A topic of general interest to population ecologists and of particular interest to fisheries managers is the concept of production. How much biomass (or fish flesh) is a population producing, and how much of this is available to predators, including humans, without causing the population to crash? Can production be predicted from such measurable population traits as the birth and death schedules of different age classes (i.e., from calculations using life table characteristics discussed above)? Production is calculated as the growth rate of individuals over a time period multiplied by the biomass of the age class, corrected for mortality occurring during the time period (Ricker 1975; Gulland 1983; Wootton 1990; Ross 1997).
Natural production values for different populations of temperate freshwater fishes vary widely, from <0.1 g/m2/ year for Sockeye Salmon in an Oregon lake to 155 g/m2/ year for desert pupfish (Cyprinodon nevadensis, Cyprinodontidae) in a desert stream in California. Most populations fall near the lower end of these values, in the 1–10 g/m2/year range. Tropical and fertilized ponds often show higher values (Chapman 1978). Knowing production also allows one to calculateannual turnover, which is the ratio of production to biomass (P : B). Turnover is an index of how productive populations and subpopulations are; it can be quite useful in understanding ecosystem processes. Among different age classes, very young fishes, although constituting relatively little of the biomass of the overall population, contribute 60–80% of population production because of their high P : B ratios. Young fish have very high growth rates relative to their sizes and hence have high turnover rates.
Yields to predators are therefore relatively higher when predators feed on young fish rather than eating older, slower growing fish. Whether high rates of exploitation of young age classes by predators reflect some form of optimal exploitation due to relative P : B ratios, or whether they just reflect ease of capture, would make an interesting study. Regardless, overfishing might be reduced if fisheries targeted younger age classes instead of imposing minimum size limits and targeting reproductively mature individuals. Such a management approach would more closely mimic natural predator–prey and assemblage interactions, relationships to which prey species have adjusted their life history traits over evolutionary time (Helfman 2007).
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