PCWA-L 480.pdf - PCWA Middle Fork American River Project ...

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Bioenergetics Overview
1.1 The Modeling Strategy
1-2
Starting the process with observed growth rate is different from that of many kinds of modeling
practices. rn this case, the strategy of modei building is based on specifying rules that define the
limit conditions, Le., the maximum and minimum possible rates of growth for members of a
population. The physiological pararl!eters used to represent the rules derive from'readily and ottmeasured
processes such as temperature dependence, thermal tolerance, thermal preference
size dependence, assimilation efficiency, etc., that can be accurately measured in the laborato;y..
Those physiological parameters are assembled as empirical rules that define the effect of
temperature, body size and food quality on maximum feeding rates. 'The minimum is similarly
defined by rules describing the effect of temperature and body size on metabolic rates when food
consumption is set to zero. These limits define the boundaries of the scope for growth. Observed
growth is somewhere between those limits and allows the user to estimate how that growth rate is
being regUlated.
The hierarchy of energy allocation Is an important'component of this modeling approach.
Consumed.energy is first alioCEited to catabolic processes (maintenance and activity metabolism),
then to waste losses (feces, urine and specific dynamic action) and that lett over is allocated to
somatic storage (body growth and gonad development). This hierarchy is analogous to practical
economics. The first costs paid are those for rent or mortgage (metabolism)that sustain the
organism. The second set of costs (waste losses) are like taxes -they are proportional to income'
(food consumption) and must be paid. The energy resource remaining. may then be allocated to
savings (growth) or invested in the next generation (gonad development). In an ecological or .
evoiutionary context, it is easy to imagine selection for behaviors that maximize benefits (growth
rate or gonad development) and minimize costs. Like an account balance, a record of growth
reveals how well the organism has resolved the complexities of its environment
In a thorough review' of previous energetics work, Brett and Groves (1979) presented a
generalization about energy budgets for two ciasses of fishes. if the energy budgetis stated in the
follOWing terms:
Energy Consumed'=' Respiration + Waste + Growth,
and normalized 10 percentages when energy consumption =100, then fishes. growing at "typical"
rates would have energy budgets approximated as below.
For carnivores:
For herbivores:
Consumption = Respiration + Waste + Growth
100
100
=
=
44
37
+ 27 + 29
+ 43 + 20
These budgets reveal two important features. First, as expected, herbivores exhibit lower growth
rates and higher waste-loss rates per unit of energy consumed. That is the logicai consequence of
eating foods of Jowerenergy density and higher Indigestible content Second, both types of fishes
demonstrate high rates of ·growth efficiency compared to those known for mammals and·birds.
Although.these budgets can serve as a first approximation, the 95% confidence Intervals for each
component are substantial (e,g., plus or minus 20% of the mean). Of course, tile energy bUdget
for an average fish in a typical habitat may be very different from that of fishes in some unique
ecological context Fishes are known to exhibit among the highest growth efficiencies recorded
(approaching 50%) and are known to exhlbtt strikingly negative energy budgets, as In the case of
migrating salmon (Brett 1995). Note, too, that tile hierarchy of energy allocation operates In all
cases. Growth efficiency is not a constant, and growth rates in fishes are highly variable.
Observed growth is the integrated answer to a complex question about prey resources and