The feeding ecology of the Great Cormorant

Great Cormorants inhabit coastal and inland freshwaters preying on fish, their main food source. In the Alpine foreland these birds initially were only overwintering, nowadays they have also established breeding colonies there. Although the feeding ecology of cormorants has been examined previously, important issues such as the actual spectrum of fish prey, and temporal, spatial or sex-specific variations in their feeding habits remain poorly addressed, mainly due to a lack of adequate methodology. The current proposal directly addresses these gaps of knowledge for cormorants in the Alpine foreland. Its aims are three-fold: 1. To assess the temporal and spatial variation in the trophic relationship between cormorants and their fish prey for both the overwintering and the breeding period. 2. To determine the characteristics of foraging grounds preferred by Great Cormorants in the Alpine foreland. 3. To examine if and how the feeding habits differ between male and female cormorants. There are five objectives to this three-year research programme: (i) to develop a DNA-based approach for identifying fish DNA in cormorant faeces and regurgitates (pellets) and to compare these outcomes with prey hard part analysis, (ii) to determine the diet of overwintering and breeding cormorants and to track temporal dietary changes in relation to fish phenology, (iii) to provenance the fish prey by analysing ear stones (otoliths) and to examine when foraging grounds are changed and how this is related to fish phenology, (iv) to assess the environmental parameters of preferred and avoided feeding grounds, and (v) to examine, based on molecular sexing of dietary samples, whether male and female cormorants differ in their feeding habits. Using a combination of non-invasive methods, feeding experiments and a two-year field study, this work will provide a detailed picture of the cormorant`s feeding ecology in the Alpine foreland. Aside from their ecological significance, the results obtained will provide a scientific basis for developing new management strategies for cormorant populations in Central Europe. Moreover, the molecular and micro-chemical methodology developed within this project will not be restricted to the analysis of cormorant-fish trophic interactions but it will also provide a new means for studying the feeding ecology of a wide range of other piscivorous vertebrates.

Great Cormorants are birds which inhabit coastal and inland freshwaters where they prey on fish. In Europe, cormorant numbers saw a steep increase over the past decades. Nowadays cormorants not only overwinter in Central Europe but have also established breeding colonies. This has increased the predation pressure on fish which has been controversially discussed between those perceiving cormorants primarily as pests and others which see cormorants as part of Europes native bird fauna. Key to a better understanding how cormorants impact fish populations is their feeding ecology. Although studies in the diet of cormorants have been conducted previously, important issues such as the actual spectrum of fish prey, and the temporal, spatial or sex-specific variations in their feeding habits remain poorly addressed. This is mainly due to a lack of adequate methodology to species- specifically identify fish prey and to assign it to its specific origin.This project directly addressed these gaps of knowledge for cormorants in the Alpine foreland. We developed new and highly sensitive molecular prey detection systems which allow to species-specifically identify fish DNA from cormorant pellets and faeces and faecal material of other fish-eating animals such as European otters. The molecular approach revealed that the diet of cormorants comprises a much larger range of fish species than detectable by conventional analysis of fish hard part prey remains. The molecular analysis also significantly increased the number of prey fish species detectable in the gut content of shot cormorants. By molecularly sexing cormorant pellets, we found that females consumed significantly smaller fish than males which also have a broader prey spectrum than females. As such, females and males potentially have a different effect on fish populations in the Alpine foreland. By analysing ~2,900 cormorant pellets collected fortnightly during two years at a cormorant colony at lake Chiemsee, we found strong evidence that cormorants directly react to fish phenological events such as spawning, when fish are preferentially taken. Most of the consumed fish were small to medium sized and only 20% of the prey was in adult, reproductive stage. Albeit the diet of cormorants is mainly composed of fish species of low commercial value, whitefish, which are commercially important, were mostly consumed in adult stage during the spawning period.Differences in water chemistry between rivers and lakes were reflected in otoliths of fish which dwell in these waters. By micro-chemically analysing fish otoliths retrieved from cormorant pellets, we were able to identify the clusters of water bodies from which cormorants were taking their prey. Most of the fish eaten by cormorants from the colony at lake Chiemsee were taken