Episodic zooplanktivory by largemouth bass (Micropterus salmoides) on Daphnia: a 25-year natural history record from a small northern temperate lake

Abstract

We observed episodic zooplanktivory by largemouth bass (Micropterus salmoides; LMB) ≥ 150 mm total length (TL) on the cladoceran Daphnia spp. over two decades in a small oligotrophic lake. While zooplanktivory is beneficial for young-of-year and juvenile LMB, dietary utilization of Daphnia by adults is unusual because zooplankton are not energetically favorable and the LMB buccal cavity is poorly adapted for filter feeding. We classified the threshold of ≥ 38 Daphnia per feeding event as an episode of zooplanktivory based on data from a zooplankton specialist, the bluegill sunfish (Lepomis macrochirus). In this study, 1718 LMB were caught, marked, sampled, and released. Gut contents at the time of capture were collected via gastric lavage. We captured the same 612 LMB 3 or more times. Of these, approximately 26% had at least one episode of zooplanktivory and approximately 6% had multiple episodes. There was no relationship between the population sizes of Daphnia and LMB, but the yearly percentage of individual LMB with at least one episode of zooplanktivory significantly increased with LMB population density. Episodes of zooplanktivory were more likely to occur later in the season when Daphnia were more abundant in the lake. The probability of an individual LMB having a repeat episode of zooplanktivory was not significantly different from expected; however, the probability of a LMB not having a single episode was significantly greater than expected by random chance signifying that some LMB were better than others at eliminating Daphnia from their diets. The probability of episodic zooplanktivory significantly decreased with increasing fish TL and weight but significantly increased with increasing Daphnia abundance. LMB without episodes of zooplanktivory achieved significantly greater maximum weight and body condition factor than those with them, but there was no statistical difference in their maximum TL or longevity (duration between first and final capture of the same LMB). In contrast, LMB with episodes of zooplanktivory had significantly greater instantaneous growth rates than those without. These results suggest that LMB within the resource-limited lake partitioned prey resources when some LMB sporadically targeted under-utilized, energetically unfavorable prey.