Eurasian otters and prey populations in freshwater habitats

Salmonid fishes were an important component of the Eurasian otter's diet in Scotland (see Chapter 7). In most freshwater streams in the north-east, brown trout Salmo trutta and Atlantic salmon S. salar dominated the fish fauna (Durbin 1997; Kruuk etal. 1993a). Their age-class distribution showed many 1-year-olds (3-5 cm in summer), fewer 2-year-olds (6-10 cm) and even fewer that were older than 2 years (Fig. 8.11). These and many other data were collected mostly by my colleague David Carss, by electro-fishing.

During electro-fishing, a section of, for example, 50 or 100 metres of stream is netted off on both sides, and fish are stunned by a pulsing electric current of 400 V between mobile electrodes a few metres apart. The operators fish out the entire section of stream, and all fish that are stunned are netted, measured, recorded and collected. The procedure is repeated three times, with 30 minutes between, with progressively fewer fish being stunned and netted. The total numbers of fish caught, and the differences between the three catches, are used in a formula to calculate the total numbers of fish present (Zippin 1958). The method provides a fairly accurate assessment of fish populations (Bohlin etal. 1989).

We found in one of the streams, the Beltie Burn, a tributary of the River Dee, that salmonid biomass varied between 9.2 and 14.4 g/m2. This is a commonly occurring density, and similar to values found elsewhere in Scotland, England, Denmark, Norway and North America (Bergheim and Hesthagen 1990; Egglishaw 1970; Elliott 1984; Mortensen 1977; Newman and Waters 1989). The biomass of other fish in the Beltie Burn, mostly eel, was only 0.5-1.6 g/m2.

The total salmonid population in our study areas was not evenly distributed through the rivers and streams, but, interestingly, it was concentrated in the narrower streams and tributaries. These small tributaries had a higher biomass per area than the wider rivers (Fig. 8.12). Durbin (1993, 1997) established that, of the two species studied, brown trout concentrated in narrower streams, whereas salmon tended

□ Salmonids present in stream -•- Salmonids in otter food

□ Salmonids present in stream -•- Salmonids in otter food

50 100 150

Fork length (mm)

Figure 8.11 Salmonid size selected by Eurasian otters in Beltie River, north-east Scotland. Otters ignore Atlantic salmon and brown trout of size less than 5 cm. Electrofishing results from 2718 fishes; diet calculated from 40 salmonid vertebrae in spraints. (After Kruuk etal. 1993a.)

50 100 150

Fork length (mm)

o lm

Figure 8.11 Salmonid size selected by Eurasian otters in Beltie River, north-east Scotland. Otters ignore Atlantic salmon and brown trout of size less than 5 cm. Electrofishing results from 2718 fishes; diet calculated from 40 salmonid vertebrae in spraints. (After Kruuk etal. 1993a.)

10 100 Width of stream (m)

Figure 8.12 Narrow streams carry more fish: biomass of Eurasian otter prey in Scottish streams and rivers of different widths (electro-fishing results on salmonid fish) (r = -0.71, P < 0.001).

to stay in wider waters. The differences in prey biomass in streams of different widths were reflected in the behaviour of the otters (see Chapter 4).

In addition, there were other fish species, such as minnows Phoxinusphoxinus, sticklebacks Gasterosteus aculeatus and lamprey Lampetra planeri, that were all eaten by otters. However, the species that, next to salmonids, was most important as otter prey in many places in the north-east Scotland up to the 1990s was the eel Anguilla anguilla (Fig. 8.13). David Carss showed, again with electro-fishing, that in some of the small streams there were as many as 2-16 eels per 100 m2 (in one stream as many as 80 per 100 m2). In terms of biomass this was 0.8-5.1 g/m2, and electro-fishing for eels in shallow Scottish lochs showed 1.4-4.0 g/m2. These figures are similar to those for eel biomass in streams elsewhere, in England, Ireland, Denmark and Norway (between 1.0 and 7.5 g/m2; references in Carss etal. 1999), and only a little lower than our estimates of benthic fish biomass along the coasts of Shetland (see above). In the Scottish lakes, there were also many perch Perca fluviatilis and pike Esox lucius. In a Portuguese stream, Beja (1996a) found up to 20 eels per 100 m2, as well as some 140 cyprinids of various sizes.

