Striped bass (Morone saxatilis) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

Of the various habitats used by striped bass during its life cycle, the most important to the maintenance of a population seems to be its spawning, incubation and rearing habitat (Jessop 1990, 1991; Melvin 1991; Dudley and Black 1978; Van den Avyle and Maynard 1994).

Egg survival to hatching is closely tied to the physicochemical properties of the incubation habitat, particularly temperature, dissolved oxygen and the presence of a moderate current (Cooper and Polgar 1981). The duration of incubation is a function of temperature. The highest hatching rates (87%) and larval survival rates in the first 24 hours (76%) are obtained at 18°C (Morgan et al. 1981). At that temperature, the eggs hatch approximately 48 hours after fertilization (Pearson 1938; Raney 1952). Egg survival rapidly declines when the water temperature exceeds 23°C and gradually declines as water temperature drops below 17°C, with few eggs surviving at temperatures below 12°C(Morgan and Rasin 1973; Rogers et al. 1977).

The two other factors, i.e., sufficient dissolved oxygen levels and the presence of a current, may have a combined effect on egg survival. The eggs are generally heavier than water and, in the absence of current, sink to the bottom, where they are more exposed to anoxia (Chittenden 1971; Rawstron et al. 1989). The presence of a moderate current creates low turbulence levels, which keep the eggs in suspension in the water column during incubation.

The survival of the larvae, like the eggs, also depends on physical variables, including temperature and dissolved oxygen. However, an additional requirement, i.e., a sufficiently abundant food supply, comes into play upon resorption of the yolk sac and the onset of feeding (Cooper and Polgar 1981). This key period occurs on about the eighth day of existence of the larvae, when they measure 6 to 7 mm. In natural environments, the rate of survival of larvae that have exhausted their yolk reserves is directly related to the abundance of zooplankton in their environment (Kernehan et al. 1981).

In most striped bass populations, spawning, incubation and early larval development occur in fresh or slightly brackish waters. However, the Shubenacadie River population spawns in a section of the river affected by a tidal bore: the initial stages of development seem to be adapted to these conditions and tolerate greater temperature and salinity variations than U.S. striped bass populations (Cook 2003).

Immature and adult striped bass frequent coastal and estuarine habitats (Bain and Bain 1982). During their first two years, they feed primarily on invertebrates, but gradually become piscivorous. In summer, striped bass movements seem to be associated primarily with those of their preferred prey.

Canadian striped bass populations migrate upstream in the fall and overwinter in fresh or brackish water. The reason for this behaviour appears to be to avoid the low ocean temperatures in winter (see “Biology, Movements/Dispersal”). Southern Gulf striped bass enter a number of estuaries along the New Brunswick coast in the fall. Telemetric tracking of striped bass overwintering under the ice in the Kouchibouguac and Kouchibouguacis rivers has revealed that they moved about little (less than one kilometre) and selected areas where the temperature remained above –0.4°C and where salinity did not exceed 15 ppm (Bradford et al. 1997a).

The confinement of striped bass to overwintering sites could increase the risk of mortality due to environmental conditions or illegal fishing. St. Lawrence striped bass, for example, were the target of a major ice fishery on Lake Saint-Pierre. This fishery was prohibited in 1951, but some fishers reportedly continued to fish illegally (A. Michaud, pers. comm.; Robitaille 2001).

Trends

In recent decades, habitat changes in three rivers may have limited striped bass abundance and contributed to the decline in its populations.

Striped bass spawning grounds in the Saint John and Annapolis rivers appear to have been affected by changes in water quality or physicochemistry, related perhaps to changes in flow due to the construction of impoundments (see the section “Limiting Factors and Threats”). On both rivers, the cessation of spawning has been attributed to inadequate physicochemical conditions, namely high levels of PCBs or DDTs on the Saint John River, and agricultural pollution or overly low pH on the Annapolis River. However, changes in the circulation of water masses due to impoundments may have affected spawning and rearing habitat quality (Douglas et al. 2003).

The disappearance of the St. Lawrence Estuary striped bass population also seems to be associated with habitat changes. In this case, however, the spawning, incubation and rearing habitat does not appear to have been affected, but rather the summer rearing habitat of immature fish, located off several islands in the St. Lawrence. These areas appear to have been modified by the dumping of dredged material (Robitaille 2001). As a result of the habitat changes, striped bass became concentrated at several locations along the south shore which quickly become very popular fishing sites. Biologists tried unsuccessfully to persuade fishers to limit their catches and fishing continued without any real restrictions until 1968, the year of the last catches (Robitaille and Girard 2002).

Protection/ownership

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