The interactions between cormorants and wild fish populations : analytical methods and applications

Sammanfattning: Predation is the core in ecology, as a function in food webs which regulate both populations and communities. Seabirds are at the top of the food chain and key players in many aquatic food webs. So are humans, and in certain cases conflicts of resources arise. Cormorant predation on fish is probably one of today’s most well-known and wide spread human-wildlife conflict. Different species of cormorants have independently increased in numbers in several areas of the world. For some species, their predation has created a human conflict concerning resource competition (real or perceived competition) with both commercial and recreational fisheries. Though there is extensive research on cormorant diet we are far from reaching a consensus about how cormorant predation affects the environment. The aim of this thesis was to investigate how cormorants interact with wild fish communities and human fisheries. This was achieved by investigating cormorant diet composition, changes in diet over time, and between areas. The thesis also includes the first meta-analysis on cormorant diet, in which previous research investigating the effects of cormorant abundance on fish parameters were analysed. The results shows that cormorants generally have negative effects on fish populations, and control measures to limit predation generally have positive effects. Especially vulnerable to cormorant predation are species within the Percidae and Cyprinidae families. To some degree fish species and sizes in the diet overlap with those in fisheries catches (commercial and recreational). The predation on smaller sized fish however, is for some fish species more important in terms of competition with fisheries, as it results in less recruitment to commercial sizes. The diet analyses support earlier studies on temporal and spatial variation in the diet of cormorants. Essential knowledge for the management of fish, fisheries and cormorants is how cormorants affect fish populations. A misdirected effort in cormorant research is emphasized. Most studies fail to identify effects as they don’t relate diet with cormorant abundance and predation pressure. There is a need for systematically designed research, where cause and effects are studied. Future research should also consider an ecosystem approach, where indirect effects of predation are considered.

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