Marine Biology Research Division

The sea-skater Halobates was collected in eastern Indonesian waters during the SNELLIUS II Expedition in August 1984 (SE monsoon) and February/March 1985 (NW monsoon).  About 200 specimens were caught in 18 tows at offshore sites in August, in the upwelling season, but more than 1,000 in 18 tows in February, when the mixed layer was nutrient depletedand chlorophyll-poor. The Indo-Pacific species H. germanus predominated in thecatches during both cruises, its density being inversely related to surface chlorophyll concentration.  Reproductive activity of H. germanus was highest in oligotrophic conditions, the relative abundances of nymphs and of cast skins being much higher in February than inAugust. Shifts in the population structure in February suggested a development time of about 7 days or less per stadium at a surface seawater temperature of 29°C. The cosmopolitan H. micans and the coastal H. flaviventris and H. princeps were absent or rare in August but present in most catches in February; some specimens of the latter two species were found at stations more than 150 km from the nearest shore.