The Otter Lutra lutra in Essex 1996-2002
The most significant direct acting pollutants are those which accumulate in food chains, causing
mortality, compromising health and reducing reproductive success. Dieldrin, mercury and PCBs
have all been implicated in the decline of Otters. The evidence for the effects of dieldrin is circum-
stantial. While fish, and especially Eels, from Essex rivers were widely contaminated with mercury
(Mason & Barak 1990), it is unlikely that levels arc sufficiently high enough to adversely affect
Otters, at least when acting alone. In experiments with mammals, it has been shown that PCBs cause
disturbance to female reproductive function, affecting the reproductive tract, the neuro-endocrine
system controlling puberty, oestrus and ovulation, and foetal and neonatal survival. There are also
effects on male reproduction (Mason & Wren 2001). PCBs at levels frequently found in the environ-
ment are known to inhibit reproduction of the closely related American mink (review in Macdonald
& Mason 1994; Mason & Wren 2001). Across Europe as a whole there is an inverse relationship
between mean PCB concentrations in Otter populations and the size of the population. In the British
Isles there is an inverse relationship between the regional distribution of Otters (the number of sites
found positive for the species in standard field surveys) and the average concentration of PCBs in
spraints (Mason 1995). High concentrations of PCBs are found where Otters are scarce, low con-
centrations where they are widespread. In Otters from Denmark, there was a strong negative corre-
lation between concentrations of vitamin A and PCB concentrations expressed as dioxin equivalents
(Murk et al. 1998). The reduction in vitamin A coincided with a higher incidence of diseases, en-
doparasites and pathological changes. A similar relationship between vitamin A and high concentra-
tions of organochlorine s, including PCBs, has been found in southwest England (Simpson et al.
2000). Concentrations of PCBs were high enough to cause adverse effects on the health of Otters.
Otter spraints in East Anglia were found to be widely contaminated with PCBs, and at levels of
concern, in 1989-91 (Mason & Macdonald 1993a). The sample included a small number of spraints
from the upper River Cam but no other sites in Essex were positive at that time. Spraints were also
collected over a six-month period following a release of Otters on the River Stort in 1991 and there
was a rapid increase in PCB contamination. One of these animals was killed by a car shortly after
release and tissue analyses revealed already elevated concentrations of PCBs (Mason 1995).
There is evidence, however, that there has been a general decline in PCB levels in Otters in the UK
(Mason 1998), Denmark (Mason & Madsen 1993) and Sweden (Roos et al 2001). It seems likely that
amounts of PCBs within Otters have fallen below critical levels (Mason 1998), allowing populations in
some regions to begin to recover. A sample of 25 spraints collected from the study area in 2001 had an
average PCB concentration of 4.27 mgkg-1 in lipid (G M. Troisi and C.F. Mason, unpublished data),
about half the average levels of spraints collected from East Angliaa decade earlier (Mason & Macdonald
1993a). There has been little change in the concentration of DDT. It seems unlikely that these current
levels of organochlorines will be having an adverse impact on Otter populations.
Traffic accidents
Road deaths are a problem throughout Europe. In Denmark for example, at a time when Otter
distribution was limited to one region, Hammershoj et ai (1996) recorded 60 deaths in 1994-95, 45
of which were due to collisions with vehicles. Green (1991) collated traffic accident figures. These
showed that 101 animals were reported killed in Britain (excluding Ireland) during the 1970s and
270 in the 1980s. Clearly these figures only represent minimum numbers of casualties and, within
that time, peoples' awareness and likelihood of reporting road kills increased but there still appears
to be a trend. As traffic volumes increase, numbers of deaths are also bound to increase and as Otter
populations recover, individuals will increasingly live close to people and traffic. In Essex the level
of traffic and density of busy roads is high and increases from north to south. Road kills will pose a
Essex Naturalist (New Series) 20 (2003)
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