Striped bass (Morone saxatilis) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

In the United States, where striped bass populations are more abundant, it is estimated that overfishing, pollution and spawning habitat alteration caused by changes in flow conditions can contribute, to varying degrees, to declines in abundance. It appears that Canadian bass populations are subject to the same threats. In addition, they could be exposed to additional limiting factors because they occur at the northern limit of the species’ range. For example, the fact that it overwinters in freshwater could pose risks because the bass are concentrated in small areas for several months, making them vulnerable to poaching and various other mortality factors.

Climatic constraints

Striped bass populations often undergo natural abundance fluctuations due to characteristics of the species’ dynamics: the production of large numbers of offspring by a given number of spawners is closely related to the occurrence of favourable climatic and environmental conditions (Ulanowicz and Polgar 1980; Rutherford and Houde 1995; Rutherford et al. 1997; Bulak et al. 1997), which do not occur every year. However, once they have reached maturity, striped bass may spawn several times, thereby offsetting the effect of variable recruitment (see the section “Survival”).

The first critical period for the production of a strong bass cohort seems to occur when the larvae exhaust their yolk reserves and begin to feed. At that time, they must find an abundance of zooplankton (Cooper and Polgar 1981). A second important condition, at least for more northern populations, is believed to be growth during the first summer. In the Gulf, it is estimated that juveniles that reach at least 100 mm in length by the end of their first growing season have a better chance of surviving the prolonged fast of their first winter under the ice than do smaller individuals (Bernier 1996; Bradford and Chaput 1998; Hurst and Conover 1998).

Fishery

Depending on its intensity, fishing can limit the number of individuals that reach maturity and, for those that reach maturity, it can reduce the probability of repeated participation in spawning (Williamson 1974; Jessop and Doubleday 1976; Hogans and Melvin 1984; Secor 2000), thus reducing the capacity of the population to mitigate the impact of irregular recruitment.

The effect of fishing on bass abundance had long been underestimated. For example, migratory populations from Chesapeake Bay were decimated for two entire decades (1970 and 1980). Many studies had been conducted to attempt to identify and address the causes of this decline, without success. Given that the decline in abundance continued and that certain coastal states were little inclined to limit their fishing, the U.S. federal government intervened and imposed a moratorium on all forms of harvesting. The rapid recovery of the populations that followed confirmed that the cause of the problem was the overfishing (Field 1997). It was observed that the rebuilding of these populations began with spawning by large bass, some of which were over 30 years old and were protected by the fishing regulations (Secor 2000). A similar situation may have occurred in southern Gulf bass. Only one population is known; it spawns in the Miramichi River. Outside the spawning season, fish from this population migrate along the coasts, where they can be caught in fishing gear. They overwinter in several rivers, where they are exposed to illegal harvesting (Douglas et al. 2003).

The low abundance of bass in the southern Gulf led to the closure of the commercial fishery in 1996, followed by the other fisheries. The number of spawners entering the Miramichi River is reported to have increased considerably when a first strong cohort, produced in 1998, reached maturity (Douglas et al. 2003).

However, there continues to be evidence of mortality related to the fishery: a significant bycatch of young-of-the year still occurs in the fall and winter rainbow smelt fisheries (Osmerus mordax) and in eel traps (Anguilla rostrata) (Bradford et al. 1995, 1997). A similar situation has been observed in the St. Lawrence, where large numbers of juvenile bass have perished in eel traps (Trépanier and Robitaille 1995; A. Michaud, pers. comm.)

High mortality may also have been caused by illegal fishing, although this is impossible to assess (S. Douglas, A. Michaud, pers. comm.; Trépanier and Robitaille 1995; Douglas et al. 2003). Anecdotal accounts of illegal ice fishing activities have been reported by residents of the east coast of New Brunswick. In some communities, bass is reportedly still offered for sale door-to-door (Douglas et al. 2003).

The same problem existed in the St. Lawrence Estuary. When abundance declined sharply in the mid-1950s, managers tightened the regulations to limit bass harvesting, prompting an outcry from fishers, many of whom openly defied the regulations (A. Michaud, pers. comm.). It was common to find bass under the legal size limit for sale in Quebec City. When the winter bass fishery in Lake Saint-Pierre was closed in 1951, several fishers reportedly continued to fish illegally (A. Michaud, pers. comm.).

Habitat changes

Changes to the aquatic environment caused by human activity can increase mortality within populations, particularly in the early life stages (egg, larva). It is estimated that some habitat changes have adversely affected egg and larval survival. Contaminants, such as polychlorinated biphenyls (PCBs), aromatic hydrocarbons, pesticides, heavy metals and several other chemicals, are reported to reduce egg and larval survival in the laboratory (Korn and Earnest 1974; Bonn et al.1976; Benville and Korn 1977; Durham 1980; Cooper and Polgar 1981; Hall 1991). However, their effect on recruitment has not been clearly demonstrated in the field.

For example, the section of the Miramichi River in which bass spawning and early development occur is exposed to pulp and paper mill effluent and municipal wastewater. However, there is no evidence that this has had an impact on bass reproduction, at least not in the recent past (Douglas et al. 2003).

One possibility that has been raised is that the cessation of reproduction in the Saint John and Annapolis rivers was due to water quality changes that affected egg and larval survival: non-point source agricultural pollution, pesticides or pHdepression (Douglas et al.2003). It has also been suggested that the construction of the Royal Annapolis dam in 1960 and the Mactaquac dam in 1967 on these two rivers may have modified spawning, incubation and rearing habitat. However, there is no consensus on how these changes have affected striped bass reproduction (Dadswell 1976; Jessop and Doubleday 1976; Williams 1978; Jessop and Vithayasai 1979; Jessop 1980; Parker and Doe 1981; Dadswell et al. 1984; Williams et al. 1984; Douglas et al. 2003). Striped bass egg sampling conducted in 1975 on the Saint John River showed that 96% of the eggs captured had ruptured membranes, a phenomenon that may be due to the presence of contaminants or to a sudden change in osmotic conditions. Similarly, on the Annapolis River, spawning occurred several times in the 1980s, but none of the eggs survived in the river. However, when they were transferred to fish farms, they developed normally and produced juveniles (Jessop 1991).

The effects of flow modifications in spawning grounds has been observed in the United States. The striped bass population of the Savannah River, between South Carolina and Georgia, spawned from February to May in the channels of streams located 16 to 50 km from the sea (Van den Avyle and Maynard 1994). The dredging of a navigation channel to serve the industrial port of Savannah and the construction of a tide gate between an island and the coast resulted in displacing the salt wedge upstream and moving bass spawning grounds closer together (Van den Avyle and Maynard 1994). Since those changes, bass eggs are no longer retained in the freshwater sector, but rather transported rapidly to saltwater, which causes their mortality (Winger and Lasier 1994).

It has been suggested that similar modifications affecting incubation or larval rearing habitat may have caused the cessation of bass spawning in the St. Lawrence Estuary (Robitaille and Ouellette 1991). However, the recently rediscovered collection of specimens collected by biologists up until 1962 shows that young-of-the-year were produced in the St. Lawrence as long as spawning striped bass were present there. The analysis of mark-recapture data shows that the disappearance of this population is the result of the reduction in its range due to habitat encroachment, i.e., the sites where striped bass congregated quickly became very popular fishing sites (Robitaille 2001).

The population remained small for 12 years until catches ended completely in 1968 (Robitaille and Girard 2002).

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