Seasonal dynamics of egg laying and egg-laying strategy of the ectoparasite Argulus coregoni (Crustacea: Branchiura).

Substrate preferences, spatial aggregation patterns and seasonal dynamics in the egg laying of ectoparasitic Argulus coregoni were studied at a commercial fish farm in Finland. Pilot experiments showed that A. coregoni females selected specific types of substrates for egg laying. Significantly more A. coregoni eggs were laid on dark substrates than on light ones suggesting the use of visual cues. Therefore, egg-laying plates of dark colour were constructed for further experiments. Most A. coregoni eggs were deposited in locations in shadow and in the deepest water in a 2 m deep farming canal. Relatively more eggs were laid on bottom stones situated near each egg-laying trap than on artificial egg-laying plates indicating a preference for irregular stones in the deeper locations in the canal. The plates were located 20 cm above the bottom. However, a total of 5,863 A. coregoni egg clutches, corresponding approximately to 1.5 million unhatched metanauplii, were successfully destroyed with the plates indicating that egg-laying traps can be used as an ecological control method against argulids in certain situations. For traps to be effective, ponds should not contain stones or any other hard substrata attracting female lice. The egg laying of A. coregoni in this study started on 5 July in 2001 and extended over 3.5 months up to mid-October. The egg-laying pattern of A. coregoni population was unimodal, supporting the view that only a single A. coregoni generation occurred annually in Central Finland.