Striped bass (Morone saxatilis) COSEWIC assessment and status report: chapter 6

Biology

An anadromous species, the striped bass moves between freshwater spawning habitats and brackish or salt water feeding sites in estuaries or marine coastal waters to complete its life cycle.

Spawning

Striped bass spawn in the spring in fresh or slightly brackish water (Raney 1952). Spawning is triggered by the increase in water temperatures. The reproductive behaviour of striped bass has been described in U.S. populations (Pearson 1938; Merriman 1941; Raney 1952; Karas 1974; Setzler et al. 1980). The males are the first to reach the spawning grounds in the rivers. The females, which are generally larger and less numerous than the males, follow. Spawning begins when the water temperature rises above 10°C. When there are large numbers of spawners, spawning can last three to four weeks, with several distinct peaks if the water temperature fluctuates. Spawning occurs near the surface, at twilight. Groups of males surround each female and fertilize the eggs released (McLaren et al. 1981).

In rivers in the Maritimes, the spawning grounds are located in fresh water, sometimes in areas subject to tidal influence. They are further from the ocean in the three Bay of Fundy tributaries than in the Miramichi River in the southern Gulf. On the Shubenacadie River, a tidal bore travels upstream as far as the spawning sites (Rulifson and Tull 1999). The striped bass spawning ground in the St. Lawrence Estuary has never been located, but various sources suggest that it is in Lake Saint-Pierre or downstream from it, in the adjacent section of the upper estuary (Montpetit 1897; Vladykov and Brousseau 1957; Beaulieu 1962; Cuerrier 1962; Magnin and Beaulieu 1967; Robitaille 2001).

Incubation and rearing

The translucent green eggs measure 1.3 mm when they are released and fertilized. They swell and harden in the water in approximately 12 hours, reaching 3.4 to 3.8 mm in diameter (Pearson 1938). The eggs are semi-pelagic and require a moderate current to remain suspended in the water column throughout the entire incubation period.

On hatching, the yolk-sac larvae measure between 2.0 and 3.7 mm. The larval stage can last from 35 to 50 days, depending on the water temperature and food abundance. The feeding of the larvae changes as they grow (Humphries and Cumming 1973). They initially feed on the nauplii stages of zooplanktonic crustaceans; once they exceed 10 mm, they gradually begin feeding on larger zooplanktonic species or stages (Robichaud-LeBlanc et al. 1997).

Depending on the state of the tide, vertical migration of larvae in the water column enables them to maintain their position in the estuary and even to move downstream (Setzler-Hamilton et al. 1981).

Growth, maturation, fecundity

The larval stage ends with metamorphosis, at approximately 20 mm, at which point it takes its adult form (Mansueti 1958). Juvenile striped bass are more tolerant to changes in temperature or salinity than the eggs or larvae. Large numbers of young-of-the-year are found in the sheltered bays of estuaries, where they feed primarily on small invertebrates during their first year (Robichaud-LeBlanc et al. 1997). They gradually begin feeding on fish at about two-years-of-age (Rulifson and McKenna 1987).

The duration of the striped bass growing season has a significant effect on the size and weight reached at a given age. Canadian striped bass are smaller than U.S. bass of the same age. Size and age at maturity also vary as a function of latitude (Merriman 1941; Raney 1952; Austin 1980; Setzler et al. 1980; Bain and Bain 1982). Striped bass generally do not reach sexual maturity until three-years-of-age in males and four-, five- or even six-years-of-age in females (Berlinsky et al. 1995; Douglas et al. 2003; Powles 2003). The adults survive spawning and can spawn again, sometimes with a year of rest between successive contributions. In some rivers, fish have spawned up to their 14th year (Setzler et al. 1980). Gonad maturation in the weeks preceding spawning is dependent on the increase in water temperature (Setzler et al. 1980).

The striped bass is a highly fecund fish. In populations on the U.S. coast, estimates of the number of developing eggs range from 14,000 for a 1.4 kg female to over 3 million for a 22.7 kg female (Raney 1952). Striped bass from northern populations generally have a slightly lower fecundity, at a given size, than those in the southern part of its range (Olsen and Rulifson 1992). In Canadian populations, the fecundity of 4- to 11- year-old females (45 to 91 cm) ranges from 53,000 to 1,464,000 eggs (Paramore 1998).

