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Publication - Research and analysis

Bird stomach contents analysis - final report: Goosander and Cormorant diet on four Scottish rivers 2019 to 2020

Published
21 September 2022
Directorate
Marine Directorate, +1 more … Environment and Forestry Directorate
Topic
Environment and climate change, Farming and rural, Marine and fisheries
ISBN
9781804358337

This study analysed the stomach contents of goosanders and cormorants collected from the Rivers Tweed, Dee, Nith and Spey during 2019 and 2020 in order to to assess whether there was evidence of substantial changes in the diets of these species of fish-eating birds since the 1990s.

View supporting documents Supporting documents

4. Assessments of general diet

Monthly details of smolt run and autumn-winter period samples are show in Figures 2 and 3 and a full chronology is given in Appendix 3. The scientific names of all prey species are given in Appendix 4.

(a) Smolt run period

4a.1 River Dee Goosanders

Twenty carcases were available, 19 of them had stomach contents. Of these, all but one stomach contained Salmon (95%) and three quarters contained Trout (74%). Over half of stomachs contained Minnow (61%) and about a quarter contained 3-Spined Stickleback (28%). Two stomachs contained Eel (11%) and single stomachs each contained Lamprey and Pike (6% each, Table 3).

Table 3: The diet of Goosanders (n = 19) from the R. Dee (March-May 2019) as determined by stomach contents analysis.
No. (%) of Goosanders Fish species No. fish Estimated mass (g) % by mass
18 (95%) Salmon 142 588 50
14 (74%) Trout 44 313 26
12 (61%) Minnow 161 176 16
2 (11%) Eel 2 85 7
1 (6%) Pike 1 15 1
5 (28%) 3-Spined Stickleback 22 7 <1
1 (6%) Brook Lamprey 1 2 <1
N = 19 birds Totals 373 1186  

Comparisons with relevant historical samples (by time of year) from Marquiss et al. (1998) are given in Figure 8.

Figure 8: Diet comparisons for R. Dee Goosanders: smolt run sample period 2019 and three other broadly comparable samples (legend indicates relevant table numbers and sample sizes of stomachs with food from Appendix 5.1 of Marquiss et al. (1998).
Bar chart showing diet comparisons for River Dee Goosanders: smolt run sample period 2019 and other broadly comparable samples from historical study.

4a.2 River Nith Goosanders

Fifteen carcases were available, 13 of them had stomach contents. Of these, all stomachs contained Stone Loach and most contained Minnow (92%), Trout and Salmon (85% each). Just under half of stomachs contained 3-Spined Stickleback (46%) and single stomachs contained Eel and Lamprey (8% each), Table 4.

Table 4: The diet of Goosanders (n = 13) from the R. Nith (April-May 2019) as determined by stomach contents analysis.
No. (%) of Goosanders Fish species No. fish Estimated mass (g) % by mass
11 (85%) Trout 21 207 31
13 (100%) Stone Loach 59 174 26
12 (92%) Minnow 117 136 20
11 (85%) Salmon 22 105 16
1 (8%) Eel 2 36 5
1 (8%) River Lamprey 1 7 1
6 (46%) 3-Spined Stickleback 9 3 <1
N = 13 birds Totals 231 668  

Comparison with relevant historical samples (by time of year) from Marquiss et al. (1998) are given in Figure 9.

Figure 9: Diet comparisons for R. Nith Goosanders: smolt run sample period 2019 and four other broadly comparable samples (legend indicates relevant table numbers and sample sizes of stomachs with food from Appendix 5.1 of Marquiss et al. (1998).
Bar chart showing diet comparisons for River Nith Goosanders: smolt run sample period 2019 and other broadly comparable samples

4a.3 River Tweed Goosanders

Fourteen carcases were available, 12 of them had stomach contents. Of these, all stomachs contained Minnow, three-quarters contained Salmon (75%), and half contained Trout. Under half the stomachs contained 3-Spined Stickleback (42%), and around a third each contained Eel and Stone Loach (33% each, Table 5).

