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

Summary

Background

Assessments using best available scientific evidence to assess the potential impacts of fish-eating birds on prey populations require information on diet, numbers and behaviour of birds, and population dynamics processes of both birds and their prey fishes. This study considers aspects of the diets of fish-eating birds in some Scottish salmon rivers.

UKCEH was commissioned by the Scottish Government to assess whether there was evidence of substantial changes in diets of Goosanders (Mergus merganser) and Cormorants (Phalacrocorax carbo) since the 1990s. Samples collected under scientific licence organised by the Scottish Government, NatureScot and Fisheries Management Scotland, were available from two times of year during 2019: 1st March - 31 May 2019 (the smolt run period, a so-called "spring" sample) and from 1st September 2019-29 February 2020 (the "autumn-winter" sample).

In total, four rivers were studied- the Aberdeenshire Dee, Nith, Tweed and Spey. A total of 180 birds were examined, 108 Goosanders and 72 Cormorants. Birds were sampled from a variety of locations throughout each river catchment and all sampling locations are detailed, alongside accompanying maps. The main parameters of interest were the estimated proportions (by mass and by number) of Salmon and other fish species in the stomach contents, and the estimated length frequencies of Salmon consumed.

Diet composition

During the smolt run period, the proportions of Salmon (by relative mass) in Goosander stomach contents from 2019 varied between each of the four rivers sampled: 50% (Dee), 36% (Spey), 16% (Nith), and 15% (Tweed). This broad latitudinal trend of higher dietary proportions of Salmon in birds from northern rivers is consistent with earlier work (Marquiss et al. 1998). There were insufficient data to make the same comparison for Cormorants, or for Goosander diets in autumn/winter.

Comparisons of diet between 2019 and 1990s were made in cases where at least 12 individual birds with stomach contents were sampled. Relative mass of Salmon in the diets of Goosanders in all four rivers in spring was broadly similar between the two sampling periods, as it was for a sample of birds from the Tweed collected during autumn/winter.

There was evidence that Minnow (Phoxinus phoxinus) was significantly more commonly recorded (by mass) in the spring Goosander diet on the Tweed in 2019 than in the 1990s. A similar trend was evident in spring Goosander samples from the Dee and Spey and in spring Cormorant samples from the Tweed. Three-spined Stickleback (Gasterosteus aculeatus) also comprised a greater mass by proportion in the Tweed spring goosander sample in 2019 than in those from the 1990s.

Inadequate numbers of Cormorants were available to ensure spring 2019 samples were representative of general diet in terms of the relative proportions (by mass) of the different prey species consumed. Adequate samples, available from the 2019-20 autumn and winter for the Nith and Tweed, contained high proportions (by mass) of Grayling (Thymallus thymallus) and adult Salmon, which along with large Trout (Salmo trutta) were also recorded in the smaller number of birds sampled from the Dee and Spey.

Salmon size distribution

As yet there is no way that Salmon smolts can be differentiated from parr in the partially digested remains of most fish in stomach contents of birds. Although this and previous research does not explicitly categorise individuals as either smolts or parr, "larger" Salmon greater than 89mm in length are considered more likely to be either smolts or parr prior to smoltification. In autumn- winter 2019-20, Salmon >90mm were frequently recorded in samples of Goosanders from the Dee, Spey, and to a lesser extent on the Tweed, and Salmon > 90mm were also common in the Tweed Cormorant samples. In spring 2019, Salmon >100mm were much scarcer than they had been in the 1990s in Goosander samples from the Dee and Tweed, and spring Cormorant samples for the Tweed. Goosanders and Cormorants are considered to be 'generalist' foragers, taking the most easily available (often most abundant) local prey items. The data presented are thus consistent with evidence from other Scottish rivers of a steady, long-term (1963-2003) reduction in mean river age, and hence size, for juvenile Salmon. The results also indicate that on all rivers where there were adequate samples, predation on pre-smolts by Goosanders and Cormorants during autumn and winter could influence the strength of the smolt run in the following spring, although further research is needed to confirm and quantify this.