Resource heterogeneity, diet shifts and intra-cohort competition: effects on size divergence in YOY fish

Abstract

Most organisms exhibit a substantial size variation among individuals due to individual differences in experienced biotic and abiotic environmental conditions and because individuals undergo growth and development during most of their life time. One important issue in this context is how size variation within cohorts may develop over time. Here, we tested the hypothesis, in gape-limited animals such as fish, that size divergence among individuals within a cohort depends on the opportunity to undergo size-dependent diet shifts, by allowing initially larger individuals to make an early diet shift when the first resource becomes limiting. We used young-of-the-year perch (Perca fluviatilis) as our study organism. Competitive intensity and the opportunity to undergo a diet shift from zooplankton to macroinvertebrates affected both mean growth rates and the extent to which inter-individual variation in growth was manifested. As predicted, increased competition combined with the presence of both zooplankton and benthic macroinvertebrates increased the degree of size variation. However, size divergence was also observed among individuals when only the initial resource, zooplankton, was available. We argue that only non-exploitative interactions, such as dominance structures and social interactions, could have caused this latter pattern, as exploitative competition is expected to lead to size convergence due to the superior competitive ability of smaller individuals. Our results suggest that diet shifts are not a prerequisite for size divergence in animal cohorts, and that dominance and social interactions may have similar effects on size variation within cohorts. Finally, development of size variation is suggested to have strong implications for overall cohort performance.

Feeding ecology and ontogenetic dietary shift of yellowstripe goatfish Mulloidichthys flavolineatus (Mullidae) at Reunion Island, SW Indian Ocean

“…Differences in diet composition observed between juveniles and adults consisted of a decrease in meiofauna (nematodes, tanaids and harpacticoid copepods) and microfauna (foraminifera) prey, concurrent with an increase in macrofauna prey (gastropods, echinoderms, isopods and decapods) from a common prey pool (polychaetes, tanaids [Leptocheliidae] and harpacticoid copepods). This is consistent with the conceptual model of a relationship between fish size and the size, biomass and diversity of prey (Scotto di Carlo et al 1982, Harmelin-Vivien et al 1989, limiting intraspecific competition for similarly sized prey (Huss et al 2008). Intraspecific competition within fish cohorts, in particular following recruitment when juvenile density can affect resource density (Byström & Garcia-Berthou 1999), was found to have a large influence on the inter-individual variation in resource use (Svanbäck & Persson 2004).…”

Alternative stable states in a stage-structured consumer–resource biomass model with niche shift and seasonal reproduction

“…These models assume, however, that all individuals born in the same year mature at exactly the same age and hence do not diverge in their development. This contrast with experimental evidence (Huss et al, 2008) that considerable divergence in body size arises among individuals of the same year class. In this paper, we therefore formulate a simple model that represents reproduction as a pulsed event, but allows individuals of the same year class to mature at different times, and investigate the robustness of the occurrence of ASS as a result of ontogenetic niche shift by analyzing the dynamics of a stage-structured biomass model with pulsed reproduction of consumers.…”

Ontogenetic divergence of growth among rainbow smelt morphotypes

“…We conclude that this population of rainbow smelt displays strong phenotypic plasticity of growth in response to 1) the physical, environmental conditions experienced in early life from spawning through their first growing season, e.g., the duration of the hatching period (e.g., Snorrason et al 1994), and 2) the biological conditions related to food resources and intra-specific competition as juveniles and adults, i.e., niche shifting (e.g., Huss et al 2008). There doesn't appear to be multiple lineages, but the morphotypes are sustained to some degree by genetic isolation, i.e., larva of giant form smelt hatch and begin exogenous feeding earlier, are larger at age 1+, and thus more probable to continue as giant form smelt.…”