The first otters were probably fish-eating, Lutra-like animals. In later stages of evolution they branched out with crab-catching adaptations (Aonyx), and began to exploit echinoderms, molluscs and other invertebrates (Enhydra-like fossils). At present, all otters eat fish, as well as crustaceans, amphibians, molluscs, sea urchins and perhaps some other aquatic fauna. Their prey is relatively small; in the case of fish it is usually bottom-living and/or taken during its inactive period. In some otters there is variation in prey between the sexes, and individual otters of a given species may differ in their prey preference. Giant otters females feed their young with smaller fish than they take themselves; Eurasian otters with larger ones. To a large extent, otters of any species take fish and other suitable prey proportional to their availability.
The relationships between otter and prey populations show characteristics that appear to be general to many carnivores, and are equally applicable to predator-prey interactions in a more terrestrial environment. Where otters are unusual is in the energetic cost of their foraging trips, in the expensive requirement of keeping warm whilst fishing, even in tropical waters. To meet this, they need to eat the equivalent of 15-20% of their bodyweight every day, sea otters even 20-30%, and giant otters (in their warm waters) somewhat less (see Chapter 7).
All is well if fish occur in sufficient density for otters to be able to get their food quickly and eat. However, if they have to search for a long period, the expense of foraging increases so rapidly that otter life becomes difficult to sustain. This applies even to sea otters, which are permanently aquatic, but whose deep foraging dives take much energy, over and above what the animals need just to rest on the cold surface of the ocean.
In several areas where Eurasian otters were studied, they appeared to be living uncomfortably close to their energetic limits, and a relatively small reduction in prey availability makes, or would make, such places unsuitable for otters. This curious situation is caused by the negative relationship between foraging profitability and the length of time of cold-water foraging needed each day to stay alive. This is not linear, but approaches a negative exponential (see Fig. 9.12).
To reduce the high energy bill that otters have to face during foraging, they depend on thermo-insulation, especially in cold climates (Fig. 13.3), which is based almost entirely on the excellent qualities of the fur. This, in itself, gets otters into trouble with humans (see Chapter 14), but is not sufficient completely to prevent a considerable heat-drain in cold water. A more efficient option for thermo-insulation, such as blubber, is not available to otters, because of other demands on their mobility on land and in water (see Chapter 10).
Whilst often living close to the point where the curve of daily cost of foraging goes up sharply, otters also have to contend with the fact that their staple food is harder to catch than that of many other carnivores. Compare the skill and energy needed by an
otter to catch, for example, a trout with what a terrestrial mammal such as a badger needs to get at its earthworms, or a cat to ambush its voles. It is hardly surprising, then, that it takes a young otter more than a year to acquire these skills and efficiency, with initially a great deal of what appears to be 'teaching' by its mother (see Chapter 6). It is highly likely that this long period of dependence affects the breeding interval and lifetime reproductive output (see Chapter 11).
From the many studies on diet it is clear that otters do not take just any fish (Fig. 13.4), frog, crab or mollusc, but are selective. They do not, or only rarely, chase fast-swimming fish, and most otters leave many crabs alone as unrewarding; they concentrate on slow, bottom-living or resting fish species, preferably ones with a high lipid content such as eels or salmonids. Sea otters take those molluscs that are easy to extract, and otters that are crab specialists tend to take their prey in very shallow water, enabling fast processing.
Other efficient foraging strategies include hunting at the appropriate time of day (taking fish when they are inactive), in the sea at the right stage of the tide, hunting in shallows, avoidance of back-tracking, and the repeated use of well known, specific feeding patches, harvesting and reharvesting on an almost daily basis with the fish repopulating the empty niches before the otter returns (see Chapter 9). When
watching otters swim, dive and catch their fish, they leave us with little doubt that they have a detailed knowledge of their beat, just like an experienced fisherperson who knows the best pools and individual rocks from which to cast his or her fly.
Detailed knowledge of fishing sites and the kinds of prey likely to be encountered should have important benefits to otters. It appears that the expectation of success is expressed in the otters' success rates per dive: otters tolerate lower success rates per dive where prey is large or the water shallow. Only minimal changes in success rate occur between seasons, despite changes in fish availability. In other words, when otters dive they expect a given return for effort, and their knowledge of feeding areas enables them to maximize this (see Chapter 9).
