136
S.M. Adams
stressors (Smit et aI., 1981; Toneys and Coble, 1980; Davis and Simco, 1976).
Because electrolyte regulation is an energy-consuming process (Nordlie and
Leffler, 1975; Morris and BuB, 1968), the proportion of energy used for such
regulation may be higher at lower than at higher temperatures and may therefore
result in an additional drain on already depleted energy stores during the winter.
For example, Johnson and Evans (1996) found that the overwinter mortality of
white perch (Morone americana) at 4°C occurred primarily because of starvation
but at 2.5°C resulted from both ionic osmoregulatory imbalance as well as
starvation.
Starvation and depletion of lipids to critically low levels can result in increased
susceptibility offish to a variety of environmental stressors (Roff, 1982; Shul'man,
1974). Both Walkley and Meakins (1970) and Ghittino (1989) reported that poor
nutrition and low energy reserves from starvation can result in increased susceptibility to disease and parasites. Increased susceptibility to disease is probably
related to deficiencies in the immune system induced under starvation conditions
(Gurr, 1983). Reduction of energy stores due to spawning can also result in
mortality from parasitic infection (Roff, 1982).
Starvation due to exhaustion of energy reserves, particularly in smaBer individuals, weakens fish and also renders them more susceptible to predation (Minton
and McLean, 1982; Roff, 1982; Herting and Witt, 1967). Increased growth in
young piscivores as a result of forging on high-energy prey reduces not only
predation risk but increases their overwinter survival (Buijse and Houthuijzen,
1992; Maceina and Isley, 1986; Shelton et aI., 1979; Chevalier, 1973; Kramer and
Smith, 1962). Shortage of prey results in reduced growth, which in turn increases
the vulnerability of smaBer largemouth bass to predation and leads to increased
overwinter mortality (Wicker and Johnson, 1987). Mild winters may also result in
a prolonged period of predation risk because temperatures may be high enough to
increase metabolic demands for limited fat reserves but just low enough to depress
feeding, resulting in a negative energy balance. Even if temperatures were mild
enough during the winter such that predators could occasionaBy feed, the foraging
costs per unit of energy intake could be significantly higher when food availability
is relatively low. This situation would led to an additional energy expenditure and
would further deplete fat reserves (MacKinnon, 1972).
Species or individuals that spawn early in the year increase their probability of
overwinter survival and escaping predation by storing proportionally more fat
reserves prior to the onset of winter. Later-spawning fish may accumulate insufficient fat reserves and starve to death over a normal winter. For example, sand
smelt display density-dependent control on population size by limiting the number of fish that can spawn at the optimum time (Henderson et aI., 1988). Restricted
availability of spawning ground results in overaB population regulation because
some individuals are forced to spawn later in the spring, leaving them insufficient
time to build up fat reserves to survive the winter. This density-dependent control
acts to limit the number of individuals that can breed simultaneously at the most
desirable time and leads to a size-selective winter mortality, ultimately resulting
in a balance between spawning site availability and population size. For large-
S.M. Adams
stressors (Smit et aI., 1981; Toneys and Coble, 1980; Davis and Simco, 1976).
Because electrolyte regulation is an energy-consuming process (Nordlie and
Leffler, 1975; Morris and BuB, 1968), the proportion of energy used for such
regulation may be higher at lower than at higher temperatures and may therefore
result in an additional drain on already depleted energy stores during the winter.
For example, Johnson and Evans (1996) found that the overwinter mortality of
white perch (Morone americana) at 4°C occurred primarily because of starvation
but at 2.5°C resulted from both ionic osmoregulatory imbalance as well as
starvation.
Starvation and depletion of lipids to critically low levels can result in increased
susceptibility offish to a variety of environmental stressors (Roff, 1982; Shul'man,
1974). Both Walkley and Meakins (1970) and Ghittino (1989) reported that poor
nutrition and low energy reserves from starvation can result in increased susceptibility to disease and parasites. Increased susceptibility to disease is probably
related to deficiencies in the immune system induced under starvation conditions
(Gurr, 1983). Reduction of energy stores due to spawning can also result in
mortality from parasitic infection (Roff, 1982).
Starvation due to exhaustion of energy reserves, particularly in smaBer individuals, weakens fish and also renders them more susceptible to predation (Minton
and McLean, 1982; Roff, 1982; Herting and Witt, 1967). Increased growth in
young piscivores as a result of forging on high-energy prey reduces not only
predation risk but increases their overwinter survival (Buijse and Houthuijzen,
1992; Maceina and Isley, 1986; Shelton et aI., 1979; Chevalier, 1973; Kramer and
Smith, 1962). Shortage of prey results in reduced growth, which in turn increases
the vulnerability of smaBer largemouth bass to predation and leads to increased
overwinter mortality (Wicker and Johnson, 1987). Mild winters may also result in
a prolonged period of predation risk because temperatures may be high enough to
increase metabolic demands for limited fat reserves but just low enough to depress
feeding, resulting in a negative energy balance. Even if temperatures were mild
enough during the winter such that predators could occasionaBy feed, the foraging
costs per unit of energy intake could be significantly higher when food availability
is relatively low. This situation would led to an additional energy expenditure and
would further deplete fat reserves (MacKinnon, 1972).
Species or individuals that spawn early in the year increase their probability of
overwinter survival and escaping predation by storing proportionally more fat
reserves prior to the onset of winter. Later-spawning fish may accumulate insufficient fat reserves and starve to death over a normal winter. For example, sand
smelt display density-dependent control on population size by limiting the number of fish that can spawn at the optimum time (Henderson et aI., 1988). Restricted
availability of spawning ground results in overaB population regulation because
some individuals are forced to spawn later in the spring, leaving them insufficient
time to build up fat reserves to survive the winter. This density-dependent control
acts to limit the number of individuals that can breed simultaneously at the most
desirable time and leads to a size-selective winter mortality, ultimately resulting
in a balance between spawning site availability and population size. For large-