Feeding Aquatic Ecosystems : Whole-Lake Experimental Addition of Angler’s Ground Bait Strongly Affects Omnivorous Fish Despite Low Contribution to Lake Carbon Budget

Abstract

It is well documented that aquatic ecosystems may be subsidized by naturally derived terrestrial carbon sources. In contrast, the intentional or unintentional subsidy of animal populations by human-derived feed resources and their ecosystem effects are poorly studied. We added ground baits of the type, amount and temporal duration commonly applied by anglers targeting cyprinid fishes to a small lake, and studied behavior, diet composition and annual growth rate of the lake fish community in response to the bait addition. Based on recordings by a high-resolution ultrasonic telemetry array, common carp (Cyprinus carpio) as a model benthivore spend more time at the sites where ground baits were added, and they significantly reduced their home range relative to the period before bait addition. Furthermore, many omnivorous cyprinid fish species were regularly caught by angling at the feeding sites, indicating active ingestion of the artificially added food items. Stable isotope analyses of 13C and 15N showed that in particular larger specimens of common carp, bream (Abramis brama), white bream (Blicca bjoerkna), tench (Tinca tinca), roach (Rutilus rutilus) and rudd (Scardinius erythrophthalmus) included substantial amounts of angler baits into their diet. There was also a significant acceleration of growth in bream and white bream after bait addition, most likely attributable to the energetic benefits from bait addition. In contrast, there was no response in annual growth rate in top predators, suggesting they did not benefit from the subsidy. The amount of carbon introduced was very low (about 1%) relative to an estimate of in-lake C fixation by autotrophs. However, if the C added by bait was compared with the coarsely estimated secondary production of benthic macroinvertebrates in the lake, the C available to benthivorous and omnivorous fishes was comparable between natural resources and angler baits. We conclude that human-derived feed resources associated with recreational fishing may constitute a substantial subsidy to omnivorous fishes in lakes, in particular if the food is provided in form of particles, which are readily accessible and found at repeatable feeding places and over sufficiently long time periods during a year. The long-term consequences for the receiving ecosystems still have to be elucidated.