and population biomass increases or declines according to

the balance between these two rates.

 The feeding rate of the birds is a function of fish

density and bird biomass. It is an inverse function of

average bird weight, which decreases when nestlings join

the population but increases as they approach fledging. In

the model, breeding birds lay an average of 1.5 eggs each

in years when the laying weight threshold is exceeded by

average bird weight. The number of fledglings ultimately

produced in a given year is determined by the total weight

gain of the population.

 In Model III, both prey fish and predator fish are

decimated when isolated ponds go dry. Prey fish, in which

recruitment and growth is a direct function of water area

and solar radiation, recover more quickly than predator

populations, in which growth is a function of prey density

and predator biomass.

 Equations for Model III are shown in the legend to

Figure 13. The bases for those rate equations which differ

from those used in the simpler models are as follows.

 Consumption by prey fish (rate equation 1 in

Figure 13). To the basic equation in which food

consumption is a function of solar radiation, water surface

area, and fish biomass was added a density dependent factor

to reduce fish growth when fish density becomes high. This

addition simulates the limiting of populations at high

densities by factors such as decreased fecundity or reduced