Volume 16 Issue 2 Pages 58 - 110 (October 1999)

Citation: Madsen, A.B., Dietz, H.H., Henriksen P. & Clausen, B. (1999) Survey of Danish Free Living Otters Lutra lutra - a Consecutive Collection and Necroscopy of Dead Bodies IUCN Otter Spec. Group Bull. 16(2): 65 - 76

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Survey of Danish Free Living Otters Lutra lutra - a Consecutive Collection and Necroscopy of Dead Bodies

Aksel Bo Madsen¹, Hans Henrik Dietz², Per Henriksen^2* and Bjarne Clausen^2**

¹ National Environmental Research Institute, Department of Landscape Ecology, 14 Grenåvej, Kalø, DK-8410 Rønde, Denmark, e-mail: abm@dmu.dk
² Danish Veterinary Laboratory, Division of Wildlife Diseases, 2 Hangøvej, DK-8200 Århus N, Denmark, e-mail: hhd@svs.dk
^*present address: Danish Medical Agency, 378 Frederikssundsvej, DK-2700 Brønshøj, Denmark, e-mail: peh@dkma.dk
^**present address: Claus Nars Holm, 3520 Farum, Denmark

(received 8th July 1999, accepted 23rd July, 1999)

INTRODUCTION

The Eurasian otter Lutra lutra, is a highly vulnerable mammal in Denmark as well as in much of Europe (MACDONALD and MASON, 1994). In 1996 a national survey (HAMMERSHØJ et al., 1996) concluded that the species occurs in the northern part of Jutland; in the counties of Nordjylland, Viborg, Ringkøbing, Århus, Ribe and Vejle. On Zealand, in the county of Vestsjælland, no signs of otters were found in the national survey, but in a more detailed survey undertaken parallel to the national survey (LETH and BYRNAK, 1996), signs of otters were found at two sites (Fig. 1).

Figure 1. Geographical distribution of dead otters (n=193). The origin of one otter is unknown

It has been claimed that contaminants such as the organochlorine pesticide dieldrin, polychlorinated biphenyls, and mercury have been responsible for the rapid decline in otter populations in Europe (MACDONALD and MASON, 1994). Decreasing otter population in Denmark was thought mainly to be due to river regulation, wetland destruction, drowning in fish traps, and intensified traffic (MADSEN, 1991).

Otter carcasses have been collected annually in several European countries. In Germany e.g.. more than 50 otters were found dead each year, but only a small number of these were necropsied (ZOGALL and REUTHER, 1992). Likewise only 24 of 113 dead otters collected in Shetland were necropsied (KRUUK and CONROY, 1991). In south-west England only 77 wild otters were examined post-mortem (SIMPSON, 1997).

In this paper a comprehensive necropsy results of 145 carcasses submitted from a population of free living otters are evaluated to assess current threats to otters.

MATERIALS AND METHODS

Dead otters were received from hunters, motorists, anglers, forestmen etc. The otters were usually followed by written information about circumstantial evidence like killed on a road, died in a fish trap etc. Carcasses were frozen immediately upon arrival and stored at -18° C until necropsy was performed.

Necropsy

After thawing, the length (nose to tail) and weight was recorded. The animals were pelted followed by a routine necropsy procedure, including a search of the subcutis for lead pellets. Otters were aged as juveniles (less than about 5 months old) if tooth replacement was incomplete, as subadults (5-18 months) if the epiphyseal closure of humerus and femur at their proximal and distal ends was not complete or as adults (older than about 18 months). In males the length of the os penis was also used in ageing (van BREE et al., 1966). The craniums were cleaned from muscles etc. and the upper and lower jaw was inspected by a dentist.

Laboratory tests

Lungs and gut contents were examined for parasites, eggs and larvae from parasites using McMaster and modified Baerman techniques (HENRIKSEN, 1965, HENRIKSEN and KORSHOLM, 1984). Scrapings of epithelial lining from trachea, lungs, and urinary bladder from otters necropsied later than 1988 were examined for viral inclusion bodies using S3-staining and a routine immunohistochemical method to detect distemper virus. Bacteriological examinations (Aerobic cultures on blood agar), were performed on material from the digestive tract, lungs and kidneys.

