To obtain a quantitative expression of body condition, otter carcasses are weighed to calculate the condition index, K, which takes into account the substantial differences in overall body length between otters. A similar condition index is used for fish (Le Cren 1951), and can be calculated for any species of otter. It uses the relationship between average weight (W) and average body length (L) of animals, to be expressed as:
W = aLn in which a and n are constants, determined by the shape of the body, amongst other things. In a sample of 25 road-kills (therefore, presumably 'normal' Eurasian otters) we found that, on average, for females a = 5.02, n = 2.33, and for males a = 5.87, n = 2.39, with weight in kilograms and length (straight line from nose to tip of the tail) in metres. So, as weight-for-length, males are relatively heavier than females, on average.
The condition index for each individual is calculated as:
K = W/5.02L233 for females, and
If, in the above sample of road-kills, we use the calculated constants, the mean condition index K = 1.0. This, by definition, is for the average, normal, healthy otter. An animal that is underweight will have a value of K as low as 0.5, and a really heavy Eurasian otter may have K = 1.4 or more.
death related to body condition was found in Spain (Ruiz-Olmo etal.1998).
Shooting or trapping of Eurasian otters is illegal in almost all western European countries, but is still common in eastern Europe (Kruuk 2002). In Britain the animals were traditionally hunted with packs of specially bred otter hounds, until the 1960s. However, one still finds some otters killed by unscrupulous game-keepers (Fig. 12.9).
In Germany, Silke Hauer and colleagues (2002a) detailed the causes of death of 1067 Eurasian otters, and found 69% killed by traffic, 6% in fyke nets and 4% shot. Just as in our Scottish samples, this produces a biased view of mortality towards human causes. The study also showed a massive increase in deaths on German roads after the two Germanies joined together—the price of prosperity. Amongst other causes of death, Hauer et al. (2002a) mention pneumonia, starvation, infections and unspecified injuries, peritonitis, tumours, bites, drowning under ice and poisoning.
Otter mortality on UK roads was studied by Philcox etal. (1999), from the details of 673 dead animals. Some 56% were males, and there was a clear seasonality: both March and November saw about five times as many casualties as June. Most of the accidents happened on major roads, close to water (but mostly where roads run alongside, not places where they cross rivers). There was a good correlation between rainfall or river flow, and numbers of otter casualties. This may be explained by otters either avoiding crossing under a road when rivers are in spate, or making more overland trips during wet
Figure 12.9 Radiograph of a Eurasian otter killed by shotgun along River Dee, Scotland, 1991.
nights (when frogs are active, which would also explain the seasonality of this kind of death). Having followed otters with radio-transmitters walking overland over considerable distances at night, I have little doubt that both phenomena are important.
The most thorough post-mortem analyses of Eurasian otters were carried out on 77 otters from south-west England, of which 83% were killed by road traffic (Simpson 1997). A fairly large proportion, 12 animals (9 males, 3 females) showed bite wounds to face, legs or scrotum, probably caused by other otters, and this was the cause of death in 5 cases.
In summary, the immediate causes of death of Eurasian otters are many. There is no evidence that the ones we see most often (i.e. death caused by road traffic) are also the most important ones affecting the populations. However, with the observed seasonality of death in otters and conclusions drawn from dissections, it is likely that at least one other, ultimate, factor is important. This is discussed further below (population limitation).
For river otters, Melquist and Dronkert (1987) and Lariviere and Walton (1998) reviewed observations of a number of mortalities. They mention human activities as a major cause, although the same reservation applies as for the Eurasian otter: human-caused deaths are much more visible. Traps are a much more common end for a river otter than for a Eurasian one. Trapping is widespread in Canada and the USA, with many river otters caught legally each year for their pelts. In addition, road traffic and fish nets take their toll, as well as predation (see below) and diseases. In Idaho, five of six deaths of river otters with radio-transmitters were caused by humans (Melquist and Hornocker 1983). Even road-kills often go unrecorded, and this kind of death may be substantial. As an example, on just 16 km of one interstate road in Florida, 15 river otters were killed over a 7-month period in 2000 (Kinlaw 2005). This is a road through prime otter habitat, used by some 60,000 vehicles per day.
