The Otter Lutra lutra in Essex 1996 - 2002
SHEILA M. MACDONALD & CHRISTOPHER F. MASON
Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester C04 3SQ
Abstract
From 1996 to 2002 all river catchments in Essex were surveyed annually to determine the distribution
of Otters Lutra lutra. In 1996 27% of 234 sites proved positive. In the final survey 31% of sites had
signs of the species. Thus, overall there has been little growth in the population. Distribution has,
however, changed over time. Initially Otters were recorded on the catchments of the Stour and
Colne, at Hamford Water, on the Cam and on part of the Lea catchment. By 2002 Otters were still
found on the Stour and Cam, had re-appeared after several years absence on the Colne, had been
lost from Hamford Water and were present, but with limited distributions, on the Chelmer, Blackwater
and Roding. The Stour is the only major Essex catchment to have held Otters throughout the study
period and may be reinforced by movements of animals from Suffolk. It is likely that most, if not all,
of the populations in Essex derive from re-stocking within the eastern region; not all releases in
Essex, we believe, have been documented. The results for the county as a whole provide little
indication that the Otter population is beginning to thrive, consolidate and expand. There are still
long stretches of river and coastal wetlands that are apparently unoccupied.
The high level of traffic on the dense road network within the county poses a major threat to Otter
survival. Modification of bridges over rivers and canals to provide safe passage for Otters is urgently
required. It is unlikely that pollution is still generally exerting an adverse impact. Riparian habitat in
much of Essex is suboptimal for the species. To increase cover and resting sites for Otters, scrub
and reed beds should be encouraged, ideally as part of an extensive restoration of these degraded
floodplains.
Introduction
From the late 1950s Otters Lutra lutra suffered a drastic decline in both range and numbers over
much of the UK and Western Europe (Macdonald & Mason 1994), a decline that went largely
unnoticed. A number of reasons have been put forward to explain this including water pollution
affecting fish stocks, the loss of riverside habitat, and especially breeding dens, hunting, road traffic
accidents, drowning in Eel nets, and general disturbance (Macdonald & Mason 1994). However flie
suddenness of such a widespread decline suggests that the cause was some contaminant that reached
critical levels in the environment in the late 1950s (Mason & Wren 2001). An analysis of hunting
records indicated that Otter numbers began to fall at the time when the organochlorine pesticide
dieldrin was introduced into agriculture (Chanin & Jefferies 1978) and such pesticides were known
to have caused widespread mortality in birds, especially birds of prey. Dieldrin is still usually given
as the cause of the Otter decline in the UK though the loose temporal correlation is the only evidence
for a relationship. PCBs (polychlorinated biphenyls) were barely recognized as environmental
contaminants at that time, but subsequent work has demonstrated that these chemicals, used in many
industrial processes, increased exponentially in the environment during the 1950s and they are now
global contaminants. PCBs (like the pesticide dieldrin) are organochlorines and accumulate in body
fats. Some are potent endocrine disrupters. The evidence that PCBs played a major role in the
decline of the Otter is discussed below. Essex, with a large amount of industry and, situated in the
east of England downwind of sources of PCB release, is likely to have been particularly vulnerable
to contamination with these compounds.
Essex Naturalist (New Series) 20 (2003)
159