The body condition (K) of otters was calculated using the equation K=W/(a x Lⁿ) where W=weight (kg) and L=total length (m) according to LE CREN (1951). The constants were those calculated by KRUUK et al. (1987) viz. a=5.02 for females, 5.87 for males; n=2.33 for females, 2.39 for males.

To test for differences in age distribution between the two sexes, a Chi²-test were used. Differences in length and weight of the two sexes and differences in body condition index were tested using a F-test.

RESULTS

194 otters were received of which 145 were necropsied. 52 otters were X-rayed. For some of the animals complete data were not received. Therefore, the number of individuals in the various examinations is inconsistent (Table 1).

The geographical origins and densities of the otters are given in Figure 1. The vast majority came from the Limfjord area. One individual found in 1979 came from the island of Funen. Half of the otters were found in or close to marine habitats. The annual number of carcasses received varied from two in 1979 to 31 in 1993 (Fig. 2). Major causes of death were identified as traffic mortality (88=45.4 %) and drowning (63=32.5 %).

Figure 2. Annual number of dead otters and cause of death (n=194)

No significant difference was found in age distribution between the two sexes, (χ²=0.43, d.f.=2, N.S.) (Table 2). Considerably more males (113) than females (79) were received during the survey. The weight and length of adult males were significantly larger than for adult females (t=9.60, df=65, P<0.001, weight; t=20.35, df=67, P<0.001, length). The condition index (K) of the otters had an overall mean value of 1.12, animals that died violently (traffic accidents and fish traps) had a value of 1.16 (Table 3).

The results of necropsy and the corresponding pathological findings are detailed in Table 4. No ectoparasites were found. Signs of endoparasites were found in only 5 individuals viz. two with one egg of Ascaridae per gram in the intestinal tract, one with one egg of Strongylidae per gram in the intestinal tract and one with Angiostrongylus vasorum larvae in the lungs. Two tapeworm Cestodae eggs per gram were found in the intestinal tract of one individual.

Inclusion bodies were found in 6 individuals, three females and three males of different age. These otters were all collected in the Limfjord area. The six otters were not believed to have suffered from clinical distemper.

Due to often severe decomposition bacteriological examination could only be applied to eight otters. Pneumonia due to bacterial infection was found in five individuals, four females and one male of which two were juveniles. One abandoned juvenile died from bacterial peritonitis two weeks after taken into captivity. Local infection with Streptococcus sp. was recorded in one animal.

Kidney stones consisting of ammonium urate were found in three adults, two males and one female, and two otters had a gall bladder enlarged by gall-stones. Two otters showed hypertrophy of the suprarenal glands. A small intestinal tumour possibly a leiomyoma (severe decomposition) and a minor umbilical hernia was seen in two otters, respectively. The eyes of one adult, male otter were completely opaque, probably causing total blindness.

Lead pellets were found in 9 otters (5%) in numbers from one to five pellets except for one individual carrying 14 pellets. The lead pellets were generally found in the pelt or subcutaneously and none were found in or close to vital organs. Parodontal disease was detected in 11 otters indicating a relatively high proportion of diseased animals.

DISCUSSION

Based on condition (K) of violent death otters there was no significant difference between otters from Denmark (Table 3) and from Shetland (KRUUK and CONROY 1991, K=1.08±0.15, n=49), (t=2.99, d.f.=171, N.S.) where thriving populations exist. The results agree with condition indices estimated by the authors from Danish data collected by JENSEN (1964) (K=1.13±0.16, n=81).

The increase in the annual numbers of submitted otters during the survey period (Fig. 2) might indicate an expanding population of otters (MADSEN et al. 1992) but a greater public awareness of otters cannot be excluded as the underlying cause of the increasing number of submissions.

The present results show that males achieve a larger overall size than females. MASON and MACDONALD (1986) classified animals weighing more than 4 kg as adults. In our study adults were classified as individuals with fully developed growth. One female with pneumonia but no emaciation weighed as little as 3.36 kg confirming that the weight and length alone may not be used as an indicator of age.

