PARENT SESSION
Oral Session #26: Aquatic Ecology: Fish - Populations, life history, autecology. Presiding: M. Fox.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.

Foraging capacity and resource synchronisation in an ontogenetic diet switcher, pikeperch (Stizostedion lucioperca).

Persson, Anders¹, Brönmark, Christer¹, ¹

ABSTRACT- Species undergoing ontogenetic diet shifts face a risk of resource competition that delays transition between feeding stages. This was studied in pikeperch (Stizostedion lucioperca), when switching from planktivorous to piscivorous feeding. We experimentally quantified foraging capacity in both feeding niches and implemented the data in an individual based growth model. For any given prey type and size, foraging capacity described a humped shaped relationship with predator size. Foraging capacity on 1mm daphnids peaked at a pikeperch length of 53 mm, suggesting a narrow scope of planktivory. The highest capacity in the piscivorous niche was reached at a predator to prey length ratio of 5. With the growth model, we derived density and size-distributions of 0+ cohorts as functions of resource levels. Discontinuous availability of prey sizes resulted in a demand of high resource density to allow diet switching the first growing season. This was accentuated by prey fish growth that caused the two niches to separate over time and limited the time-window when diet shift was possible. The relationship between foraging capacity and pikeperch size resulted in growth reduction when individuals were growing out of the zooplankton resource. Individuals failing to reach the piscivorous niche attained similar size, whereas piscivorous individuals continued to grow fast throughout the season by surfing on the wave of the prey cohort. This points to the importance of being synchronized with fluctuations in resource availability.

KEY WORDS: ontogenetic diet shift, size-structure, piscivory, planktivory