Survival

Most striped bass populations undergo fluctuations in abundance, a characteristic of species in which the survival of the early life stages, i.e., eggs and larvae, is a key factor in recruitment (May 1974; Dahlberg 1979). Striped bass are highly fecund and can spawn several times during their lives. However, the combination of conditions that results in optimum egg and larvae survival does not occur every year in most systems. Survival to the larval stage seems to be a key factor in year-class strength (Cooper and Polgar 1981; Rago et al. 1989), and thus in adult abundance several years later (Goodyear 1985; Rago et al. 1989; Ulanowicz and Polgar 1980). In the structure of the adult population, year classes that were produced in years in which conditions were favourable for spawning often dominate (Cooper and Polgar 1981; Goodyear 1985; Douglas et al. 2001).

The start of feeding, at the end of the yolk-sac stage, appears to be a determining stage. This key period is believed to occur around the eighth day after hatching, when the larvae measure 6 to 7 mm. In natural environments, the survival rate of larvae that exhaust their yolk reserves depends on the abundance of zooplankton in the environment (Kernehan et al. 1981). The physical condition of the larvae is correlated with the density of copepods and water fleas in the environment (Miller 1977; Martin et al. 1985). Starting at the juvenile stage, striped bass are better able to tolerate changes in environmental conditions. Moreover, they can move to estuarine or coastal habitats, often in schools of same-size fish, to meet their food requirements.

A particular characteristic of the northernmost populations is that climatic conditions seem to cause selective mortality in young-of-the-year. Individuals that have not reached a total length of 100 mm by the fall apparently do not survive their first winter of the prolonged fasting under the ice as well as larger individuals (Bernier 1996; Bradford and Chaput 1997; Hurst and Conover 1998).

Striped bass is long-lived (up to 30 years) (Secor 2000). The largest known specimen, captured in North Carolina in 1891, weighed 56.8 kg and measured 1.82 m (Raney 1952). The growing conditions in Canadian waters are such that the maximum size of striped bass is less than 1 m. This figure is an estimate because very few individuals survive long enough to reach their maximum size. The largest striped bass caught in the St. Lawrence Estuary measured 91.5 cm (total length) and weighed 10.9 kg (Vladykov 1953).

Physiology

Movements/dispersal

The striped bass undertakes migrations of varying distances, depending on its development, feeding, spawning and overwintering.

In the St. Lawrence Estuary, the downstream migration of young-of-the-year occurs over several weeks. Catches of juvenile striped bass measuring 20 to 35 mm have been reported near Neuville in early July (Vladykov and Brousseau 1957). In early September, individuals measuring 75 mm could be caught at the mouth of the Ouelle and Saint-Jean-Port-Joli rivers (J. Brousseau, retired biologist, 302 des Pins, Loretteville, Quebec, G2A 2L3, pers. comm.). Young-of-the-year have also been reported in large numbers in fixed gear on the Beaupré coast, north of Île d’Orléans (Vladykov 1945; Trépanier and Robitaille 1995; V. D. Vladykov, unpublished manuscript).

In the southern Gulf, first year growing habitats (40 to 200 mm) are found in estuaries and lagoons along the coast (Rulifson and Dadswell 1995; Douglas 2003). The presence of young-of-the-year striped bass has been reported in eel, smelt and tomcod fishing gear set in the estuaries of several rivers (Douglas et al. 2003).

With respect to the Bay of Fundy group, the growing habitats of juvenile bass are known primarily for the population of the Shubenacadie-Stewiacke system; young-of-the-year are caught in the lower reaches of the river in the early summer, and later on the north shore of Cobequid Bay in August and September (Rulifson et al. 1987; Douglas et al. 2003). Little data are available on the development and movements of young striped bass in the Annapolis River (Williams et al. 1984; Stokesbury 1987).

