature tolerance is that cod in different areas are adapted to their local environments.  This conclusion is supported by the finding that cod in different areas of coastal Newfoundland possess different levels of antifreeze protein (see Physiology section below), a physiological adaptation that would influence the tolerance of cod to low water temperatures.

From a spawning perspective, it is not known if cod have specific habitat requirements.  Cod spawn in waters ranging from tens (Smedbol and Wroblewski 1997) to hundreds of metres in depth (Hutchings et al. 1993).  Atlantic cod in Canadian waters are known to spawn extensively throughout the inshore, nearshore, and offshore waters (McKenzie 1940; Scott and Scott 1988; Hutchings et al. 1993; Morgan and Trippel 1996), a conclusion also supported by fishers (Neis et al. 1999).  Although cod spawning appears to be associated with the bottom (Morgan and Trippel 1996; Hutchings et al. 1999), this may have more to do with the cod mating system (a lek mating system has been hypothesized; Hutchings et al. 1999; Nordeide and Folstad 2000) rather than any physical requirements for the offspring, given that cod neither build egg nests nor provide parental care.  Perhaps the factor most beneficial to the survival of their offspring is the presence of physical oceanographic features (e.g., water currents) that would serve to entrain the buoyant eggs and prevent them from being dispersed to waters poorly suited to larval cod, e.g., waters off the continental shelf.  It is highly unlikely that spawning habitat is limiting for Atlantic cod.

Thus, the habitat most likely to be critical and potentially limiting for Atlantic cod may well be the vertical, ‘three-dimensional’ structures provided by plants, rocks, physical relief, and corals.  In addition to providing protection from predators, such physical heterogeneity would almost certainly provide habitat for small fish and invertebrates, organisms upon which juvenile cod could feed.

Trends

If physical structure is critically important to the survival of juvenile cod, notably in the form of plants, bottom physical relief, and corals, there may be less habitat available today than decades ago in some parts of the range of this species.  Any reduction in physical heterogeneity on the bottom since the 1960s can be attributed to the increased use of bottom-trawling gear to catch groundfish such as cod, haddock, pollock, and several species of flatfish.  Repeated trawling in a given area tends to ‘smooth’ and flatten the bottom, reducing vertical and physical heterogeneity (Collie et al. 1997, 2000; Kaiser and de Groot 2000).  The destruction of deep-dwelling corals off Nova Scotia--first reported and well-documented by fishermen--is another product of bottom-trawling and, to a considerably lesser degree, long-lining (Mortensen et al. submitted). Although physically heterogeneous areas frequented by juvenile cod may not have been heavily trawled where gear damage or loss was likely, no studies have been undertaken to evaluate the effects of trawling on the quantity and quality of juvenile cod habitat.