Gape-dependent Larval Foraging and Zooplankton Size: Implications for Fish Recruitment across Systems

Abstract

Small gape of zooplanktivorous larval fish limits their prey size; yet, within constraints set by gape, zooplankton size eaten influences larval growth and ultimately survival. To determine if optimal zooplankton size varied among fish species with different gapes, we conducted foraging trials with larval bluegill (Lepomis macrochirus, 10-26 mm TL) and gizzard shad (Dorosoma cepedianum, 18-31 mm TL). Larvae (n = 10) fed for 1 h on zooplankton assemblages that varied in size, after which all larvae and remaining zooplankton were preserved. Larval gape was measured; both larval gut contents and available zooplankton were quantified. Bluegill, the large-gaped species, fed on larger zooplankton than did gizzard shad with similar gapes. Further, larger bluegill fed on progressively larger zooplankton whereas all gizzard shad ate small prey (<0.60 mm). As available zooplankton size increased, bluegill prey size increased whereas gizzard shad consistently selected small prey. Therefore, differences in zooplankton size among lakes could differentially affect foraging success of larval fishes. In particular, systems with small zooplankton may represent ideal foraging environments for gizzard shad whereas lakes with large zooplankton may favor larval bluegill. If differential larval foraging translates to differential growth and survival, zooplankton size could influence recruitment success and ultimately fish community composition. La petite ouverture de la bouche chez des larves zooplanctivores de poisson les limite dans le choix des proies que ces larves peuvent ingérer; compte tenu de cette contrainte, la taille du zooplancton qui est ingéré influence la croissance des larves et ultimement leur survie. Pour déterminer si la taille optimale du zooplancton ingéré qui varie d´une espèce à l´autre de poisson en fonction de l´ouverture de la bouche, nous avons procédé à des essais d´alimentation sur des larves du crapet arlequin (Lepomis macrochirus, LT 10-26 mm) et de l´alose à gésier (Dorosoma cepedianum, LT 18-31 mm). Les larves (n = 10) ont été nourries pendant 1 h avec des groupes zooplanctoniques variables quant à leur taille; au bout de cette période, toutes les larves et le zooplancton qui restait ont été conservés. L´ouverture de la bouche des larves a été mesurée; le contenu stomacal des larves ainsi que le zooplancton ont été mesurés. Le crapet arlequin, l´espèce à la plus grande bouche, s´est nourri d´un zooplancton plus gros que celui des aloses qui avaient une bouche de la même grandeur. En outre, les plus grosses larves du crapet arlequin se sont nourries de zooplancton de plus en plus gros alors que toutes les larves de l´alose ont ingéré des petites proies (<0,60 mm). Plus le zooplancton restant était gros, plus le crapet se nourrissait de proies de plus en plus grosses; l´alose choisissait constamment de petites proies. Donc, des différences d´un lac à l´autre dans la taille du zooplancton pourraient agir différentiellement sur le succès de l´alimentation des larves de poisson. En particulier, les système...

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