Results

Diet composition

Total abundance of prey by family in cormorant diet samples (from both stomach and pellets), using minimum total number of prey ingested (8 827 individuals). Samples were pooled across years and areas

A total of 220 stomach samples and 32 pellet samples were analysed. Using prey remains found in the samples, we estimated that a minimum of 8 827 individuals had been consumed across all samples. A total of 26 families were identified. Overall, goby (Gobiidae) was the family with the highest relative abundance across all diet samples, followed by flatfish (Pleuronectidae) and codfish (Gadidae). In terms of biomass, Gadidae made up the highest proportion of the diet composition, followed by Pleuronectidae and Gobiidae.

Total weight of prey by family in cormorant diet samples (from both stomach and pellets), using minimum total number of prey ingested (8 827 individuals). Samples were pooled across years and areas.

To assess how year affected prey family biomass within fjords, four ANOSIMs were performed, comparing each fjord over two years. Only the ANOSIM performed to compare Halsefjorden between 2020 and 2021 showed a significant but slight difference in family biomass, with four families, Cottidae, Other, Cyprinidae, and Percidae, strongly associated with the year 2020.

Results from ANOSIM and multilevel pattern analysis performed to compare within-fjord prey biomass differences over time. Families with significantly strong associations with a specific group, according to the multilevel pattern analysis, have been placed in the appropriate row. The “Other” grouping represents prey from families other than the ten most abundant families.

The non-metric multidimensional scaling (nMDS) plot allows us to visualise the overlap of family biomass within samples across the different fjords, with some differences between the areas.

nMDS plot using pooled square root transformed abundance data. Each point represents one sample. The connected black points in the centre represent the centroids of the samples from each of the five areas. Large overlap of the different area groupings indicates high similarity between samples within different groups.

To assess how area affected prey family biomass, three ANOSIMs were performed on samples collected during 2020, 2021 and 2023. Only the ANOSIM performed to compare Askeröfjorden, Halsefjorden and Stigfjorden in 2020 showed a significant but slight difference between prey biomass in these areas. The “Other” group, Cottidae and Percidae were all strongly associated with Halsefjorden, while Gobiidae was strongly associated with Stigfjorden. No strong association was found for Askeröfjorden.

Results from ANOSIM and multilevel pattern analysis performed to compare between fjord prey biomass differences over time. Families with significantly strong associations with a specific group, according to the multilevel pattern analysis, have been placed in the appropriate row. The “Other” grouping represents prey from families other than the ten most abundant families.

Comparing pellet and stomach samples

Comparison of relative family biomass per sample between pellet and stomach samples sourced from Askeröfjorden in 2020.

The analysis of similarities run to assess differences in prey biomass by sample source at Askeröfjorden in 2020 found slight yet significant differences in prey families in stomach samples compared to pellet samples. A multilevel pattern analysis found that the Gadidae family was more strongly associated with pellet samples, while three families, Gobiidae, Pleuronectidae, and Labridae, had a stronger association with stomach samples.

Results from ANOSIM and multilevel pattern analysis performed to compare prey family biomass between pellet and stomach samples. Families with significantly strong associations with a specific group, according to the multilevel pattern analysis, have been placed in the appropriate row.

Comparing diet and available trawl data

Relative biomass of fish families in both diet (left) and trawl survey samples (right) sourced from Askeröfjorden in 2020.
Diet composition

In Askeröfjorden in 2020, Gadidae made up the largest proportion of both diet and trawl biomass, accounting for 66.7% and 46.8% respectively. Pleuronectidae was the second most important family, contributing 16.7% in the diet samples and 38.3% in the trawl samples. Gobiidae was the third family in terms of biomass within diet samples (5%) while Triglidae was the third family in terms of biomass within trawl samples (8.1%).

Length frequency distributions

In Askeröfjorden in 2020, the trawl survey data seemed to show a distribution centred around fish 10 cm in length. In comparison, there was no obvious pattern in stomach data, although the largest proportion of fish were between 10 and 20 cm. Gadidae caught during the trawl monitoring survey were generally shorter than those eaten by cormorants, with no individuals larger than 25 cm found, while cormorants caught individuals up to 45 cm in length.

Length frequency distributions of Gadidae (in cm) from both trawl survey (top) and diet (bottom) samples sourced from Askeröfjorden 2018. Fish caught in the trawl survey follow a relatively normal distribution centred around fish 15 cm in length. Diet data shows no distribution pattern. Cormorants caught larger Gadidae than the trawl survey.
Materials & methods
Discussion