No ectoparasites and only small numbers of endoparasites were found. This indicates that in the present situation the otter is not parasitised very often, probably due to their solitary living and the relative scarcity of the species. However, decaying before collecting the dead otters combined with freezing might have disintegrated some parasites and larvae.

Except for the larvae of Angiostrongylus vasorum all other endoparasites recorded have been described earlier to occur in otters (JEFFERIES et al., 1990; SCHIERHORN et al., 1991; WEBER 1991). Otters forage on frogs, which might act not only as paratenic but also as intermediate hosts for A. vasorum (BOLT et al., 1993, 1995). None of the parasites recorded were considered to have influenced the health status of Danish otters.

Distemper virus in captive Eurasian otters was described by GEISEL (1979) and STEINHAGEN and NEBEL (1985). Our study is the first to record distemper virus in a free living population of otters, with the exception of two individuals from Austria (LOUPAL, in press). The fact that the infected otters were collected from the Limfjord area in a period, when distemper virus was present both in the common seal Phoca vitulina (BLIXENKRONE-MØLLER et al., 1989) and in major outbreaks of distemper in farmed mink in this area, indicates a wide range of host species for distemper virus. Negative findings in the remaining material may indicate a low propagatory rate of the virus in the population, but may also relate to the solitary life of otters and hence a low contact between animals.

Two cases of hepatitis probably causing severe health problems were seen. Pneumonic changes were found in five of 145 necropsied free living Danish otters. This corresponds to the findings of KRUUK and CONROY (1991) who found one case among 24 necropsied otters. Pneumonia has not hitherto been recorded in captive animals (ROGOSCHIK and BRANDES, 1991). One individual was recorded as blind in our study. WILLIAMS (1989) also reported blind otters from Britain during the period 1957-80.

Based on our study we would argue that only the two animals with hepatitis, and the five animals with pneumonia were likely to have died because of the diseases detected. In addition, one animal with peritonitis definitely died from this disease.

Since 1967, the Danish otters have been protected by law. During the period 1967-1982, fish farmers could be granted a special permission to kill otters at fish ponds but this exemption was terminated in 1982. However, this study shows that totally protected animals are still shot at. To the less experienced hunter an otter may be mistaken for a free living mink of which more than 8.000 are shot annually in Denmark (ASFERG, 1999).

The level of PCBs in otters from Denmark (MASON and MADSEN, 1993) is at the same level as found in 1988 among young common seals in the Limfjord area (STORR-HANSEN and SPLIID, 1993) and much lower than the 50 mg/kg which causes reproductive failure among mink in laboratory studies and which is assumed to be a critical level for otters as well (KEYMER et al., 1988; SMIT et al., 1994).

It is seen (Fig.2) that the number of otters dying in fish traps has decreased. It is believed that this is the successful effect of a 1986 compulsory use of stop grids in fish traps for fishermen (MADSEN and SØGAARD, 1994). It should be noted that traffic mortality constitutes 45% of the total mortality (males as well as females, young as well as adults) indicating the need for preventive measures where roads are crossing rivers in Denmark.

In conclusion, our results suggest that the population of otters seems healthy and in good reproductive condition (ELMEROS and MADSEN, 1999), although traffic mortality may constitute a threat to the spread of the population..

ACKNOWLEDGEMENTS - We wish to thank all the people who have delivered otters for this study and the staff at Natural History Museum, Aarhus for assistance with the preservation of specimens. Thanks are due to B. Gaardmand, and P. Mikkelsen for help with age determination and handling of the data. Parodontal examination was carefully done by Dr. F. Hjortkær. Thanks to Drs. J.W.H. Conroy and H. Kruuk for valuable comments on the paper during the first authors stay at the Institute of Terrestrial Ecology, Banchory Research Station, Scotland and especially thanks to the reviewer. The study was financially supported by the Danish Animal Welfare Society, the World Wide Fund for Nature and the National Nature and Forest Agency.

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