Abstract
Abstract Perch (Perca fluviatilis) is an important prey species of the great cormorant (Phalacrocorax carbo sinensis) in the coastal waters of the northern Baltic Sea. The annual predation mortality caused by cormorants was estimated based on the consumed numbers of perch in relation to the abundance of perch in vulnerable age groups in the sea. We used existing stock assessment data (Archipelago Sea) or a simple population model in other areas (Gulf of Finland, Bothnian Sea, and Quark), based on commercial and recreational perch catches, and estimated fishing and natural mortalities, with distributions describing our understanding about the likelihood of different values for every variable. The median predation mortality was 4–10% annually, depending on the sea area. The area-specific 80% likelihood limits ranged from 3 to 21% annually. Compared to a recent perch tagging study in the Quark, the cormorant-induced mortality estimates were considerably lower, but the likelihood distributions partly overlapped. The results were sensitive to the estimated number of vulnerable perch in the population, which in turn largely depends on the natural mortality in young perch age groups.
Highlights
Cormorants’ (Phalacrocoracidae) effects on natural fish populations continue being a subject of debate, and even if several studies have been published, there seems to be no general agreement on the issue (Ovegård et al, 2021)
The estimated annual consumption of perch by cormorants was highest in the Bothnian Sea and Quark, the 80% probability interval ranging from about 500 to nearly 1000 tonnes, and lowest in the Archipelago Sea (200–500 tonnes; Table 5), due to the low abundance of cormorants (Figure 2)
The median estimate of the annual mortality caused by cormorants in the perch size classes vulnerable to cormorant predation was highest in the Gulf of Finland (9.6%), less in the Bothnian Sea and Quark (5.6%), and lowest in the Archipelago Sea (4.4%; Table 5, Figure 4)
Summary
Cormorants’ (Phalacrocoracidae) effects on natural fish populations continue being a subject of debate, and even if several studies have been published, there seems to be no general agreement on the issue (Ovegård et al, 2021). One reason to this is in many cases the lack of data on the prey fish populations (Arlinghaus et al, 2021). Perch is a freshwater species and favours relatively sheltered and shallow sea areas (Kallasvuo et al, 2016) It is a desired prey species for human fishers, and economically important for coastal commercial fisheries, which tends to cause conflicts between the fishing industry and environmental organizations (Svels et al, 2019). Perch is abundant in the coastal waters and preyed upon by many other predators such as pike (Esox lucius), pikeperch (Sander lucioperca), and large conspecifics (Heikinheimo et al, 2018)
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