No abstract

It is a fundamental feature of evolution that natural selection acts on individuals to adapt to predation pressure via the development of anti‐predator mechanisms. As such mechanisms are costly in terms of energy and time, species living in habitats where predators are rare or absent were expected to show reduced predator responses. Such a reduction was expected for larvae of the native whitefish (Coregonus wartmanni) in Upper Lake Constance, as these pass their initial stage in a historically predator‐free pelagic habitat and only start to encounter predators in a later ontogenetic stage during which they shift to littoral habitat. However, a recent invader of the pelagic waters of the lake, the three‐spined stickleback (Gasterosteus aculeatus), is implicated in a severe decline in whitefish recruitment through larval predation. The predator‐naivety effect and its consequences on predation were experimentally tested by comparing predator defenses of different age classes of whitefish with those of roach (Rutilus rutilus) and perch (Perca fluviatilis), two species whose larvae grow up in a predator‐holding habitat. As predator, the three‐spined stickleback (Gasterosteus aculeatus) was selected. Sticklebacks actively hunted all prey species but predation was high on whitefish only. The addition of zooplankton (Daphnia) as alternative prey for sticklebacks in the experimental setup did not reduce predation on the whitefish larvae, indicating a clear preference for whitefish in this invasive predator. The escape responses of roach and perch were found to be clearly more complex and diverse to those of the predator‐naïve whitefish larvae. Particularly striking was that whitefish often did not show any apparent behavioral response to the approaching predator. Only large whitefish larvae (Table 1: length = 40.8 ± 0.6 mm) appeared capable of escaping predation, and this was correlated with a change in complexity of predator escape responses. Thus, differences in predation pressure for the different larvae species were highly related to the observed interspecific differences in predator defenses, and with the ontogenetic intraspecific change in whitefish from poor and inefficient predator escape responses to complex and more efficient predator escape responses. The evolutionary consequences of stickleback as an invasive predator on whitefish larvae behavior are discussed.

The three-spined stickleback Gasterosteus aculeatus invaded Lake Contance in the 1940s and expanded in large numbers from an exclusively shoreline habitat into the pelagic zone in 2012. Stickleback abundance is very high in the pelagic zone in winter near the spawning time of pelagic whitefish Coregonus wartmanni, and it is hypothesized that this is triggered by the opportunity to consume whitefish eggs. Field sampling has qualitatively confirmed predation of whitefish eggs by stickleback, but quantification has proven difficult due to stormy conditions that limit sampling. One fundamental unknown is if freshwater stickleback, known as visual feeders, can successfully find and eat whitefish eggs during twilight and night when whitefish spawn. It is also unknown how long eggs can be identified in stomachs following ingestion, which could limit efforts to quantify egg predation through stomach content analysis. To answer these questions, 144 individuals were given the opportunity to feed on whitefish roe under daylight, twilight, and darkness in controlled conditions. The results showed that stickleback can ingest as many as 100 whitefish eggs under any light conditions, and some individuals even consumed maximum numbers in complete darkness. Furthermore, eggs could be unambiguously identified in the stomach 24 hours after consumption. Whitefish eggs have 28% more energy content than the main diet of sticklebacks (zooplankton) based on bomb-calorimetric measurements, underlining the potential benefits of consuming eggs. Based on experimental results and estimates of stickleback abundance and total egg production, stickleback could potentially consume substantial proportions of the total eggs produced even if relatively few sticklebacks consume eggs. Given the evidence that stickleback can feed on eggs during nighttime spawning and may thereby hamper recruitment, future studies aimed at quantifying actual egg predation and resulting effects on the whitefish population are urgently needed.

The fish community of Lake Constance, a large, deep, oligotrophic lake has undergone drastic changes in recent years, with the sudden rise to dominance of invasive three-spined stickleback (Gasterosteus aculeatus) in the pelagic zone, a rather atypical habitat for this species in Central Europe. The core objective of this study was to compare the feeding ecology of stickleback and young Eurasian perch (Perca fluviatilis) in this unique situation to identify reasons for this unexpected dominance, a possible food niche and feeding time overlap, and to discuss consequences for the reshaped pelagic fish community. The diel feeding patterns and prey compositions of pelagic sticklebacks and juvenile (0+) perch sampled in October 2018 and March 2019 were studied analyzing stomach contents. The diets of both species mostly comprised zooplankton, with copepods appearing in the greatest numbers. Benthic and airborne insects were consumed occasionally, mostly by sticklebacks. Both species exhibited peaks of feeding activity early in the morning, afternoon and dusk, and in both species, stomachs were fullest at dusk. Stickleback stomachs contained about 20% more prey at night than perch, and mean estimated nocturnal stomach fullness values were almost 50% greater. Night feeding in sticklebacks was confirmed by digestive states, pointing to a possible competitive advantage over perch. Dietary composition varied over a 24-h cycle and the pattern of consumption of different prey varied between the species. Perch consumed more comparatively small cladocerans (Bosmina spp.), while larger Daphnia appeared more often in stickleback stomachs. In both species, seasonal variation in diet mirrored food availability, indicating some degree of opportunism. A Morisita-Horn index value of 0.95 confirmed dietary niche overlap between species, suggesting the large population of sticklebacks may exert a competitive effect on juvenile perch when resources are limited. Both the longer feeding periods and greater intake of nutritive high quality prey like daphnids can contribute to the rapid success of stickleback in dominating the pelagic zone of Lake Constance.