DAO 46:123-128 (2001)  -  doi:10.3354/dao046123

Spatial distribution and hatching of overwintered eggs of a fish ectoparasite, Argulus coregoni (Crustacea: Branchiura)

V. N. Mikheev1,*, A. F. Pasternak2, E. T. Valtonen3, Y. Lankinen4

1A.N. Severtsov Institute of Ecology & Evolution, Russian Academy of Sciences, 33 Leninskii Prospekt, Moscow 117071, Russia
2Institute of Oceanology, Russian Academy of Sciences, 36 Nakhimov Avenue, Moscow 117853, Russia
3Department of Environmental and Biological Science, University of Jyväskylä, PL 35, 40351 Jyväskylä, Finland
4Savon Taimen OY, 77700 Rautalampi, Finland

ABSTRACT: The habitat distribution of overwintered eggs, which were found to be the only source of spring recruitment of Argulus coregoni Thorell, was studied at a commercial fish farm in Central Finland. The frequency of occurrence of egg clutches in the deep parts of the canals and ponds was 50 to 80% and the percentage cover of the surface of stones with egg clutches was 1.7 to 6.4%, while in the shallow parts these values were 8 to 27% and 0.1 to 0.3%, respectively. A greater proportion of empty egg-shells was observed in shallow water in the mid-May, suggesting an earlier hatching there stimulated by the increased temperature and higher illumination. Under laboratory conditions, only elevated UV illumination, but not diurnally fluctuating temperature, significantly accelerated hatching. Normally overwintered eggs produced a pronounced peak of hatched larvae at the end of May and hatching continued at a much slower rate throughout the summer. Eggs that overwintered twice, first normally and then for a second time buried under sediments, were exposed to the same laboratory conditions simultaneously with normally overwintered eggs, but their hatching was delayed until August. The hatching rate was low, but markedly increased in December.

KEY WORDS: Overwintering · Argulus coregoni · Ectoparasite · Egg distribution · Hatching

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