134
S.M. Adams
plenished over the nonfeeding periods, lipid levels are typically reduced to very
low levels by the end of the winter. Exhaustion of lipid reserves can have a variety
of negative effects on fish condition and health and compromise individual survival and ultimately population success.
Mortality due to exhaustion of stored energy has been reported for a number of
fish species (Henderson et aI., 1988; Adams et aI., 1985, 1982; Isley, 1981; Oliver
et aI., 1979). Fish must maintain a critical or minimal level of fat to survive (Hoar,
1983). Minimal fat levels for survival have been reported for various species as
1.4% wet wt. in herring (Clupea ham/gus) (Wilkins, 1967),2.2% dry wt. in yellow
perch (Percajlavescens) (Newsome and Leduc, 1975),3.2% dry wt. in sand smelt
(Atherina boyerO (Henderson et aI., 1988), and I % wet wt. in gizzard shad
(Domsoma cepedianum) (Adams et aI., 1985). Several studies also indicate that
overwinter starvation mortality occurs when a fixed proportion (approximately
50%; i.e., the 50% rule) of the initial body weight is lost (Kleiber, 1961). For
example, weight loss percentages in the 50% range for mortality resulting from
starvation have been reported for several species (57%, Johnson and Evans, 1996;
55%, Shuter et aI., 1980; 56%, Savitz, 1971). The assumption for the 50% rule is
that a starved fish depletes the usable portion of its body energy (mainly TAGs),
leaving only phospholipids, protein, and small amounts of carbohydrates.
The association between critical lipid (TAG) levels and survival may be related
to the period when all the neutral lipids have been exhausted and fish begin to use
phospholipids as an energy source. Even though phospholipids can be metabolized to provide some minimal energy during starvation (Love, 1980; Wilkens.
1967), the biochemical and physiological consequences for doing so appear to be
severe for the organism. These structural lipids playa key role in the activity of
various enzyme complexes that oxidize intermediates in the tricarboxylic acid
cycle.
Structural lipids are also essential in cellular metabolism as they relate to
osmotic and electrolyte homeostasis in the cell. Alterations in membrane permeability and ion transport mechanisms can occur under starvation, reducing the
ability of fish to osmoregulate, particularly at low temperatures (Stanley and
Colby, 1971; Morris and Bull, 1968). Nordlie and Leffler (1975) have suggested
that reduced survival of fish during the winter may be due to this impaired
osmoregulatory ability.
Over the winter nonfeeding period, most temperate zone fish undergo physiological changes that result in declining body condition and depletion of energy
reserves (Cunjak, 1988). Many authors have shown the close relationship between
the lipid content of fish and their ability to survive the winter. To survive through
the winter, fish either accumulate a critical amount of fat or feed periodically to
maintain their reserves. Fish that can switch their allocation of available energy
from somatic growth to fat reserves before winter have a higher probability of
overwinter and early spring survival (Thompson et aI., 1991). In populations of
temperate zone fish near the northern limit of their range, overwinter mortality is
often high (Shuter and Post, 1990; Oliver et aI., 1979). Freshwater fish at higher
latitudes are particularly more vulnerable to overwintering mortality not only
S.M. Adams
plenished over the nonfeeding periods, lipid levels are typically reduced to very
low levels by the end of the winter. Exhaustion of lipid reserves can have a variety
of negative effects on fish condition and health and compromise individual survival and ultimately population success.
Mortality due to exhaustion of stored energy has been reported for a number of
fish species (Henderson et aI., 1988; Adams et aI., 1985, 1982; Isley, 1981; Oliver
et aI., 1979). Fish must maintain a critical or minimal level of fat to survive (Hoar,
1983). Minimal fat levels for survival have been reported for various species as
1.4% wet wt. in herring (Clupea ham/gus) (Wilkins, 1967),2.2% dry wt. in yellow
perch (Percajlavescens) (Newsome and Leduc, 1975),3.2% dry wt. in sand smelt
(Atherina boyerO (Henderson et aI., 1988), and I % wet wt. in gizzard shad
(Domsoma cepedianum) (Adams et aI., 1985). Several studies also indicate that
overwinter starvation mortality occurs when a fixed proportion (approximately
50%; i.e., the 50% rule) of the initial body weight is lost (Kleiber, 1961). For
example, weight loss percentages in the 50% range for mortality resulting from
starvation have been reported for several species (57%, Johnson and Evans, 1996;
55%, Shuter et aI., 1980; 56%, Savitz, 1971). The assumption for the 50% rule is
that a starved fish depletes the usable portion of its body energy (mainly TAGs),
leaving only phospholipids, protein, and small amounts of carbohydrates.
The association between critical lipid (TAG) levels and survival may be related
to the period when all the neutral lipids have been exhausted and fish begin to use
phospholipids as an energy source. Even though phospholipids can be metabolized to provide some minimal energy during starvation (Love, 1980; Wilkens.
1967), the biochemical and physiological consequences for doing so appear to be
severe for the organism. These structural lipids playa key role in the activity of
various enzyme complexes that oxidize intermediates in the tricarboxylic acid
cycle.
Structural lipids are also essential in cellular metabolism as they relate to
osmotic and electrolyte homeostasis in the cell. Alterations in membrane permeability and ion transport mechanisms can occur under starvation, reducing the
ability of fish to osmoregulate, particularly at low temperatures (Stanley and
Colby, 1971; Morris and Bull, 1968). Nordlie and Leffler (1975) have suggested
that reduced survival of fish during the winter may be due to this impaired
osmoregulatory ability.
Over the winter nonfeeding period, most temperate zone fish undergo physiological changes that result in declining body condition and depletion of energy
reserves (Cunjak, 1988). Many authors have shown the close relationship between
the lipid content of fish and their ability to survive the winter. To survive through
the winter, fish either accumulate a critical amount of fat or feed periodically to
maintain their reserves. Fish that can switch their allocation of available energy
from somatic growth to fat reserves before winter have a higher probability of
overwinter and early spring survival (Thompson et aI., 1991). In populations of
temperate zone fish near the northern limit of their range, overwinter mortality is
often high (Shuter and Post, 1990; Oliver et aI., 1979). Freshwater fish at higher
latitudes are particularly more vulnerable to overwintering mortality not only