It is important, therefore, for animals to be able to use the same resource-rich patches again and again, without interference from others. This need has implications for the social and spatial structure of the otter population, affecting even behaviour patterns such as scent marking or sprainting, which all otters (except Enhydra) do unusually frequently, compared with other carnivores.
I have argued that, on the one hand, a Eurasian otter in Shetland needs to be able to use different stretches of a sea coast, because various important prey species occur in given sites and seasons. Some fish need sheltered areas, some need exposed sites, and different vegetations of algae play a role. But, on the other hand, once an otter has a range encompassing such a variety of sites, it can allow others to share, without competition for food, provided they can avoid exploiting exactly the same patches (see Chapter 5). This, it appears, is achieved by organization into core areas, and a signalling system with spraints that informs other otters that someone is already exploiting a particular site (see Chapter 6). This enables newcomers to keep out, to the benefit of the animal who was there first as well as to the newcomer itself (the latter because it does not have to waste energy by exploring an already partly emptied site).
One may expect, a priori, that food is likely to have its main impact on populations of otters, and on the behaviour of the animals, during periods of shortage or low abundance—hence the increase in sprainting during such periods (see Chapter 6). A first ecological effect to look for is the choice of prey; during shortages, low-preference food (prey such as rabbits, birds or fish carrion in the case of Eurasian otters) is taken more often. This may be the case also for other otter species. The role of amphibians in the diet, especially frogs, is somewhat ambiguous; they are taken mostly, and in large numbers, during times when fish stocks are low, but that is also the time when frogs themselves are much more available to otters. At least for Eurasian otters, frogs appear to be an excellent prey, of the right size and in the right kind of places (and for some populations frogs are a staple).
Studies on Eurasian otter feeding ecology in fresh water show that the quantity of fish consumed by an otter population may be large in relation to the fish populations themselves. Otters in some areas may eat considerably more per year than the actual total 'standing crop' of fish. Annual fish productivity is high, often larger than the standing crop itself (see Chapter 8), but even if we express otter predation as a proportion of fish productivity, rather than in relation to biomass, it is still very considerable—in our Scottish study area, more than half. This is likely to be true in many areas with good populations of otters, dealing with different species of fish. In the case of sea otter predation, it was clearly demonstrated that this seriously depletes prey populations of sea urchins and shellfish (see Chapter 8).
Under such conditions it is likely that prey numbers will determine numbers of otters, and, indeed, we found that there are more Eurasian otters where there is more fish, in a significant positive correlation. In this complicated scenario, otters may affect fish numbers, but at the same time fish numbers are likely to be determined partly by food conditions in the streams and lakes. Given the high rate of predation, under certain conditions, otters could be competitors for food (fish) with other predators, such as humans. This has been demonstrated only for sea otters and shellfish, but the ingredients are there for such competition between other species with fisheries elsewhere. There is, for instance, suggestive evidence that Eurasian otters possibly affect populations of Atlantic salmon and brown trout (see Chapter 8). They are certainly perceived to damage salmonid and cyprinid populations (see Chapter 14), especially also in fish farms, which is an important factor in conservation management. Further, spotted-necked otters and other species take large numbers of fish from nets in Africa and elsewhere (see Chapter 11).
One interesting complication, in the relationship between populations of otters and fish, is the fact that, at least in species such as the North American river otter and the Eurasian one, male and female otters do not use the same habitat. They live in different waters, and eat different sizes of fish. Males of Eurasian otters are shown to live along more exposed coasts and in larger rivers, feeding on larger prey, but there is, of course, much overlap between the sexes. In winter in mainland Scotland, it is the male Eurasian otters that often take large salmon (themselves also mostly male fish). Strikingly, in the otter 'female areas' in the sheltered bays in Shetland, the main food species, eelpout, showed a large increase in numbers in summer, and we demonstrated that this was closely associated with otters' reproductive success. It is possible, therefore, that fluctuations in numbers of given prey species affect male and female otters differently, but much more data are needed before we begin to understand the implications of this.
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