Control of freshwater fish louse Argulus coregoni: a step towards an integrated management strategy.

Harmful infections by ectoparasites of the genus Argulus occur repeatedly in freshwater fish farming operations where the management has largely been ineffective. Preventative methods and regular monitoring are rarely applied, so that chemical interventions become necessary. According to the Integrated Pest Management (IPM) approach, a sustainable management or control program for a parasite should be based on knowledge of the ecology of the parasite along with adoption of several prevention and control methods, the application of which is dependent upon the prevailing infection level. The application of multiple management tactics is especially important because parasites can develop resistance to chemical treatments. We took a step towards sustainable management of Argulus populations and tested the effect of several types of treatments on survival of A. coregoni at different stages in its life cycle. Parasite juveniles and adults were highly sensitive to potassium permanganate treatments (0.01 g l(-1)), which lead to 100% mortality, whereas treatments with formalin (0.6 ml l(-1)), sodium chloride (20 g l(-1)) or malachite-green/formalin were not effective. Mechanical treatment by shaking infected fish in a hand net was an effective means of detaching parasites from the fish, and resulted in > 80% decreases in parasite numbers. Compared to eggs in control treatments, both drying over a minimum period of 24 h and formalin treatments (120 ml l(-1)) led to significantly higher mortality of A. coregoni eggs. Other treatments, i.e. drying over a period of 15 h, baths in potassium permanganate (1 g l(-1)) or sodium chloride (50 g l(-1)), did not significantly affect the viability of eggs. Based on the present results and previously published papers, we present an initial framework showing how A. coregoni populations could be managed effectively.

Satiation-dependent, intra-cohort variations in prey size selection of young roach (Rutilus rutilus).

Feeding rate and behaviour of juvenile roach in a non-depleting patch consisting of Daphnia magna of two size classes (optimal and much smaller) were studied in 20-min experiments. The medium size group (standard length 26-28 mm), large (28-32 mm), and small (24-26 mm) fish differed significantly in the proportion of small prey eaten. During a short initial period of burst feeding (1-5 min) only large prey were selected by all fish. The switch from large to small prey occurred much earlier in small than in large roach. During the initial period small fish had higher feeding rates. In small fish, small prey made up about 30% of the food biomass consumed during the whole 20-min feeding period, whereas in large fish small prey played a minor role (about 2%). Within the medium-sized group, one subgroup of fish, characterized by early switching to small prey, showed a feeding behaviour similar to small fish, whereas the other subgroup, which switched later, behaved like large fish. Neither the apparent prey size model nor optimal foraging theory, even in its dynamic version, were supported by the data. It is suggested that vigilance against predators changes the initial energy-maximizing strategy in hungry fish to a time-minimizing strategy in partially satiated fish. Intra-cohort variations in prey size selection might be related to social status of the fish. Social status may affect prey size selection and feeding rate, and thus result in different growth rates and size differentiation.