7. Role of Lipids in Fish Populations
139
history of a species (Larson, 1991). There are both costs and benefits to the
organism, however, in regard to not only how energy allocated to reserves is
applied to other immediate uses such as growth and reproduction, but how these
reserves are ultimately used (Slobodkin, 1962). Costs are balanced against the
benefits of reserve utilization, and the trade-offs are evaluated in the context of
resource allocation to current and future reproduction (Fisher, 1958). For example, in most poikilothermic species, fecundity increases with body size (Munro,
1990; Bell, 1980), and hence, optimizing the age at first reproduction means
trading off future increased egg production associated with growth against the
advantages of early maturity (Roff, 1981, 1980).
Timing of breeding may be dependent on the quantity and quality of available
energy. In years of low food availability, retaining lipids in mesenteries and other
tissues increases the prospect of maternal survival and greater production of
progeny in future, more optimal (food production) years, perhaps at the expense
of the present year class (MacFarlane et ai., 1993). Some fish may defer reproductive maturity, rather than maturing during the present year, to store the necessary
reserves for overwinter survival, particularly in years of low food supply. One
advantage of shifting reproductive effort from the end of the growing season in the
fall to the following spring is that of optimizing both fecundity and offspring
fitness. Lipid storage allows the production of a large clutch early in the season
compared with smaller clutches that may have resulted from a later spawning
(Reznick and Braun, 1987). Young fish that are born earlier in the season have a
longer time interval for growth during favorable periods when prey abundance is
the highest. Early breeding, therefore, enhances the probability of individual
survival and thus increases the recruitment of individuals into the next year class.
Overwinter lipid reserves, therefore, permit a useful temporal shift in reproductive
effort, allowing fish to deter energy allocation to a time that may be more propitious for reproduction (Reznick and Braun, 1987).
Resource allocation strategies in fish involve trade-offs between available energy partitioned among growth, reproduction, maintenance, and storage (Fig. 7.1).
The optimization of energy allocation strategies is a relatively long-term phenomenon operating over many generations of a species. The optimal level of reproductive effort is a trade-off between current reproduction and survival coupled with
future reproduction (Steams, 1992). Surplus energy can be used for reproductive
tissue, which represents an investment in current reproductive success, or it can be
used to build up somatic tissue, which is an investment in future reproduction at
the expense of current reproductive success.
There appears to be two conflicting or alternative strategies of energy resource
allocation in fish. Either a fish may maintain a constant body weight over time and
adjust gamete quantity and quality accordingly, or it may produce a constant
number of eggs and sacrifice energy reserves and somatic tissue to meet reproductive requirements. Very few species appear to adopt either of these extreme
reproductive strategies, rather they typically employ some type of intermediate
strategy. The range of energy allocation strategies used by fish populations is
discussed below and represents strategies near the end of these extremes and some
that demonstrate intermediate patterns. In all these examples, food resources are
139
history of a species (Larson, 1991). There are both costs and benefits to the
organism, however, in regard to not only how energy allocated to reserves is
applied to other immediate uses such as growth and reproduction, but how these
reserves are ultimately used (Slobodkin, 1962). Costs are balanced against the
benefits of reserve utilization, and the trade-offs are evaluated in the context of
resource allocation to current and future reproduction (Fisher, 1958). For example, in most poikilothermic species, fecundity increases with body size (Munro,
1990; Bell, 1980), and hence, optimizing the age at first reproduction means
trading off future increased egg production associated with growth against the
advantages of early maturity (Roff, 1981, 1980).
Timing of breeding may be dependent on the quantity and quality of available
energy. In years of low food availability, retaining lipids in mesenteries and other
tissues increases the prospect of maternal survival and greater production of
progeny in future, more optimal (food production) years, perhaps at the expense
of the present year class (MacFarlane et ai., 1993). Some fish may defer reproductive maturity, rather than maturing during the present year, to store the necessary
reserves for overwinter survival, particularly in years of low food supply. One
advantage of shifting reproductive effort from the end of the growing season in the
fall to the following spring is that of optimizing both fecundity and offspring
fitness. Lipid storage allows the production of a large clutch early in the season
compared with smaller clutches that may have resulted from a later spawning
(Reznick and Braun, 1987). Young fish that are born earlier in the season have a
longer time interval for growth during favorable periods when prey abundance is
the highest. Early breeding, therefore, enhances the probability of individual
survival and thus increases the recruitment of individuals into the next year class.
Overwinter lipid reserves, therefore, permit a useful temporal shift in reproductive
effort, allowing fish to deter energy allocation to a time that may be more propitious for reproduction (Reznick and Braun, 1987).
Resource allocation strategies in fish involve trade-offs between available energy partitioned among growth, reproduction, maintenance, and storage (Fig. 7.1).
The optimization of energy allocation strategies is a relatively long-term phenomenon operating over many generations of a species. The optimal level of reproductive effort is a trade-off between current reproduction and survival coupled with
future reproduction (Steams, 1992). Surplus energy can be used for reproductive
tissue, which represents an investment in current reproductive success, or it can be
used to build up somatic tissue, which is an investment in future reproduction at
the expense of current reproductive success.
There appears to be two conflicting or alternative strategies of energy resource
allocation in fish. Either a fish may maintain a constant body weight over time and
adjust gamete quantity and quality accordingly, or it may produce a constant
number of eggs and sacrifice energy reserves and somatic tissue to meet reproductive requirements. Very few species appear to adopt either of these extreme
reproductive strategies, rather they typically employ some type of intermediate
strategy. The range of energy allocation strategies used by fish populations is
discussed below and represents strategies near the end of these extremes and some
that demonstrate intermediate patterns. In all these examples, food resources are