7. Role of Lipids in Fish Populations
135
because of shorter periods for feeding and accumulation of energy stores, but
also because of extended nonfeeding periods when any remaining lipids are used.
The ability of fish to store energy and survive the winter generally decreases,
therefore, from south to north as the growing season becomes shorter and the
starvation period is extended (Miranda and Hubbard, 1994). The likelihood that
young fish will die from exhaustion of energy stores is reduced at low latitudes
where the growing season is longer and winters are shorter and milder. Shuter and
Post (1990) surmised that overwinter starvation mortality of young fish may be
the critical factor limiting the northern distribution of both yellow perch and
smallmouth bass (Micropterus dolomieui) throughout large parts of their ranges.
The northern distribution limits for other species may similarly be influenced by
winter starvation dynamics.
Overwinter mortality appears to be a size-dependent phenomenon, with the
smaller fish in a cohort experiencing higher levels of mortality than larger individuals (Hutchings, 1994; Adams et aI., 1982; Shuter et aI., 1980; Toneys and Coble,
1979). Size-dependent overwinter mortality is based, in part, on metabolic allometry (Miranda and Hubbard, 1994; Thompson et aI., 1991; Post and Evans,
1989). Smaller fish, with lower energy stores and relatively higher metabolic
rates, generally display higher mortality rates during winter when energy stores
are often depleted and fish cannot meet basic metabolic demands (Post and Evans,
1989). Small individuals use proportionally more of their fat stores to maintain basal metabolic rate during periods of low food than do larger individuals
(Schmidt-Nielsen, 1984; Shuter et aI., 1980). A positive relationship between
body size and lipid content has been found for yellow perch (Toneys and Coble,
1979), smallmouth bass (Shuter et aI., 1980; Oliver et aI., 1979), sand smelt
(Henderson et aI., 1988), and largemouth bass, Micropterus salmoides (Adams et
aI., 1982; Isley, 1981). Also, fish appear to have a compensatory reaction that
raises the metabolic rate when temperatures approach critically low levels
(Shul'man, 1974), with the rate of fat metabolism increasing at very low temperatures (Prosser, 1973). Thus, metabolism plays a significant role in lipid dynamics
of the fish during the winter and has a major influence on survival rate.
Impairment of osmoregulatory ability can occur not only as a result of phospholipid utilization during periods of severe starvation but also as a result of biochemical changes in membrane structure at low temperatures. Lipids have a large
influence on membrane permeability with fatty acid chain length, percentage
unsaturated fatty acids, and number of double bonds per chain regulating fluidity,
ion fluxes, and enzyme kinetics (Friedman et aI., 1986). As temperatures decrease, the amount of unsaturated fatty acids in the plasma membrane increases
along with concomitant changes in osmotic sensitivity, fluidity, and permeability.
At low temperatures, replacement of native phosphatidy1choline by more saturated species considerably modifies membrane properties, resulting in a progressive increase in osmotic fragility and permeability (Leray et aI., 1986). Changes in
membrane permeability and impaired functioning of ion transport mechanisms at
lower temperatures generally reduce the ability of fish to osmoregulate, which
either results in death or increases the organism's vulnerability to additional
135
because of shorter periods for feeding and accumulation of energy stores, but
also because of extended nonfeeding periods when any remaining lipids are used.
The ability of fish to store energy and survive the winter generally decreases,
therefore, from south to north as the growing season becomes shorter and the
starvation period is extended (Miranda and Hubbard, 1994). The likelihood that
young fish will die from exhaustion of energy stores is reduced at low latitudes
where the growing season is longer and winters are shorter and milder. Shuter and
Post (1990) surmised that overwinter starvation mortality of young fish may be
the critical factor limiting the northern distribution of both yellow perch and
smallmouth bass (Micropterus dolomieui) throughout large parts of their ranges.
The northern distribution limits for other species may similarly be influenced by
winter starvation dynamics.
Overwinter mortality appears to be a size-dependent phenomenon, with the
smaller fish in a cohort experiencing higher levels of mortality than larger individuals (Hutchings, 1994; Adams et aI., 1982; Shuter et aI., 1980; Toneys and Coble,
1979). Size-dependent overwinter mortality is based, in part, on metabolic allometry (Miranda and Hubbard, 1994; Thompson et aI., 1991; Post and Evans,
1989). Smaller fish, with lower energy stores and relatively higher metabolic
rates, generally display higher mortality rates during winter when energy stores
are often depleted and fish cannot meet basic metabolic demands (Post and Evans,
1989). Small individuals use proportionally more of their fat stores to maintain basal metabolic rate during periods of low food than do larger individuals
(Schmidt-Nielsen, 1984; Shuter et aI., 1980). A positive relationship between
body size and lipid content has been found for yellow perch (Toneys and Coble,
1979), smallmouth bass (Shuter et aI., 1980; Oliver et aI., 1979), sand smelt
(Henderson et aI., 1988), and largemouth bass, Micropterus salmoides (Adams et
aI., 1982; Isley, 1981). Also, fish appear to have a compensatory reaction that
raises the metabolic rate when temperatures approach critically low levels
(Shul'man, 1974), with the rate of fat metabolism increasing at very low temperatures (Prosser, 1973). Thus, metabolism plays a significant role in lipid dynamics
of the fish during the winter and has a major influence on survival rate.
Impairment of osmoregulatory ability can occur not only as a result of phospholipid utilization during periods of severe starvation but also as a result of biochemical changes in membrane structure at low temperatures. Lipids have a large
influence on membrane permeability with fatty acid chain length, percentage
unsaturated fatty acids, and number of double bonds per chain regulating fluidity,
ion fluxes, and enzyme kinetics (Friedman et aI., 1986). As temperatures decrease, the amount of unsaturated fatty acids in the plasma membrane increases
along with concomitant changes in osmotic sensitivity, fluidity, and permeability.
At low temperatures, replacement of native phosphatidy1choline by more saturated species considerably modifies membrane properties, resulting in a progressive increase in osmotic fragility and permeability (Leray et aI., 1986). Changes in
membrane permeability and impaired functioning of ion transport mechanisms at
lower temperatures generally reduce the ability of fish to osmoregulate, which
either results in death or increases the organism's vulnerability to additional