Table 5: The diet of Goosanders (n = 12) from the R. Tweed (April-May 2019) as determined by stomach contents analysis.
No. (%) of Goosanders Fish species No. fish Estimated mass (g) % by mass
12 (100%) Minnow 155 305 38
4 (33%) Eel 6 174 22
6 (50%) Trout 15 147 19
9 (75%) Salmon 21 120 15
5 (42%) 3-Spinedd Stickleback 45 26 3
4 (33%) Stone Loach 5 22 3
N = 12 birds Totals 224 794  

Comparison with relevant historical samples (by time of year) from Marquiss et al. (1998) are given in Figure 10.

Figure 10: Diet comparisons for R. Tweed Goosanders: smolt run sample period 2019 and four other broadly comparable samples (legend indicates relevant table numbers and sample sizes of stomachs with food from Appendix 5.1 of Marquiss et al. (1998).
Bar chart showing diet comparisons for River Tweed Goosanders: smolt run sample period 2019 and other broadly comparable samples

4a.4 River Spey Goosanders

Fifteen carcases were available, 13 of them had stomach contents. Of these, most stomachs contained Salmon (92%) and Trout (92%), just less than half contained Minnow (46%), just less than a third each contained Eel and 3-Spined Stickleback (31% each), and one stomach contained Lamprey (8%, Table 6).

Table 6: The diet of Goosanders (n = 13) from the R. Spey (March-April 2019) as determined by stomach contents analysis.
No. (%) of Goosanders Fish species No. fish Estimated mass (g) % by mass
12 (92%) Trout 31 245 40
12 (92%) Salmon 33 219 36
4 (31%) Eel 4 87 14
4 (31%) 3-Spined Stickleback 42 34 6
6 (46%) Minnow 32 27 4
1 (8%) Brook Lamprey 4 3 <1
N = 13 birds   146 615  

Comparison with relevant historical samples (by time of year) from Marquiss et al. (1998) are given in Figure 11.

Figure 11: Diet comparisons for Spey Goosanders: smolt run sample period 2019 and three other broadly comparable samples (legend indicates relevant table numbers and sample sizes of stomachs with food from Appendix 5.1 of Marquiss et al. (1998).
Bar chart showing diet comparisons for River Spey Goosanders: smolt run sample period 2019 and other broadly comparable samples

4a.5 River Dee Cormorants

No birds sampled in spring 2019.

4a.6 River Nith Cormorants

March/April 2019

Six carcasses were available, one was likely to have regurgitated its stomach contents (only a single [32mm/0.3g] 3-Spined Stickleback was found in the gullet), and two were categorised as 'empty' containing only a few, very eroded bones (sea fishes in one stomach and probably a large Salmonid in the other). The remaining three stomachs with food contained:

3 Perch (42% by mass), 12 Trout (20%), 22 Salmon (19%), 1 unidentified Flatfish sp. (11%), 2 Grayling (6%), 1 Stone Loach (<1%), and 2 Minnow (<1%). All three stomachs contained Trout and Salmon, two stomachs contained Minnow (66%), and one stomach each contained Perch, Flatfish, Grayling, and Stone Loach (33% each).

Due to the constraints of small samples (current and historical), no comparison is possible with previous data.

4a.7 River Tweed Cormorants

April/May 2019

Nine carcasses were available, 6 of them had stomach contents. Of these, most contained Trout and Salmon (83% each), two-thirds contained Minnow, and single stomachs each contained Grayling and 3-Spined Stickleback (17% each, Table 7).

