Interactions between fish, mysis, and zooplankton in Lough Neagh

Abstract

Interactions between fish, Mysis and zooplankton were studied in a large eutrophic lake in Northern Ireland, between January and December 1993. Routine fish sampling revealed that pollan, perch and roach numerically dominated the fish community, but pollan provided the majority of biomass (c. 90%). Juvenile fish dominated the population structure of the three fish species.

Water temperature was the most important factor which controlled growth in roach and perch, and the lengths attained by the O+ and 1+ cohorts of both species at the end of the growing season were lower than those achieved between 1985 and 1987 (warmer years). Fish distribution varied with season. The majority of the roach and perch populations migrated inshore during the summer, whereas pollan exhibited a homogenous distribution between sites. The majority of fish inhabited deeper waters in winter. The three species showed different distributions in the vertical plane; perch displayed a benthic distribution whereas pollan and roach were more pelagic.

The zooplankton was dominated by Cyclops abyssorum, but the calanoid Eudiaptomus gracilis was also present throughout the annual cycle. Daphnia hyalina was the most abundant dadoceran but its appearance was restricted between April and October. Cyclops and Eudiaptomus density was low over winter but rose quickly in spring when environmental conditions became suitable. The vertical distribution of zooplankton was studied over a diel period in April and August. Copepods did not how clear or consistent diel movements but Daphnia migrated in summer. The mechanisms which induced movement are discussed.

Fish diet was studied in detail and measures of seasonal prey preferences, feeding intensity (fullness cores and the proportion of fish feeding), concordance, prey species and size selectivity, and diet overlap were made. All fish showed a preference for Daphnia during the summer months but also selected the largest sizes of the dadoceran (except juvenile perch which selected the smallest sizes). In contrast the smallest sizes of Mysis were always eaten. The effects of fish predation on zooplankton communities are discussed.

The importance of competitive interactions between the fish species within the laugh are discussed in relation to other ecosystem process (including trophic interactions).

Date of Award	Jan 1996
Original language	English