The interactions between the macro-invertebrates, fish, and diving ducks of Lough Neagh, Northern Ireland

Abstract

Macro-invertebrates in Lough Neagh were monitored from October 1997 to March 2000,and fish from October 1998 to March 2000. The macro-invertebrates showed seasonal trends in abundance at a number of different sites within the lough, although no consistent overwinter declines were recorded. Depth had the greatest influence on the macro-invertebrate fauna, while specific differences between Antrim and Ballyronan Bay were recorded. Asellus aquaticus was absent, and ValvaJa spp. present in reduced numbers in Antrim Bay, compared to Ballyronan Bay.

Roach and pollan, both in terms of numbers and biomass, dominated the fish population. Large individuals dominated the roach population, a situation not previously observed. The pollan population was dominated by two age-classes.

The numbers of tufted duck and pochard declined during the mid 1980's, but since that time numbers have been consistently higher. The distribution of the ducks within Lough Neagh shows a number of patterns, both during winters, and over successive winters.

Fish diet and selectivity was investigated, with particular reference to the influence offish size, location of feeding, and season. Larger roach generally fed on larger prey items, with non-chironomid (Valvata spp. & A. aquaticus) prey being of greater importance.' An overwinter shift in the roach diet was observed, with Tanytarsini becoming more important as winter progressed. The different diets recorded at the various depths reflected differences in the macro-invertebrates present. The diet of visually feeding fish (pollan, perch, and trout) was dominated by Mysis relicta, with Gammarus spp. and A. aquaticus occurring in considerable numbers in trout.

The diets of the diving ducks were examined with respect to differences between species, sexes, and location of capture. It was found that pochard generally fed exclusively on chironomid larvae, while molluscs and A. aquaticus were also taken by other diving duck species. There was little difference in the diet of ducks caught on the same occasion, although considerable differences were recorded for diving ducks caught on separate occasions.

There is substantial dietary overlap among the fish species, particularly among the visually feeding fish (pollan, perch, trout)-and among the cyprinids (roach, bream),although overlap between these two groups is low. Dietary overlap among the diving duck species is high due to the small number of prey taxa consumed. Dietary overlaps also occur between the diving ducks and the fish, although such overlaps are less than that recorded within both groups.

A bioenergetics approach has been used to assess the amount of macro-invertebrates consumed by the fish and diving ducks. The quantity of macro-invertebrates consumed by fish and ducks was similar, but only comprised a small percentage of the total macroinvertebrate resource of the lough.

Through an assessment of the dietary overlap of the fish and diving duck species and the calculation of food consumed, it is clear that the potential for competition does exist between these vertebrates. However, as a result of the large quantities of macroinvertebrates present in Lough Neagh it is unlikely that competition is occurring at the present time.

Date of Award	Dec 2000
Original language	English
Supervisor	DAVID GRIFFITHS (Supervisor) & CLARE CARTER (Supervisor)