Table 7: The diet of Cormorants (n = 6) from the R. Tweed (April-May 2019) as determined by stomach contents analysis.
No. (%) of Cormorants Fish species No. fish Estimated mass (g) % by mass
5 (83%) Trout 27 323 45
5 (83%) Salmon 39 289 40
4 (67%) Minnow 41 71 10
1 (17%) Grayling 1 42 6
1 (17%) 3-Sp Stickleback 2 1 <1
N = 6 birds Totals 110 726  

Some samples of Cormorants with food from the R. Tweed were small (including the April/ May 2019 one) and therefore too small to be sure they are representative of general diet. Nevertheless, the comparison with historical samples is of interest (Figure 12).

Figure 12: Diet comparisons for Tweed Cormorants: smolt run sample period 2019 and three other broadly comparable samples (legend indicates relevant table numbers and sample sizes of stomachs with food from Appendix 5.3 of Marquiss et al. (1998). Note: apart from the 1995 sample, others are too small to be sure they are representative of general diet.
Bar chart showing diet comparisons for River Tweed Cormorants: smolt run sample period 2019 and other broadly comparable samples

4a.8 River Spey Cormorants

April 2019

One carcase contained:

1 Salmon kelt (estimated FL = 47.5cm, estimated wt. = 907g).

(b) Autumn-winter period

4b.1 River Dee Goosanders

Two carcases during autumn 2019, 2 with stomach contents contained:

33 Salmon (estimated mass 259g, 72% by biomass), 8 Trout (93g, 26%), and 3 Minnow (7g, 2%).

Both stomachs contained Salmon and Trout, one of them contained Minnow.

Due to the constraints of small samples (current and historical), no comparison is possible with previous data, including that from the 2019 smolt run period.

4b.2 River Nith Goosanders

Ten carcases during autumn-winter 2019/20 were available, 10 of them had stomach contents. Of these, all contained Trout and most contained Salmon (80%), followed by Minnow and 3-Spined Stickleback (70% each). Over half the stomachs contained Grayling (60%) under half contained Stone Loach (40%), and two contained Common Frogs (20%, Table 8).

Table 8: The diet of Goosanders (n = 10) from the R. Nith (October 2019/February 2020) as determined by stomach contents analysis.
No. (%) of Goosanders Fish species No. fish Estimated mass (g) % by mass
10 (100%) Trout 81 546 41
6 (60%) Grayling 20 458 34
2 (20%) Common Frog 3 111 8
8 (80%) Salmon 11 98 7
7 (70%) Minnow 74 54 4
4 (40%) Stone Loach 10 37 3
7 (70%) 3-Sp Stickleback 64 25 2
N = 10 birds Totals 200 1329  

Due to the constraints of small samples (current and historical), no comparison is possible with historical data. Although the autumn-winter stomachs containing food is <12, a comparison is nevertheless made with data from the 2019 smolt run period (presented in Table 4) in Figure 13.

Figure 13: Diet comparisons for R. Nith Goosanders: smolt run sample period 2019 and subsequent autumn-winter (Oct 2019 – Feb 2020) sample (legend indicates sample sizes of stomachs with food).
Bar chart showing diet comparisons for River Nith Goosanders: smolt run sample period 2019 and subsequent autumn-winter (Oct 2019 – Feb 2020) sample

4b.3 River Tweed Goosanders

Twenty-four carcases during autumn 2019 were available, 24 of them had stomach contents. Of these, almost three-quarters contained 3-Spined Stickleback, and rather fewer contained Minnow (62%), and Trout and Salmon (58% each). Fewer than a third of stomachs (29%) contained Grayling, even fewer contained Stone Loach (21%), and one stomach (4%) contained Flatfish (Table 9).

Table 9: The diet of Goosanders (n = 24) from the R. Tweed (Sep/Nov 2019) as determined by stomach contents analysis.
No. (%) of Goosanders Fish species No. fish Estimated mass (g) % by mass
14 (58%) Trout 37 699 40
7 (29%) Grayling 18 606 34
14 (58%) Salmon 36 229 13
5 (21%) Stone Loach 22 99 6
17 (71%) 3-Sp Stickleback 177 75 4
15 (62%) Minnow 40 47 3
1 (4%) Flatfish 2 7 <1
N = 24 birds Totals 332 1762  

Comparison with relevant historical samples (by time of year) from Marquiss et al. (1998) are given in Figure 14.