A documented example of the extent of mortality caused by trappers every year is the tally for 1978, which was 47,000 river otters in the USA and Canada (Kruuk 2002). The Nebraska Game and Parks Commission website (www.ngpc.state.ne.us/ wildlife/otters.asp, accessed April 2005) mentions that about half of the American states, and all of the
Figure 12.9 Radiograph of a Eurasian otter killed by shotgun along River Dee, Scotland, 1991.
Canadian provinces, have otter-trapping seasons. In some recent years, more than 50,000 otters have been taken in North America, and the annual otter harvest in Louisiana alone sometimes exceeds 10,000 animals, usually surpassing that in any other state.
Because of the large number of available carcasses from trapped river otters, there has been a considerable amount of research on ectoparasites and endoparasites, as well as bacterial, fungal and viral diseases. There have also been many relocation and reintroduction schemes for this species when otters were kept in captive conditions for some time (see Chapter 14), and in which there was a great deal of veterinary interest. The results have been reviewed extensively by Kimber and Kollias (2000). Most of the parasites and diseases appear to have little effect on individual otters, but some may be fatal. They include trematodes, salmonella, canine distemper, rabies, feline pan-leukopenia, hepatitis, pneumonia, tuberculosis and many others, but even for the potentially fatal ones the importance in otter populations is not known. Otters in captivity appear to be more vulnerable, as they lose part of their resistance against many different afflictions.
Sea otter mortality is often highly seasonal. At northern latitudes, for instance in the Aleutian islands, many were found to die especially in late winter and early spring, with carcasses washing up on the beaches (Bodkin etal. 2000; Kenyon 1969). The actual cause of death in these cases was not identified, but was likely to be related to starvation. Around Kodiak Island, Alaska, most deaths in sea otters with radio-transmitters was caused by (legal) shooting, by Alaskan natives (Monson and DeGange 1995). In later years, the role of predation has become much more prominent, and is discussed below. In contrast, the mortality rate of sea otters along Californian coasts is higher from spring to late summer (Estes etal. 2003a), and appears to have quite different causes.
One striking population decline occurred amongst Californian sea otters from 1976 until 1984, when numbers dropped from about 1800 to 1200. This coincided with a large increase in in-shore fish-netting (carried out mostly in summer), and many otters were caught and drowned. Once the practice was legally stopped, and nets were allowed only much further offshore, the sea otter decline ended (Estes etal. 2003a). However, another decline started in 1994 (see Chapter 11), and its cause proved more difficult to pin down.
Several researchers have noted that many Californian sea otters succumb to disease, and hundreds of beached carcasses have been analysed. Estes et al. (2003a) found that a large proportion of animals died during their prime middle age. They also noticed a considerable number killed by sharks (see below), and large numbers of carcasses in which the cause of death could not be identified (therefore quite likely to be from disease). At the same time, Kreuder et al. (2003), in a highly detailed analysis, pointed out that in 64% of the otter carcasses found along the Californian coast the animal had died from disease as a primary cause. Amongst the pathogens, the most prominent was Toxoplasma gondii, which turned out to be a highly significant affliction. Not only does this protozoon cause encephalites in sea otters and in many other terrestrial mammals, including humans, and is thereby an important direct cause of death, but interestingly it is also closely associated with other causes of death, such as heart disease, acanthocephalan infection (thorny-headed worm) and shark attack (see below). None of the Alaskan otters appears to have been in contact with this protozoon.
Toxoplasma is best known from domestic cats, and infections in people are usually a result of contact with cats. In California, many domestic cats may have been exposed to the pathogen, or actually carry the disease. The route of infection goes through faeces, and this is the way in which other mammals become involved. It seems quite probable that, given the high human (and domestic cat) population density along areas of the Californian coast, the sea otters there are prominently exposed to Toxoplasma. Coasts where sea otters have especially high exposure to the pathogen are those with a high run-off of fresh water.
In summary, evidence suggests that in California a sluggish speed of increase in numbers, then a decline in the population of sea otters, is largely due to increased mortality amongst adult animals. It is likely to be caused by a disease organism, Toxoplasma gondii, which may well ride on the coat-tails of our civilization. In other parts of their range, sea otters are disappearing even faster, a consequence of predation that may also be driven by the activities of humans (see below).
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