Diet composition
The diet of cormorants in the 8-fjords is composed of both marine and freshwater fish species, the proportion of which vary across time and space. Although Gobiidae was the most abundant family, Gadidae was the most important in terms of biomass. This suggests that although fewer in numbers, Gadidae are important in allowing cormorants to reach their daily energy requirements [1]. While fish were the most numerous type of prey, crustaceans and even amphibians also appear to be targeted by foraging cormorants, although to a much lesser extent.
There was a considerable amount of overlap between the sample areas and years. The differences between areas, although significant, were very slight, which suggests that cormorants target the same prey families regardless of the area. Another hypothesis is that the species assemblages in the sampled fjords are not significantly different, which would explain the similarities between cormorant diets across the areas. The differences found between years could be due to cormorants changing their diet in response to temporal changes in fish assemblages in the area [2].
Comparing stomach and pellet diet data
Gadidae remains were found in almost the same proportions in both pellet and stomach samples, but in terms of biomass they were significantly more associated with pellets. This may be due to the overrepresentation of larger bones and otoliths in pellets, as they are usually more robust and will erode less dramatically in the digestive system [3,4]. This in turn explains the lack of more fragile remains from smaller fishes in pellets, such as stickleback spines, which are known to erode completely [5]. Thus, pellets may lead to an overestimation of the importance of codfish in the diet and an underestimation of other families, particularly those with small and fragile bones. However, stomachs may also lead to underestimations of fish proportions and sizes, depending on the digestion rates of the prey remains [5].
Comparing diet with monitoring survey data
The relative abundances of fish families were quite different between the trawl data and diet data, with cormorants eating a larger variety of fish families than were found in the trawl, and consuming many more goby than were caught by the trawl, at least in Askeröfjorden.
Codfish were much more abundant in the trawl than in the diet, but cormorants caught fish from a larger size range, particularly in Askeröfjorden, with several over 25 cm and some reaching over 40 cm – length classes that were not present in the trawl data. This could be due to cormorants hunting in more diverse habitats that the trawl cannot reach, as older juvenile cod are known to inhabit areas with rocky features and more complex relief [6-9].
Limitations
Due to the proximity of the fjords within this study, as well as the high mobility of foraging cormorants, there is no certainty that the birds shot in a specific fjord consumed fish from that fjord. This is most obvious when freshwater species are present in diet samples. Additionally, the lack of overlap between trawl data and diet data adds another level of uncertainty to prey provenance.
Erosion of important otolith features prevented identification to species level for many otoliths, thus impeding efforts to determine which species cormorants may be targeting, particularly in the case of codfish and flatfish as those are the species of interest in the 8-fjords area.
Conclusions
It is difficult to make any conclusions about the ecological impact of cormorants in the 8-fjords based only on trawl monitoring data, as this method does not offer a complete view of the available habitats and species present in the area and is not performed in lakes or rivers.
Biotelemetric tagging of cormorants may lead to a better understanding of their geographical foraging habits, while performing more extensive monitoring surveys using different methods may increase the knowledge of local species assemblages.
Further studies are needed to elucidate these aspects to gain a better understanding of the influence of cormorants on protected fish in the 8-fjords.
References
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- Boström, M. K., Östman, Ö., Bergenius, M. A. J., & Lunneryd, S.-G. (2012). Cormorant diet in relation to temporal changes in fish communities. ICES Journal of Marine Science, 69(2), 175–183. https://doi.org/10.1093/icesjms/fss002
- Larsson, A. (2017). A diet study of post-breeding Great cormorants (Phalacrocorax carbo sinensis) on Gotland. 2017:13. https://stud.epsilon.slu.se/11054/
- Fleet, F. (2021). Diet Composition of Great Cormorants (Phalacrocorax Carbo) in the 8-Fjord Area and Gullmarsfjord. Master’s Thesis. University of Gothenburg.
- Carss, D. (2012). The INTERCAFE Field manual: Research methods for Cormorants, fishes, and the interactions between tehm. COST, European Cooperation in Science and Technology.
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