Figure 14: Diet comparisons for R. Tweed Goosanders: autumn sample period 2019 and two other broadly comparable samples (legend indicates relevant table numbers and sample sizes of stomachs with food from Appendix 5.1 of Marquiss et al. (1998).
Bar chart showing diet comparisons for River Tweed Goosanders: autumn sample period 2019 and other broadly comparable samples

For the sampling periods of the current study, it was also possible to compare the autumn diet of Goosanders on the R. Tweed with data from previous smolt run period (presented in Table 7) in Figure 15.

Figure 15: Diet comparisons for R. Tweed Goosanders: smolt run sample period 2019 and subsequent autumn (Sep-Oct 2019) sample (legend indicates sample sizes of stomachs with food).
Bar chart showing diet comparisons for River Tweed Goosanders: smolt run sample period 2019 and subsequent autumn (Sep-Oct 2019) sample

Sample sizes of Goosanders from the R. Tweed were large enough in the current study to look for differences in diet between early and late autumn (Sep-Oct vs Nov 2019, respectively), as shown in Figure 16.

Figure 16: Diet comparisons for R. Tweed Goosanders: early autumn (Sep-Oct 2019) and late autumn (Dec 2019) samples (legend indicates sample sizes of stomachs with food).
Bar chart showing diet comparisons for River Tweed Goosanders: early autumn (Sep-Oct 2019) and late autumn (Dec 2019) samples

4b.4 River Spey Goosanders

Eight carcases were available, 7 of them had stomach contents contained:

57 Salmon (estimated mass 450g, 64% by biomass), 23 Trout (240g, 34%), and 13 Minnow (17g, 2%). All stomachs contained Trout, most contained Salmon (89%), and around a third contained Minnow (29%, 2 birds).

Due to the constraints of small samples (current and historical), no comparison is possible with previous data, including that from the 2019 smolt run period.

4b.5 River Dee Cormorants

Four carcases during winter 2019/20 were available, 3 of them had stomach contents contained: 1 adult Salmon (estimated mass 1020g, 60% by biomass), and 2 Pike (668g, 40%). One stomach contained an adult Salmon and the other two each contained a single Pike.

Due to the constraints of small samples (current and historical), no comparison is possible with previous data, including that from the 2019 smolt run period.

4b.6 River Nith Cormorants

Twelve carcases during autumn-winter 2019/20 were available, 12 of them had stomach contents. Three quarters of stomachs contained Grayling and most contained Minnow (58%). Half the stomachs contained Trout and Salmon, and a third contained 3-Spined Stickleback and adult Salmon. Single Rainbow Trout, Grey Mullet, and Stone Loach were each found in one stomach (8%, Table 10).

Table 10: The diet of Cormorants (n = 12) from the R. Nith (Nov 2019/Feb 2020) as determined by stomach contents analysis.
No. (%) of Cormorants Fish species No. fish Estimated mass (g) % by mass
4 (33%) Adult Salmon 4 1748 37
9 (75%) Grayling 48 1599 34
6 (50%) Trout 22 753 16
1 (8%) Rainbow Trout 1 432 9
6 (50%) Salmon 8 111 2
7 (58%) Minnow 24 54 1
4 (33%) 3-Sp Stickleback 21 8 <1
1 (8%) Grey Mullet 1 8 <1
1 (8%) Stone Loach 1 3 <1
N = 12 birds Totals 130 4716  

Due to a lack of historical samples and a small sample from the 2019 smolt run period, no comparisons are possible with previous data.

4b.7 River Tweed Cormorants

Twenty-seven carcases during autumn 2019 were available, 24 of them had stomach contents. Most stomachs contained Trout (58%) and just over half contained 3-Spined Stickleback. Just less than half of the stomachs contained Salmon and Minnow (46% each) or Grayling (42%). A single large Salmonid, adult Salmon, large Trout, Flatfish, and Gudgeon were each found in one stomach (4%, Table 11). Due to a lack of historical samples, no comparisons are possible with previous data.