In summer, Bay of Fundy striped bass populations mix with migratory populations from the U.S. Several large populations from the U.S. eastern seaboard undertake long coastal migrations between the Bay of Fundy and northern Florida. In early spring, some individuals originating in Chesapeake Bay tributaries leave the bay and move northward along the coast to the Bay of Fundy (Melvin 1978, 1991; Waldman et al. 1990). These migratory bass are believed to congregate in the eastern part of the Bay of Fundy, along the coast of Nova Scotia, for the first half of the summer, and then in the western part for the latter half (Dadswell et al. 1984). Some enter the Saint John River estuary and migrate to the Reversing Falls area, where they remain until the end of summer (Dadswell 1976). In September, these migrants return south. Some individuals tagged in the Saint John River have been recaptured along the states of Massachusetts, New Jersey, New York, Delaware and Maryland (Melvin 1978). There is no indication that these individuals can breed with striped bass from Canadian rivers.

Southern Gulf of St. Lawrence striped bass populations also move along the coasts in summer (Bradford and Chaput 1996; Douglas et al. 2003), but they migrate over shorter distances than bass from the U.S. Movements of some fish beginning in the spring from the Kouchibouguac River to Miramichi Bay, 50 km to the north, or to Tabusintac, 125 km, have been reported (Hogans and Melvin 1984). At the end of summer, they enter Nepisiguit Bay, in southern Chaleur Bay, where fishing, when permitted, was at its best in September. These individuals are part of the population that spawns in the Miramichi River (Douglas et al. 2003).

Movements related to feeding have been observed in several rivers. Prior to the construction of the Mactaquac Dam on the Saint John River, immature bass that appeared to be foraging were reported to migrate upstream to fresh water, 320 km from the mouth (Jessop 1991). On the Miramichi River, bass catches in a Millbank index trapnet show three peaks in abundance. The largest peak is observed in the spring (May-June) and corresponds to spawning; a second peak in summer and a third in fall are believed to correspond to movements for feeding (Chaput and Randall 1990).

Movements of bass from the St. Lawrence have been described on the basis of recaptures of tagged fish (Beaulieu 1962; Magnin and Beaulieu 1967; Robitaille 2001). In the fall, spawners migrated up the river to Lake Saint-Pierre, where they overwintered (Montpetit 1897; Vladykov 1947; Vladykov and Brousseau 1957; Magnin and Beaulieu 1967). It is assumed that spawning took place in this sector or downstream, between mid-May and mid-June; the spawners then migrated downstream to the estuary, where they fed and regained condition during the entire summer. The fall upstream migration to Lake Saint-Pierre seemed to be made by fish that were preparing to spawn, because bass under three-years-old did not undergo this migration. Rather, they overwintered in the estuary, downstream from Quebec City, in the freshwater plume of the river (Montpetit 1897; Robitaille 2001).

Striped bass from the southern Gulf and Bay of Fundy also migrate upstream to freshwater in the fall. However, in their case, bass of all ages undertake the migration seemingly to escape the cold marine waters during the winter, which would be fatal (Rulifson and Dadswell 1995; Bradford et al. 1995; Douglas et al. 2003). Fishers differentiate Shubenacadie River striped bass that have overwintered in fresh water by their black colour; fish that are newly arrived from the sea are greenish in colour (Rulifson and Dadswell 1995; Paramore and Rulifson 2001).

In the southern Gulf, striped bass of all sizes entered several rivers along the coast in the fall. It was long believed that this movement was made by bass originating in each of these rivers (Rulifson and Dadswell 1995). However, it is now known that all of these fish were part of the Miramichi River population (Bradford et al. 1995; Robichaud-LeBlanc et al. 1996; Douglas et al. 2003). In the Kouchibouguac River, adult bass overwinter in deep sections of the river (Hogans and Melvin 1984). When the ice breaks up, they migrate downstream to salt water, where they feed during the summer. Movements along the river and in the estuary seem to be relatively closely related to temperature and availability of prey. When the water temperature begins to fall, the bass gradually migrate upriver where they overwinter.

Striped bass from the Shubenacadie River migrated upstream to overwinter in Grand Lake, where they can be caught in the winter ice fishery (Jessop 1991). In May, they migrate downstream to Minas Basin in the southern Bay of Fundy. Striped bass spawn in the Stewiacke River, a tributary of the Shubenacadie River.

Local migrations of striped bass from the Saint John Riverfollowed the same general pattern. Striped bass from this river also overwintered in freshwater, in Belleisle Bay, Washademoak Lake and in other deep sectors of the estuary. In May and June, they spawned in the tributaries, upstream from the tidal influence zone, and then migrated downstream to saltwater to feed for the entire summer (Dadswell 1976). In the fall, they returned to the river and, as their activity slowed, gathered in deep waters (Melvin 1978).

