Fish Eating Birds and Salmonids in Scotland, page:14

Fish Eating Birds and Salmonids in Scotland

FIGURE 3.4. Length frequency distributions of salmon in bird stomachs

	Winter	Spring

	Red-breasted merganser


	Goosander


	Cormorant



	Fork length (mm)

Salmon remains were found in 761 stomachs, 52% of those containing freshwater fishes. The salmon eaten by birds were 38-179mm, the majority less than 140mm, much smaller than the trout consumed. Only 5% of the variation in salmon length was attributable to differences between bird species, 8% to location, 3% to time of year and 9% year. Overall, the salmon taken by red-breasted mergansers averaged 83mm, compared with 92mm fish taken by goosanders and 101mm fish by cormorants (F_{2, 729} = 24.3, p<0.001) (Figure 3.4). Larger salmon were taken in the winter months (F_{5, 729} = 7.9, p<0.001) (Figure 3.4). The average length of salmon in most years was 90 or 91mm; the significant difference between years (F_{6, 729} = 16.8, p<0.001) was mainly due to much smaller fish (mean 82mm) taken in 1993 and larger fish (100mm) in 1991. Thus, although variation amongst salmon lengths was highly significant, its magnitude was still small compared with that amongst trout lengths.

The numbers of trout and salmon consumed by sawbills and cormorants

The numbers of salmonids consumed by sawbills and cormorants (Chapter 4) were calculated from the abundance of birds and their daily food consumption, multiplied by the number of trout or salmon consumed per unit mass. Here we estimated the latter directly, expressing it as the numbers of salmon or trout per 100g fresh weight of stomach contents. As anticipated from trends in the other parameters, the variation between bird species and between locations accounted for most of the overall variation (Table 3.11). Less was attributable to year and time of year. These four factors could explain only about a third of the variation in the numbers of trout consumed compared with over half of the total variation in the numbers of salmon.

This was mainly due to the greater number of salmon consumed by birds on northern rivers, and more of the larger salmon consumed on northern rivers, in some years. Using latitude as a covariate in the analysis, 62% of the overall variation in the numbers of salmon taken was explicable in terms of bird species, latitude and season. When it came to the numbers of larger (>90mm) salmon taken, 68% of all variation was explicable in terms of bird species, latitude, time of year and year.

Table 3.11. Partition of the variation amongst samples (n = 185) in the numbers of trout and salmon taken by birds for every 100g of food consumed, associated with bird species, location, time of year and year.

Factor	Variable
	Number of salmon/100g	Number of large (90mm) salmon/100g	Number of trout/100g	Number of large (200mm) trout/100g
Bird spp.	0.28	0.15	0.14	0.07
Location	0.12	0.23	0.12	0.11
Time of year	0.08	0.10	0.02	0.01
Year	0.01	0.07	0.06	0.08

Multivariate ANOVA, bird spp (F_{8, 294} = 25.6), location (F_{84, 583} = 2.6), time of year (F_{24, 514} = 4.9) and year (F_{24, 514} = 3.7). All factors across all variates are highly significant (p<0.001).
Within cells, the variation is statistically insignificant (p0.05) for values of 0.02 or less.

The numbers of all trout consumed per 100g of fish intake varied ten-fold amongst locations and eight-fold between bird species, with most taken by red-breasted mergansers and fewest by cormorants (Table 3.12). Large trout were taken mainly by cormorants (on the Rivers Deveron and Laggan, and on Cobbinshaw Loch), goosanders took a few, and red-breasted mergansers none. The numbers of salmon consumed also varied considerably. Most were taken on northern rivers, by red-breasted mergansers and goosanders with the highest figures for ducklings and juveniles, in July and August. At that time of year relatively few of these salmon were 90mm or more. Birds took more of these larger salmon in winter and spring on some rivers but the figures only exceeded 2 per 100g intake, for cormorants on the Beauly, and sawbills north of the Tay. Note, however, these figures do not take into account differences in the Daily Food Intake of bird species, which is considered in Chapter 4.