TABLE OF CONTENTS Pages Symposium 1 - the National Sea ...

will have a large effect upon the cost of product. Fixed costs, which are an amalgamation
of all costs that are independent of the rate of production, also represent a large part of the
cost of production. In reality, most costs are a combination of a fixed and a variable
component, and thus estimates of fixed costs will vary greatly depending upon which costs
are considered fixed costs.
Losordo and Westerman (1994) state that capital costs and depreciation of capital
equipment account for approximately 34 percent of the total production cost, but these
costs are just two of the larger fixed costs. Thus total fixed costs would represent a much
larger percentage of the cost of production than the 34 percent noted previously.
O’Rourke (1996) divided all of his costs into either fixed or variable, and calculated that
fixed cost were 56 percent of total costs. Included in the list of fixed costs were wages,
water, and maintenance, which usually would not be considered total fixed. It is fair to say
however that fixed cost represent somewhere between 34 and 56 percent of total
production costs. By definition, once a production facility is operational, fixed expenses
remain constant. Fixed or overhead costs per unit of production change as the production
rate changes, thus fixed costs as a percentage of total production costs can be reduced by
increasing the production rate.
The production rate for a given facility can be increased by either increasing the stocking
density or the growth rate. While increasing stocking densities is an obviously attractive
goal, there are physiological as well as engineering and physical limitations that limit the
culture intensity. Stocking density has been shown to negatively affect growth rates in
many species of fish, although the responses vary. Poorer growth rates have been
associated with higher stocking densities of tilapia raised in ponds, irrigation ditches or
cages (Coche 1982; Carro-Anzalotta and McGinty 1986; D'Silva and Maughan 1996; Yi
et al. 1996; Siddiqui et al. 1989), but little research has be conducted on tilapia in
recirculating systems. Huang and Chiu (1997) researched the effects of stocking density
on growth rates on tilapia fry smaller than 10 g, and observed that increasing stocking
densities had a significantly negative effect upon size, size variation and survival rate.
Regardless, the effect of stocking density is beyond the scope of the experiments reported
herein. For this research project, a density of 5 gallons per fish was selected to minimize
the effects of crowding.
Fish physiologists do not fully understand the biological mechanisms that control the
growth rates of fish. Although there have been numerous mathematical models to
describe the growth characteristics of fish, most of these models do not take into account
such important factors as availability of food, food quality or the effect of environmental
factors such as temperature. This has given rise to the scientific field of bioenergetics,
which partitions the energy consumed into various metabolic processes including
production or growth, and waste products. While the bioenergetic models account for
changes in energy consumption, measuring each of the energy partitions is labor intensive,
and are usually measured as a function of only one variable; thus ignoring the interactions
with other biological factors. Bioenergetics provides a reasonable growth projection for
fish in the natural environment, but its usefulness for describing growth in recirculating
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