The ecology of a threatened fish: the Pollan (Coregonus Autumnalis) in lough Neagh, Northern Ireland

Abstract

Owing to its recent glacial history, Ireland has a depauperate native freshwater fauna of only 14 species. Of the native species, only one, the pollan (Coregonus autumnalis Pallas 1776) is not found elsewhere in Western Europe, imparting it with a unique conservation standing e.g. it is a UK Biodiversity Action Plan Priority Species. Although Ireland has more than 4000 loughs larger than five hectares, pollan only occur in four large lowland loughs (Lough Neagh, Lower L. Erne, L. Derg and L. Ree) The ecology of these relict Irish populations contrasts markedly with that of the species in the remainder of its typically Arctic distribution, as they are non-anadromous, short-lived, and display early maturation and grow rapidly.

Recent work has shown that Lough Neagh supports the last abundant population of C. autumnalis in Western Europe, with pollan in Lower Lough Erne, Lough Derg and Lough Ree contributing less than I % of survey catches. The Lough Neagh population underwent marked shifts in size structure and spawning behaviour in the mid-1990s and it was feared that it was undergoing a decline following continued cultural eutrophication and the establishment of a large, dominant roach (Rutilus rutilus) population.

The study involved a robust and detailed assessment description of the autecology of the Lough Neagh pollan population between November 1997 and December 1999. Examination of a series of ecological characteristics known to be sensitive indicators of population decline (e.g. growth, mortality, age structure, condition, reproductive investment, diet) provided little evidence of a decline. When the changes in the population were considered in the context of long-term population dynamics, it became apparent that the shifts in size structure were a result of poor recruitment and elevated adult mortality rate. The study included a detailed examination of the factors regulating the pollan population, including a range of abiotic (temperature, dissolved oxygen) and biotic (human exploitation, interspecific competition, parasitism, predation) factors. A major component of the study involved the description the infection dynamics of Ichthyocotylurus erraticus, the principal parasite of pollan. This trematode worm encysts in elevated numbers on the heart and in the pericardia! membrane of pollan. Although thought likely to harm hosts, no evidence of parasite-mediated mortality was shown.

Several observers have suggested that the decline of pollan elsewhere in Ireland was due to interspecific competition with the introduced roach. The relative competitive abilities of 0+ pollan and roach were examined in a series of tank studies, and pollan were shown to outperform 0+ roach. There was a positive relationship between recruitment of roach and pollan in Lough Neagh, indicating that additional factors than interspecific competition were influencing the performance of juvenile roach and pollan in Lough Neagh.

Finally, the study included an assessment of the conservation status of pollan, and concluded that there was a pressing need for prompt conservation action in order to ensure the long-term survival of pollan in Western Europe.

Date of Award	2001
Original language	English
Supervisor	DAVID GRIFFITHS (Supervisor)