Cormorant-induced mortality in Western Baltic Cod (KoMoDo)

Poor Condition of Cod Population

The Western Baltic cod stock is in historically poor condition. The stock includes fewer spawning individuals than at any time since the 1980s, and juvenile production has been well below the long-term mean since 2015; the last strong year class was produced in 2016 (ICES 2022). Despite a sharp reduction in fishing effort and closure of the cod directed fishery, there is no sign of recovery. It is likely that non-fishing factors such as warming, eutrophication, habitat loss, and increases in natural mortality play an important role in the continued trend (Bryhn et al. 2022). In this context, predation of cod by cormorants is of particular interest.

Potential Role of Cormorants

The cormorant population of the southwestern Baltic Sea region (Denmark, Germany with the federal states of Schleswig-Holstein and Mecklenburg-Western Pomerania) increased sharply from 1970 to 1995 and has remained largely constant at just below 50,000 breeding pairs since then (Koop 2020). Fisherman perceive this relatively high number of cormorants as a source of competition and a threat to fish stocks and their management (DFV 2022). Currently, the level and trends of fish predation by cormorants are not considered when modeling stock dynamics of western cod. However, the results of recent studies suggest that cormorants may have meaningful impacts on fish stocks and fisheries along the Baltic coast (Bryhn et al. 2022; Haase et al. 2021; Pietrock et al. 2021).

A recently completed study by the IfB highlighted that cormorants had an unexpectedly high proportion of juvenile cod in their diet at a roosting site on Dassower See near the Bay of Lübeck (Pietrock et al. 2021) and brought into focus the possible role of predation in the poor state of the western Baltic cod stock. The monthly diet of cormorants at this site consisted of 25% to 96% cod (based on biomass), with predation concentrated mainly on one- to two-year-old fish between 12 and 43 cm (Pietrock et al. 2021).
However, these results originated from one single site and the temporal variability was very high. Other studies along the coast of Mecklenburg-Vorpommern have showed local variations of cod biomass fractions between 0 and 46 % (H. Winkler, personal communication). The representativeness of the results of Pietrock et al. (2021) for other locations and years thus remains unclear. Additional studies are needed for a more reliable assessment of the possible influence of cormorant predation on the dynamics, status, and rebuilding potential of the Western Baltic cod stock and other fish species in the region. A better understanding of the causes of stock dynamics is important to better assess the recovery prospects of the western cod stock and, consequently, the prospects for its future fisheries exploitation.

Study Aims

This study will focus on the following main areas:
1) quantification of cormorant predation on selected sections of the Baltic Sea coast with a particular focus on cod,
2) comparison of conventional and novel genetically-based methods for quantifying fish predation from cormorant pellet analyses, and
3) estimation of the potential impact of natural mortality by cormorants on the population dynamics of western Baltic cod.

This project is based on close cooperation between the IfB, the Thünen Institute for Baltic Sea Fisheries, the Fraunhofer Institute for Cell Therapy and Immunology: Branch of Bioanalytics and Bioprocesses, and several ornithologists. The work is thus strongly interdisciplinary, including expertise on cod and cormorant ecology, Baltic Sea and inland fisheries, population dynamics, and genetic analyses.

The project is funded by the fishery levy of the state of Schleswig-Holstein.

Literature:

Bryhn, A. C., Bergek, S., Bergström, U., Casini, M., Dahlgren, E., Ek, C., Hjelm, J., Königson, S., Ljungberg, P., Lundström, K., Lunneryd, S. G., Ovegård, M., Sköld, M., Valentinsson, D., Vitale, F., & Wennhage, H. (2022). Which factors can affect the productivity and dynamics of cod stocks in the Baltic Sea, Kattegat and Skagerrak? Ocean & Coastal Management, 223, 106154. https://doi.org/10.1016/j.ocecoaman.2022.106154
ICES. 2022. Cod (Gadus morhua) in subdivisions 22–24, western Baltic stock (western Baltic Sea). ICES Advice: Recurrent Advice. Report. https://doi.org/10.17895/ices.advice.19447868.v1
Koop, B. 2020. Ornithologische Begleituntersuchungen zum Kormoran: Bericht für 2020 vorgelegt im Auftrag des Ministeriums für Energiewende, Landwirtschaft, Umwelt, Natur und Digitalisierung des Landes Schleswig-Holstein, Kiel.
Deutscher Fischerei-Verband e.V. (DFV). 2022. Kormorane fressen mehr Dorsch, als Fischer fangen dürfen. Natürlich Jagd. https://www.natuerlich-jagd.de/im-original/kormorane-fressen-mehr-dorsch-als-fischer-fangen-duerfen/
Haase, S., K. Hüssy, M. Casini, K. Radtke, & U. Krumme. (2021). Eaten by a cormorant: Unexpected return of a tagged Baltic cod. 2021 International Workshop on Metrology for the Sea; Learning to Measure Sea Health Parameters (MetroSea), 283–287. https://doi.org/10.1109/MetroSea52177.2021.9611573
Pietrock, M.; Sternberg, N. 2021. Analyse von Speiballen zur Ermittlung der Nahrungs-zusammensetzung von Kormoranen in den Gebieten Plöner Seen, Untertrave und Schlei. Bericht im Auftrag des Ministerium für Energiewende, Landwirtschaft, Umwelt, Natur und Digitalisierung des Landes Schleswig-Holstein. Institut für Binnenfischerei e. V. Potsdam-Sacrow, 90 pp.
Kieckbusch, J.J. & B. Koop. 1996. Brutbestand, Rastverbreitung und Nahrungsökologie des Kormorans (Phalacrocorax carbo sinensis) in Schleswig-Holstein. Corax 16: 335-355.