The ecology of the Roach (Rutilus rutilus (L.)) of Lough Neagh, Northern Ireland

Abstract

Roach were introduced into the eutrophic Lough Neagh catchment in the early 1970's and underwent a rapid population expansion over the following decade. The ecology of this introduced species was investigated from 1985 to 1988.

The roach population is dominated by young fast growing fish. As in many other recently colonised lakes in Ireland, the diet is mainly animal food. The most important prey items are chironomid larvae and pupae for adults, and Cyclops for underyearling roach. Food does not appear to be limiting and potential competition between roach and perch in the lough is reduced by partitioning of habitat and food.

The roach population undergoes seasonal onshore-offshore migrations, in addition to diel horizontal and vertical migrations, possibly linked to the avoidance of avian predators. Spawning occurs mainly in the rivers of the catchment in the latter half of May and early June.

During the years of this study the population experienced a dramatic decline in numbers, caused by the infestation of a high proportion of the younger year-classes with the ces tode Li g u la intestinalis. Multiple infections were common and the parasites·were found to comprise up to 33% of the host weight. Heavily infested fish show restricted seasonal and diel migrations and are located closer to the water surface than their healthy conspecifics. As a result high mortalities due to avian predation are thought to have occurred, especially in the first and second winter of life. 20% of a sample of piscivorous birds examined were host to
mature Lig ula. In addition, infected fish did not become sexually mature. Impaired recruitment as a result of the above effects led to the decline in roach abundance over the period of this study.

It is concluded that parasitism has had a marked influence on roach population dynamics during this invasive period, an influence that has been greatly reduced in more recent years. However, further oscillations in the population may be anticipated. The interaction of factors which are responsible for the overall success of roach in Lough Neagh are examined and the impact of this introduction on the fish community and the ecosystem of the lough as a whole is discussed.

Date of Award	Jan 1990
Original language	English