The Otter Lutra lutra in Essex 1996-2002
although a decline was recorded in the interim. The Brett has consistently performed well as the
most preferred of the tributaries. The Colne, having initially been colonized almost throughout, lost
its Otters by 1999 and only in the spring of 2002 were spraints again recorded but only in a limited
section of the river. It was supposed that these derived from a transient Otter possibly from the Stour.
The release of Otters to Hamford Water appears to have failed. Animals arc present on the Chelmer/
Blackwater system but seem to be struggling to increase in range and are making little use of tribu-
taries or headwaters. The Lea/Stort population still survives but again with little feeling of sub-
stance or consolidation while the sudden spurt of activity on the Roding has collapsed. Overall, the
Otter is still a rare mammal in Essex and although it has been present for the last seven years, there
is no obvious sign of genuine re-establishment.
The Otter population performance on all rivers, as revealed by the index, was much lower than that
found on rivers in the Welsh Marches, which were recolonized naturally (Mason & Macdonald
1993b). For example, on the River Lugg an index value of 17 was recorded in the sixth year follow-
ing colonization. The highest value in the current survey was 13.8 on the Stour in 1996, which
thereafter declined. No river has shown the consistent increase that was recorded on western rivers.
It could be argued that the rivers of eastern Britain, which are much more highly managed than those
in the west, could not be expected to have similar Otter population index values. However the River
Black Bourne, a Suffolk tributary of the Little Ouse, and the location of the first Otter releases in
Britain in 1983, has consistently produced an index value of greater than 18 over the last 10 years
(authors' unpublished observations). The headwaters of the Black Bourne are close to those of the
Stour catchment (especially the Brett) and this river is the most likely source of any colonization into
the Stour and rivers further south in Essex. We therefore believe that the low and fluctuating index
values in our study catchments represent a genuine poor performance in the Otter populations of
Essex.
Monitoring
The current situation of the Otter in Essex requires that the population be monitored on an annual
basis. This could either be done using a professional surveyor or by forming an Essex Otter Group
of volunteers under the auspices of the Essex Wildlife Trust. It is essential that volunteers receive
adequate training and reinforcement of that training. Otter monitoring is not as simple as it may
appear from a few hours spent in the field with an expert.
Monitoring should be conducted between February and mid-May, avoiding spate conditions. Records
of sites visited, sites found positive and the number of spraints per sprainting site should be held
centrally so that an annual distribution map can be produced, and the percentage of sites positive per
catchment and a catchment Otter index (as described above) can be calculated.
Otters in Essex - threats and conservation measures
Pollution
Macdonald & Mason (1994), Mason (1995) and Mason & Wren (2001) summarized the role of
pollution in influencing the distribution of the Otter. Effects may be indirect or direct. Indirect
effects, the most significant of which in Essex are organic effluents, reduce the food supply (princi-
pally fish) of Otters. The results of routine fisheries surveys, undertaken by the Environment Agency,
suggest that the impacts of organic effluents are minimal, apart from the South Essex catchment
(Crouch, Roach, Mardyke etc.) where fish stocks are poor. They may render the future colonization
by Otters in this region difficult.
172
Essex Naturalist (New Series) 20 (2003)