Table 11: The diet of Cormorants (n = 24) from the R. Tweed (Sept/Nov 2019) as determined by stomach contents analysis.
No. (%) of Cormorants Fish species No. fish Estimated mass (g) % by mass
14 (58%) Trout 35 1292 26
1 (4%) Large Salmonid 1 1020 20
1(4%) Adult Salmon 1 694 14
10 (42%) Grayling 20 572 11
1 (4%) Large Trout 1 481 10
11 (46%) Minnow 215 446 9
11 (46%) Salmon 26 401 8
1 (4%) Flatfish 1 108 2
13 (54%) 3-Sp Stickleback 54 18 <1
1 (4%) Gudgeon 1 18 <1
N = 24 birds (100%) Totals 355 5050  

For the sampling period of the current study, it was possible to compare the autumn diet of Cormorants on the R. Tweed with data from the previous smolt run period (presented in Table 6), as shown in Figure

17. However, any interpretation must be made with caution as the latter sample (n = 5 stomach with food) is likely to be too small to be representative of general diet.

Figure 17: Diet comparisons for R. Tweed Cormorants: smolt run sample period 2019 and subsequent autumn (Sep-Oct 2019) sample (legend indicates sample sizes of stomachs with food). Note the sample size in April/May is likely to be too small to be representative of general diet.
Bar chart showing diet comparisons for River Tweed Goosanders: early autumn (Sep-Oct 2019) and late autumn (Dec 2019) samples

Sample sizes of Goosanders from the R. Tweed were large enough in the current study to look for differences in diet between early and late autumn 2019 (Sep-Oct vs Nov, respectively), as shown in Figure 18.

Figure 18: Diet comparisons for R. Tweed Cormorants: early autumn (Sep-Oct 2019) and late autumn (Dec 2019) samples (legend indicates sample sizes of stomachs with food).
Bar chart showing diet comparisons for River Tweed Cormorants: early autumn (Sep-Oct 2019) and late autumn (Dec 2019) samples

4b.8 River Spey Cormorants

Thirteen carcases during winter 2019/20 were available, 8 of them had stomach contents contained:

7 large Trout (estimated mass 4856g, 59% by biomass), 2 adult Salmon (2245g, 27%), 1 large Salmonid (1148g, 14%), 1 Salmon (7g, <1%), and 1 Minnow (3g (<1%). Most stomachs contained large Trout (88%), two (25%) contained adult Salmon, and single stomachs (12.5%) each contained a single large Salmonid, Salmon, and Minnow.

Due to the constraints of small samples (current and historical), no comparison is possible with previous data, including that from the 2019 smolt run period.

(c) Statistical analysis for some stomach contents samples

For a limited number of samples, it was possible to investigate statistically any differences in diet assessments (using K-S tests as described in section 2) with the dietary data under comparison being the estimated mass of each fish species recorded in each stomach that contained food. Historical data in the required form (i.e. mass of each prey species per stomach with food) were only available for a single previous sample (Tweed Goosanders, April 1992, N = 55 birds containing food). Consequently, statistical comparisons of general diet were made between samples of Tweed Goosanders in the smolt run period 2019, spring 1992, autumn 2019, and both early and late autumn 2019 separately.

Statistical comparisons between pairs of samples could not be made if all of the stomachs in either sample contained no records of a particular species: this was the case for Lamprey, Gudgeon, Flatfish and Grayling. Statistical comparisons were possible for one of the pairs of Eel samples and some of the Stone Loach ones but none were significant. In this necessarily restricted analysis, significant dietary differences were found for Salmon, Trout, Minnow, and 3-Spined stickleback (Table 12).

