Cannibalism in a size-structured population:: Energy extraction and control

Recent size-structured cannibalistic models point to the importance of the energy gain by cannibals and also show that this gain may result in the emergence of giant individuals. We use a combination of a 10-year field study of a perch (Perca fluviatilis) population and quantitative within-season modeling of individual and population-level dynamics to investigate which mechanisms are most likely to drive the dynamics of the studied perch population. We focused on three main aspects to explain observed discrepancies between earlier model predictions and data: (1) introduction of more than one shared resource between cannibals and victims, (2) whether or not several victim age cohorts are necessary to allow giant growth, and (3) the intensity of inter-cohort competition between young-of-the-year (YOY) perch and 1-yr-old perch. At the start of the study period, the perch population was dominated by stunted perch individuals, and recruitment of perch to an age of 1-yr-old was negligible. Following a major death in adult perch, strong recruitments of perch to 1-yr-old were thereafter observed for a number of years. As 1-yr-olds these successful recruiters subsequently starved to death due to competition with the new YOY. The few surviving adult perch accelerated substantially in growth and became giants. At the end of the study period, the perch population moved back to the situation with stunted individuals. There was a high agreement between observed diets of cannibalistic perch and those predicted by the model for both the stunted and the giant phases. Analyses of growth rates showed that cannibalistic perch could become giants on a diet of YOY perch only, but that a supplement with the second shared resource (macroinvertebrates) was needed to reach the observed sizes. Modeling of growth and diet in the giant phase showed an exploitative competitive effect of YOY perch on 1-yr-old perch, but a restriction in habitat use of 1-yr-old perch had to be assumed to yield the observed growth rate and diet. The resource dynamics of zooplankton and macroinvertebrates were both accurately predicted by the model. Also, YOY perch mortality was accurately predicted and, furthermore, suggested that one of the trawling methods used may underestimate the number of YOY perch when they increase in size. We conclude that the presence of a second shared resource and the restricted habitat use and absence of cannibalistic consumption by 1-yr-old perch individuals are two important mechanisms to explain the discrepancy between model predictions and data. Our results also point to the fact that that the dynamics observed may be explained by complex dynamics not involving the presence of a giant and dwarf cycle.