The Annapolis River striped bass were found primarily in the vicinity of the Royal Annapolis dam, in summer and fall (Jessop and Doubleday 1976). Some individuals from this population migrated upstream to fresh water to overwinter (Dadswell et al. 1984).

Nutrition and interspecific interactions

Striped bass initially feed primarily on invertebrates and, as they grow, on fish (Rulifson and McKenna 1987; Boynton et al. 1981; Robichaud-LeBlanc et al. 1997; Brousseau 1955; Robitaille 2001).

In their feeding and prespawning rearing areas, striped bass move in groups along the coasts, chasing schools of fish, particularly juvenile clupeids (Manooch 1973). The species found in bass stomach contents vary depending on location and season. The primary prey of Hudson River bass are blueback herring (Alosa aestivalis), American sand lance (Ammodytes americanus) and bay anchovy (Anchoa mitchilli) (Gardinier and Hoff 1982; Dew 1988). Other coastal populations feed primarily on blueback herring or Atlantic menhaden (Brevoortia tyrannus) (Manooch 1973; Trent and Hasler 1966). Tomcod (Microgadus tomcod) can often be found in the stomach contents at certain periods, but not throughout the annual cycle.

In the Kouchibouguac River estuary, the preferred prey of striped bass are mummichog (Fundulus heteroclitus), juvenile clupeids, threespine stickleback (Gasterosteus aculeatus), sand shrimp (Crangon septemspinosa) and marine worms (Hogans and Melvin 1984).

The organisms most frequently found in the stomach contents of St. Lawrence striped bass over two-years-old were Atlantic tomcod, rainbow smelt (Osmerus mordax), juvenile clupeids (American shad, Alosa sapidissima, gaspereau, Alosa pseudoharengus, Atlantic herring, Clupea harengus) and flounders (Pseudopleuronectes americanus and Liopsetta putnami) (Brousseau 1955; Robitaille 2001).

Behaviour/adaptability

There are two different points of view on the adaptability of striped bass, depending on their life stage, i.e., early life stages (eggs and larvae) or the stages after metamorphosis (juveniles and adults).

Adult bass tolerate and withstand variations in salinity, temperature, pH or turbidity (Talbot 1966; Auld and Schubel 1978; Setzler et al. 1980). The striped bass is highly fecund, opportunistic in its feeding habits and fast growing; these characteristics facilitate the rapid increase in its numbers in favourable environments. However, adult bass appear to avoid temperatures over 24°C, which may result in their confinement, on the hottest days of summer, to small refuges in certain reservoirs and estuaries in the United States (Coutant 1985). A similar phenomenon, but at the other end of the range of temperatures tolerated by this species, is believed to explain the river overwintering behaviour typical of Canadian populations. By overwintering in rivers, they appear to avoid the cold ocean waters in winter (Rulifson and Dadswell 1995).

By comparison with the adults, striped bass eggs and larvae are sensitive to minor changes in environmental variables (Cooper and Polgar 1981). Ambient conditions can therefore have a significant effect on the population dynamics of this species.

The abundance of progeny for a given number of spawners can vary from year to year (Merriman 1941; Raney 1952; Koo 1970; Van Winkle et al. 1979). The most important factor in year-class strength appears to be egg and larval survival. The abundance of a year class is reported to be already largely determined at the time of metamorphosis, i.e., at the end of the larval stage (Chadwick et al. 1977).

Bass populations are often characterized by variable recruitment (Merriman 1941; Raney 1952; Koo 1970; Van Winkle et al. 1979; Setzler et al. 1980; Ulanowicz and Polgar 1980; Kernehan et al. 1981; Cooper and Polgar 1981; Polgar 1982). Catch data show significant interannual variability; years in which catches are high correspond to the passage of strong year classes through the exploited segment of the population.

Periodicities of 6, 8 and 20 years have been detected in commercial striped bass landings on the Atlantic coast of the U.S. (Van Winkle et al. 1979). Abundance peaks in U.S. populations are not necessarily in phase with peaks in Canadian populations. Some biologists believe that heavy fishing can either cause or amplify variations in the abundance of bass populations.

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