Table 12: Comparisons (D-statistics, KS- FFT p-values) between general diet assessments for a variety of Tweed Goosander samples in different seasons, details of which are given for either Sample 1 or 2. Statistically significant differences are highlighted. The large Autumn 2019 sample could be split in to an early (E) and a late (L) one. The only biologically meaningful comparison that could be made with the April 1992 sample was with that from the same time of year (Apr/ May) in 2019.
Tweed Goosander Salmon Trout Minnow 3-Sp s'back
Sample1 Sample2 D p D p D p D p
April 1992 Apr/May 2019 0.24 0.24 0.11 0.87 0.72 1.9e-7 0.27 0.03
Apr/May 2019 Autumn 2019 0.17 0.37 0.21 0.13 0.71 6.3e-13 0.29 0.01
Apr/May 2019 Autumn 2019 E 0.37 0.02 0.13 0.64 0.76 1.0e-8 0.35 0.02
Apr/May 2019 Autumn 2019 L 0.20 0.51 0.47 3.8e-3 0.75 4.8e-7 0.24 0.17

Salmon

The only significant difference for the Salmon component of the Tweed Goosander diet samples was between those from Apr-May 2019 and early Autumn 2019 (Table 12).

Boxplots for the mass (g) of Salmon in stomachs for the Tweed Goosander samples are shown in Figure 19a and ecdf plots in Figure 19b. As can be seen in the boxplots, the Salmon component of the diet in the Apr-May 2019 sample had a median mass of 5.5g, higher than the median mass of the early Autumn 2019 sample which was 0g.

Figure 19: Boxplots for the mass of Salmon from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods.
Boxplots for the mass of Salmon from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods.

Trout

The only significant difference for the Trout component of the Tweed Goosander diet samples was between those from Apr-May 2019 and late Autumn 2019 (Table 12).

Boxplots for the mass (g) of Trout in stomachs for the Tweed Goosander samples are shown in Figure 20a and ecdf plots in Figure 20b. As can be seen in the boxplots, the Trout component of the diet in Apr-May 2019 sample had a median mass of 3g, lower than the median mass of the late Autumn 2019 sample which was 46g (see also Figure 15).

Figure 20: Boxplots for the mass of Trout from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods.
Boxplots for the mass of Trout from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods.

Minnow

There were significant differences for the Minnow component of the Tweed Goosander diet samples for all comparisons (Table 12).

Boxplots for the mass (g) of Minnow in stomachs for the Tweed Goosander samples are shown in Figure 21a and ecdf plots in Figure 21b. As can be seen in the boxplots, the Minnow component of the diet in the Apr-May 2019 sample had a median mass of 22.5g, higher than the median mass of the April 1992 sample which was 0g (see also Figure 10), and of the Autumn 2019 sample when it was 1g (see also Figure 15), as it was in both early and late autumn samples.

Figure 21: Boxplots for the mass of Minnow from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods
Boxplots for the mass of Minnow from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods

3-Spined stickleback

There were three significant differences for the 3-Spined stickleback component of Tweed Goosander diet samples (Table 12).

Boxplots for the mass (g) of 3-Spined stickleback in stomachs for the Tweed Goosander samples are shown in Figure 22a and ecdf plots in Figure 22b. As can be seen in the boxplots, the 3-Spined stickleback component of the diet in both the smolt run period 2019 and April 1992 samples had a median mass was 0g, the median mass in Autumn 2019 was 1g, and in early autumn 2019 sample it was 2g.

Figure 22: Boxplots for the mass of 3-Spined stickleback from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods
Boxplots for the mass of 3-Spined stickleback from the stomach contents of R. Tweed Goosanders from various samples (a, left), and plots of empirical Cumulative Distribution Functions (ecdf) for the same data (b, right). Further sample details in text, note on boxplot presentation in Methods

(d) Broad summary: proportions of Salmon in the diet

Overall, samples for the two bird species on each of the four study rivers, collected over two time periods (the smolt run and autumn-winter), give a total of 16 samples (Table 13). However, three 'samples' (3/16 = 19%, indicated in red in Table 13) were void, either because there were no birds sampled (N/a) or no Salmon were recorded in those birds that did contain food (None).

