Effects of food limitation and emigration on self-thinning in experimentalminnow cohorts

1. The theory of food-regulated self-thinning (FST) for mobile animals pred icts population density (N) to be an inverse function of mean body mass (W) scaled to an exponent (b), such that N = k W-b, where k is a constant. FST also predicts energy requirements (or energy now) to remain constant over time (termed energetic equivalence) as losses to cohorts (e.g. emigration a nd mortality) are balanced by increased growth of surviving individuals. 2. To test these predictions, we analysed the dynamics of six experimental minnow cohorts. Replicate populations of fish were held under identical con ditions with a constant and limited supply of food over a 126-day period. H alf of the cohorts were open to emigration, and half were closed so that fi sh could only be lost through starvation mortality. 3. Patterns of self-thinning indicated non-linear changes in population den sity and energy flow in relation to changes in mean body mass and time, res pectively. Nonlinear patterns of self-thinning were probably due to a delay ed growth response to changes in population density effected through mortal ity and/or emigration. Contrary to results of similar experiments on other fish, emigration did not have a significant influence on the pattern of sel f-thinning. 4. These results may be attributed to trophic interactions within cohorts a nd the importance of social behaviour to cohort dynamics. Both population d ensity and energy flow in our experimental populations appeared to cycle, w ith episodes of starvation and mortality alternating with food recovery and weight gain, as predicted by recent models of stepwise die-off and stunted growth in animal cohorts. 5. Most of the support for FST in mobile animals comes from observational d ata on mean body mass and population density. Potentially important mechani sms, including the manner in which individuals are lost or retained in popu lations, are usually not investigated directly. Such tests of FST can only provide equivocal support. Detailed observational study and controlled expe riments are needed to understand casual mechanisms.