Food, feeding and growth of the eel (Anguilla anguilla L.) in a Dutch eutrophic lake.

This thesis describes the food, foraging and growth of the eel (Anguilla anguilla). Attention is paid to the abundance and distribution of the food organisms and the feeding and growth of other fish species. The investigations were carried out in the Tjeukemeer (21 km2) in the northern part of the Netherlands. This lake is the main object of study of the Tjeukemeer laboratory, part of the Limnological Institute of the Royal Netherlands Academy of Arts and Sciences. The Royal Academy funded the research on eel, the Ministry of Health and the Environment funded the zooplankton research.The Tjeukemeer is a shallow (1-2 m) , turbid (Secchi-disc 0.30 - 0.40 m), hypertrophic lake. It is part of a system of canals and reservoirs (lakes) receiving surplus water in late autumn, winter and early spring from a watershed covering 3060 km <sup>2</SUP>. Via this system the water is drained into the Waddensea or pumped into the IJsselmeer. When during spring and summer the evaporation exceeds precipitation, the whole system is drained with water from the IJsselmeer. These inputs have great impact on the physico-chemical conditions, but also on the fish fauna of the lakes. The smelt (Osmerus eperlanus) immigrates passively from the IJsselmeer as a massive flow of larvae. This immigration is dependent on weather conditions, therefore the abundance of the smelt is highly variable.The shoreline of the Tjeukemeer is poorly developed, the emergent vegetation covers 0.5% of the surface; since the early 1970s the submerged aquatic vegetation is greatly reduced and now virtually absent.The eel population in the Tjeukemeer, like in other large Dutch lakes, is heavily exploited. The population is dominated by small eels between 200 and 300 mm. Eels smaller than 120 mm are scarce. The proportion of eels larger than 300 mm ranges between 7 and 30% of the catch.Irrespectively their body length, the eels prefer as food invertebrates, at least 7 mm large, living on the water bottom surface. In the Tjeukemeer the larvae (later instars) and pupae of the chironomids <u>Chironomus</u><u>plumosus</u> , <u>Einfeldia</u><u>carbonaria</u> and <u>Glytotendipes</u><u>pallens</u> , and the amphipod <u>Gammarus</u><u>tigrinus</u> are most frequently consumed.In 1979 larger bivalve mollusks are of secondary importance in the eel diet, while in 1980 and 1981 the>200 mm eels can switch to predation on smelt ( <u>O. eperlanus</u> ). Eels smaller than 340 mm become gape- limited in predation on smelt because the smelt grows faster than the eel mouth.The searching and catching behaviour of small eels foraging on chironomids was investigated in aquaria. The feeding efficiency, expressed as the time interval between finding and swallowing the larva, rapidly decreases when larvae are burrowed in the substrate. The eel has no preference for pupae, if larvae and pupae are offered in the same way, i.e. both easily available for the eels.In the stomach contents of the eels from the lake the proportion of pupae in the total chironomid biomass is 60-90%. The preference for pupae is explained by the availability of pupae, in comparison with the chironomid larvae, which are often deeply burrowed in the bottom substrate.The growth of eel was investigated by means of otolith readings and cohort analysis. The otoliths from 905 eels were sawn and polished or manually ground to determine the age by counting winter rings. The growth of &lt;250 mm eel amounts 16 to 33 mm per year. The 250-400 mm eels grow faster, up to 43 mm per year. The length-frequency distributions of the eel are used for cohort analysis to check the accuracy of the conclusions about growth. This analysis validated the results of the otolith readings.Earlier studies on fish and zooplankton showed the great impact of the predation by smelt on the density of the <u>Daphnia</u> , in the crustacean zooplankton. <u>Daphnia</u> also is an important part of the diet of larger bream ( <u>A. brama</u> ) . In 1980 and 1981, when the smelt population increases, the zooplankton is hardly available to the bream. A large part of the bream population switches to a diet of chironomids. The chironomid population becomes overexploited.The overexploitation of the chironomids caused bad feeding conditions for eels confined to prey on invertebrates. The low growth rate of small eels is explained by these feeding conditions. We observe that even small eels start to prey on smelt in 1980 and 1981, but become gape-limited. Larger eels (>340 mm) show a better growth when feeding on fish.The effects of the inputs of water from other origins in the Tjeukemeer on the horizontal distribution of crustacean zooplankton were studied. Only during short periods in summer, the population densities appeared to be influenced by the inflow of water from the IJsselmeer. In periods with a precipitation surplus (autumn and winter) the zooplankton community was "diluted" by the inflow of surplus water from the watershed. The influence on the rest of the aquatic fauna of the lake was only slight.Patchiness in the zooplankton distribution occurs in the Tjeukemeer due to inhomogeneities in physico-chemical parameters. However, the lake is shallow and strongly influenced by windinduced turbulence. The zooplankton appears to be more homogenous than in other lakes elsewhere in Europe. These lakes were often deeper or contained more sheltered parts.In the last chapter the role of the eel in the ecosystem is discussed. Although competition between eel and bream for food (chironomids) occurs, it is suggested that changes in the vegetation and nutrient loading of the lake and the heavy commercial exploitation of the eel population have a negative influence on the amount of eel, while the bream is favoured by the effects of eutrophication.<TT>