Demographic modeling of selected fish species with RAMAS

The microcomputer program RAMAS 3 developed for EPRI, has been used to model the intrinsic natural variability of seven important fish species: cod, Atlantic herring, yellowtail flounder, haddock, striped bass, American shad and white perch. Demographic data used to construct age-based population models included information on spawning biology, longevity, sex ratio and (age-specific) mortality and fecundity. These data were collected from published and unpublished sources. The natural risks of extinction and of falling below threshold population abundances (quasi-extinction) are derived for each of the seven fish species based on measured and estimated values for their demographic parameters. The analysis of these species provides evidence that including density-dependent compensation in the demographic model typically lowers the expected chance of extinction. This is because if density dependence generally acts as a restoring force it seems reasonable to conclude that models which include density dependence would exhibit less fluctuation than models without compensation since density-dependent populations experience a pull towards equilibrium. Since extinction probabilities are determined by the size of the fluctuation of population abundance, models without density dependence will show higher risks of extinction, given identical circumstances. Thus, models without compensation can be used as conservative estimators of risk, that is, ifmore » a compensation-free model yields acceptable extinction risk, adding compensation will not increase this risk. Since it is usually difficult to estimate the parameters needed for a model with compensation, such conservative estimates of the risks of extinction based on a model without compensation are very useful in the methodology of impact assessment. 103 refs., 19 figs., 10 tabs.« less