Fewer than half of these samples (7/16 = 44%, indicated in green in Table 13) were large enough to be confident that they were representative of general diet (i.e. at least 12 stomachs with food, Marquiss & Carss, 1997). This proportion was highest for diet assessments during the smolt run period for Goosanders from all study rivers, and the autumn-winter period for both Goosanders and Cormorants from the R. Tweed, and Cormorants from the R. Nith. Some dietary information was available for the remaining samples (6/16 = 38%, indicated in amber in Table 13) and, although samples were small, it appeared to follow the patterns described below for other samples.

From Table 13, the proportions of Salmon in Goosander stomach contents from the smolt run period sample were highest from the Dee (50% by mass), followed by the Spey (36%), Nith (16%), and Tweed (15%). At this time of year, Salmon comprised the highest proportion of diet by mass on the Dee, and the second highest after Trout (36%) on the Spey. On the Nith and Tweed, four species were taken in higher proportions than Salmon (Trout, Stone Loach, Minnow on the Nith; Minnow, Eel, Trout on the Tweed).

Proportions of Salmon were low in the autumn-winter samples of Tweed Goosanders (13% by mass) and Cormorants (8%), and of Nith Cormorants (2%). In none of these samples were Salmon the main species in the diet by mass. Two species (Trout, Grayling) were taken in higher proportions by Tweed Goosanders, six others (Trout, large Salmonid, adult Salmon, Grayling, large Trout, Minnow) by Tweed Cormorants, and four others (adult Salmon, Grayling, Trout, Rainbow Trout) by Nith Cormorants.

Clearly, Cormorant diet in autumn-winter was often dominated by large Salmonids – presumably adult Salmon and Trout around the spawning period. In general, apart from the Dee sample in the spring and those (albeit with small sample size) from the Dee and Spey in autumn, the proportions of Salmon in Goosander stomach contents in either the smolt run or autumn-winter periods were not particularly high, and Salmon seldom comprised the highest proportion of the diet, other species often being consumed in larger proportions by mass. The same was true for the two autumn-winter Cormorant samples from the Tweed and Nith.

Table 13: The estimated proportion of Salmon in the diet assessments of 16 samples of Goosanders and Cormorants from four study rivers in the smolt run and autumn-winter periods 2019/20. For each sample, the proportion of Salmon in the diet by percentage biomass and in relation to other prey items is given.
River and bird species Smolt run period Autumn-winter period
Proportion of Salmon in diet assessment
By mass In relation to other species By mass In relation to other species
Dee Goosander 50%(1) Highest proportion(1) 72%(2) Highest proportion(2)
Nith Goosander 16%(1) 4th after Trout, Stone Loach, Minnow(1) 7%(2) 4th after Trout, Grayling, Common Frog(2)
Tweed Goosander 15%(1) 4th after Minnow, Eel, Trout(1) 13%(1) 3rd after Trout, Grayling(1)
Spey Goosander 36%(1) 2nd after Trout(1) 64%(2) Highest proportion(2)
Dee Cormorant N/a(3) - (3) None(3) - (3)
Nith Cormorant 19%(2) 3rd after Perch, Trout(2) 2%(1) 5th after adult Salmon, Grayling, Trout, Rainbow Trout(1)
Tweed Cormorant 40%(2) 2nd after Trout(2) 8%(1) 7th after Trout, large Salmonid, adult Salmon, Grayling, large Trout, Minnow(1)
Spey Cormorant None(3) - (3) 1%(2) 4th after large Trout, adult Salmon, large Salmonid(2)

(1) Samples of 12 or more stomachs with food

(2) Samples with fewer than 12 stomachs with food

(3) Samples with either no birds or no Salmon

Contact

Email: SalmonandRecreationalFisheries@gov.scot

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