Ranking fish biomass by species in a number of river and lake sites in Scotland, Carss found that salmonids dominated in rivers and streams, in some more than in others, with eels coming second. In two lochs eels dominated, in one more than in another, followed by perch, then pike. When then we assessed otter diet from spraints collected in these same places, we found the same ranking of fish species as

Figure 8.13 Eels (Anguilla anguilla) in a Scottish stream-an important, but disappearing, prey.

in the electro-fishing results: prey was taken according to availability (Carss et al. 1998b). This result was similar to that of earlier studies, for example by Wise etal. (1981) and Chanin (1981) in lowland England, where otters took roach Rutilus rutilus, bullhead Cottis gobio, salmonids, eel, perch and pike largely according to their relative abundance in the ecosystem (as measured by netting and electro-fishing).

Nevertheless, Eurasian otters obviously select fish sizes, at least to some extent. The otter diet, from measurements of salmonid vertebrae in spraints collected at the same time where places from the salmonid population was assessed, showed that the predators ignored the youngest age-class, and took any fish older than that in relation to availability (Fig. 8.11). In contrast, Libois and Rosoux (1989) measured eel vertebrae from otter spraints along rivers in western France, and compared them with the size of eels caught by electro-fishing. They concluded that otters took eels of all size classes with similar frequencies as their occurrence in the population: there was no evidence of size selection.

In Hungary, Lanszki et al. (2001) studied otter predation in a species-rich ecosystem, which also included some fish ponds. Over a 6-year period they sampled the fish fauna of 19 species with small-mesh nets, and compared numbers and sizes with what they found in spraints of Eurasian otters. As expected, the researchers found generally close correlations between otter diet and fish presence, but there were interesting deviations from the general pattern of 'otters take what there is'. The animals had a significant preference for fish between 500 and 1000 g, and they avoided fish heavier than 1 kg, irrespective of species. Fish sizes below 500 g in weight were taken irrespective of size or species. In addition, the otters had a clear preference for fish that lived in dense aquatic vegetation, and in shallows. Species was not important, but habitat and size mattered— in a way similar to what we found in Shetland.

Just as in the sea, there are distinct seasonal fluctuations in the biomass of fish populations in streams and lakes that are different in various parts of the Eurasian otters' geographical range. In northern parts such as Scotland, spring is the lean time for otters; for instance, during May the biomass of salmonid populations is lowest, and highest during October (Egglishaw 1970). This is due largely to

Figure 8.13 Eels (Anguilla anguilla) in a Scottish stream-an important, but disappearing, prey.

temperature-related growth of fish, and is likely to apply to most species. However, there are compensations, provided by prey species other than fish.

Frogs and crayfish, which Eurasian otters take throughout freshwater areas in Europe and Asia, are seasonal even more than fish, and some of them are available only for short times of the year. Jean-Marc Weber (1990), for instance, noted that in Scotland frogs Rana temporaria were available to otters when they hibernated in mud at the bottom of streams and during mating, when they congregated in dense groups in marshes in early spring. Numbers taken by otters were closely correlated with numbers of frogs available in each marsh, with frogs making up to half of the prey items in the otters' diet. This occurred just at the time of year when fish numbers were lowest. Similar results were obtained by Lanszki etal. (2001), with toads Bufo bufo and frogs Rana ridibunda and R. dalmatina (Fig. 8.14). In the Iberian peninsula, too, amphibians are available to otters mostly during their breeding season, toads in winter, and frogs in spring and early summer (Beja 1991, 1995a,b).

The Louisiana crayfish Procambarus clarkii was introduced into Spain in 1973, and is now thriving in freshwater habitats throughout most of the Iberian peninsula. The species has become an important resource for many predators (Correia 2001). Amongst them are the otters, which consume large quantities (Delibes and Adrian 1987), especially in summer (Beja 1996b); the crayfish are less accessible at other, colder, times of year in their burrows.

Figure 8.14 A green frog (Rana perezi) in Portugal. After fish, frogs are the most important secondary food of Eurasian otters.

Summer is the time in Portugal when the native fish resources are at their lowest, which makes the presence of the exotic crayfish especially useful to otters as an additional resource.

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