How does variation in fish availability affect the diet of Eurasian otters along sea coasts? Which prey (species, size) do the animals select from what is available by season and site? Within the range of fish available, as in Shetland, they specialize in bottom-living species—they do not aim for open-water species such as mackerel, or saithe and pollack in summer. But does their specialization go further than that? When taking bottom-living marine animals in shallow water, do otters specialize in any particular species, or do they take them opportunistically? Specialization could consist of searching particular sites, or of decisions of whether to carry to the surface the prey items that they encountered on the sea bed.
Clearly, Eurasian otters do select, whatever the mechanism. When we compare the otters' overall diet (see Table 8.1) with what is present along the coast (see Table 7.1), there are differences, also when allowing for biasses mentioned for the latter table. For instance, the butterfish, which, after the eelpout, is the most common benthic fish, is not often taken by otters—far less than eelpout. Butterfish are, if anything, easier to find and catch than eelpout in the same habitat; they are smaller, rather sluggish, and when we were scuba-diving we very often saw butterfish, but had to search for eelpout. In contrast, throughout the seasons otters ate five-bearded rock-lings about half as often as eelpout, although eelpout were almost thirty times more common overall. Individual rocklings were therefore much more vulnerable to otter predation than eelpout, and eelpout more than butterfish.
The Shetland otter diet cannot be compared directly with overall fish-trapping results, because the traps refused fish above a given size. This was particularly evident in winter when otters frequently took fairly large fish, such as ling, cod, conger eel and lumpsucker, that could not be caught in the funnel traps. It is not possible, therefore, to calculate a 'selectivity index' for prey species. However, there was sufficient evidence to show that otters specialized in rocklings and eelpout when these were available.
Alternative prey species included saithe and pollack in winter. These were always abundant, but only in winter did they enter into the dense beds of algae in shallow inshore water, where they could be caught both by our traps and by otters. Saithe, in particular, were present throughout the year in large shoals in open water, but out of reach of the otters. They vividly demonstrated the point that otters need to catch their prey on the sea bottom or inside dense vegetation. When otters did catch saithe and pollack, they were good-sized, profitable prey items.
Spring was the tough time for otters. During spring they caught many 'other prey', including three-spined and sea sticklebacks, and butterfish—all relatively small, and somewhat spiny, and to all intents and purposes rather miserable food. Of these, only the sea stickleback was actually more widely available at that time than at other seasons; the others were eaten presumably because more profitable species were absent.
An interesting marine prey was the shore crab, especially because most of the time it appeared to be largely ignored by Shetland otters, even in spring. There were masses of crabs, and when scuba-diving they could be seen scuttling away everywhere. In the fish-traps they were caught easily, most abundantly in late autumn and sparsely in spring. I saw a few of the young, newly independent, otters eating crabs, as well as some of the cubs still dependent on their mothers; there was also one very old female, with only stumps for canines, who habitually ate crabs. Apart from these individuals, I noticed that some otters we had provided with a radio harness (which they appeared to feel as an encumbrance) ate crabs for the first week or so. I concluded that crabs must be an inferior type of food.
This result was not at all surprising. Apart from the risk of being nipped by the crabs' ferocious claws, otters had to spend much time in landing them, unlike most fish which are eaten in the water. It was then quite a skill for the otter to remove the carapace, to get at the meat. When they did take a crab, otters did not eat legs or claws, just the soft central contents, which was a relatively small reward for the lengthy handling time. Our study area was quite representative for the whole of Shetland as far as these crab observations were concerned: whenever I walked the Shetland shores elsewhere, it was obvious, from many otter spraints, that crabs were eaten only rarely.
This is in contrast to the many places along the Scottish west coast where crabs were common in the otter diet and often dominated their food, as documented by Jon Watt (1995) on Mull. He found numbers of spraints, strikingly white and pink, consisting of nothing but crab remains. Despite this, Watt showed that, because of the long handling time, it hardly paid energetically for an otter to catch crabs. In fact, unless they were diving in conditions where they had an extremely low hunting success, otters were better off not taking crabs (in terms of quantity of food per time spent hunting) because they were foregoing chances of catching more lucrative fish prey whilst dealing with the crab. Watt's elegant calculations suggested that this would be true for Shetland otters, even more than for animals in his study area on Mull, because Shetland fish as prey are generally larger and more abundant. In Watt's study area it was especially young otters that took crabs (Watt 1993).
Eurasian otters appeared to have more difficulties foraging on the Scottish west coast than in Shetland. By fish-trapping, Watt showed that there were fewer suitable fish as otter prey along the Mull shore; in consequence, otters there ate many more butterfish (a low-quality species) than their Shetland relatives (who had butterfish available, but often ignored them). If the Mull otters caught a dogfish (a shark, often some 60 cm long; Fig. 8.10), they would eat it completely, tough skin and all, whereas in Shetland otters would only whip out the large, oily liver and consume that, abandoning the rest, and I never saw them eat more of a dogfish than just the liver. The Mull otters also ate whole crabs—carapace, claws and everything—whereas in Shetland otters would eat only the contents of the thorax and the abdomen from larger crabs.
Interestingly, if the data from Mull show that all of this potential food can be used, why do Shetland otters waste such a resource, or ignore it, especially as their food is likely to be a limited resource (see Chapter 11)? The answer to this question may possibly lie in the seasonality of the various foods. Many of the fish, and also crabs, are relatively scarce in Shetland just in spring, the only time of year when food is, or might be, at a premium, and when much of the otter mortality occurs (Kruuk and Conroy 1991; see Chapter 12). At other times of year prey is abundant, and Shetland otters can afford to select the best items. Seasonality of food is probably less pronounced on Mull than in Shetland; similarly, along the Norwegian coast there is less seasonality of fish populations (Heggberget 1993).
Apart from the otters' selection of prey by species, also selection by size could be important. The estimated median sizes of fish caught by Eurasian otters in Shetland (see Chapter 7) were considerably larger than those of fish caught in our traps. This was not likely to be caused by the traps being selective for size within this range (Kruuk etal. 1988). The results suggested, therefore, that otters took the largest, most
profitable, prey items throughout most of the year, although they were less selective in spring, during low prey abundance. On Mull and along the Norwegian coast, the fish that otters caught were of similar size to those caught in the fish-traps (Heggberget and Moseid 1994; Watt 1995).
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