There are many different causes of otter mortality, and these can be classified into natural or human-originated. Deaths can also be classified into violent, such as for example following an attack by a conspecific, or non-violent where no major external event is apparent. While it is relatively easy to diagnose the causes of violent deaths, it is more difficult to assess the causes of death of those animals that die non-violently. However, overall non-violent causes of death may have a greater impact on otter populations. In particular starvation may be an important cause of death, indeed natural deaths tend to be highly seasonal and occur at times of food shortages. Here below there is a list of possible human-originated causes of mortality for otters with a brief description for each.
Road Kill
This is the most evident cause of human-induced mortality for the Eurasian otter in Europe. Otters can die immediately upon impact and remain on the road where they are seen. Studies aimed at quantifying mortality causes of L. lutra have often found that road-kills are the most frequent cause of death, therefore most of the specimens for post-mortem studies are from road-kills. The effect of road kills on the population dynamics of otters are difficult to discern unless perhaps the population is so low that any accidental death could be critical. There seems to be a marked seasonality in the number of road kills, and casualties seem to be particularly frequent in the UK when rainfall and river flow are high. Most of the accidents tend to happen on major roads, close to water, at least in the UK.
Environmental contaminants
Environmental contaminants are considered the major threat to Eurasian otters and have caused recent local extinctions from which Western European populations are only starting to recover. This threat affects the general health of otters, as well as impacting on their reproduction. The contaminants that are most frequently blamed are dieldrin, DDT/DDE, PCBs and mercury. It is seldom that the concentrations are so high that the otters actually die from toxic effects, but that has happened in some cases in the past. For example, in Hungary an accidental release of cyanine in a river in 2010 killed otters, fish and other wildlife instantly. can act either directly, by intoxicating individuals and affecting health or reproduction, or indirectly, through damaging their food supply or habitat. The main dangers of direct intoxication come from biomagnifed dieldrin, DDT/DDE, PCBs and mercury. Indirect threats that reduce food supply come from nitrate-induced eutrophication from agricultural run-off and untreated sewage, and acidification from atmospheric pollutants. Oil spills threaten coastal populations. Pollution can have different sources but for otters the main culprits are use of insecticides in agriculture, oil spills, break-up of toxic reservoirs, and mines.
Accidental Killings
Otters can be accidentally caught in fish nets or traps and drown. While only a small numbers of otter mortalities may be reported to interested authorities, the proportions of deaths by drowning in some areas may be significant. Studies where these kind of deaths were recorded reported that up to 80% of deaths can be due to by-catch.
Illegal Killings
Otters compete with us for their aquatic prey and can consume a substantial portion of fish in some rivers. Also they can do a lot of damage to fish farms that are not adequately protected. For this reason they suffer persecution and can be killed with gun shots, poisoning or trapping. The most common traps used to catch otters illegally are probably leg-hold traps and neck snares. Instances of illegal killings have been reported for a number of otter species, but it is very difficult to quantify the importance of this kind of threat on otter populations due to its illegal nature . Otters can be legally trapped and killed in several countries, although this usually happens in those countries that have healthy populations of otters.