Abundance, gut fluorescence and gut content of juvenile krill Euphausia superba (15 to 20 mm) were measured during a diel cycle in the Gerlache Strait (Antarctic Peninsula). Krill remained in the upper layers (0 to 100 m) during the day and migrated downward below this depth during the night, coinciding with the vertical ascent of the copepod Metridia gerlachei to shallower layers. Krill fed on phytoplankton during the day (as deduced from gut fluorescence measurements), whereas they switched to carnivory during the night (as deduced from gut contents). The vertical migration and the feeding behaviour of krill agree with different observations in the literature and gives an additional explanation to the observed inverse relationship between krill and non-krill zooplankton. The fact that krill is able to prey on mesozooplankton suggests that euphausiids can exert a top-down effect which structures the plankton community of Antarctic waters.
The distribution and abundance of fish larvae was studied along the eastern and southern shelf of Gran Canaria Island (Canary Islands) from July 2000 to June 2001. Oblique bongo hauls were carried out fortnightly during the daytime, coinciding with days of full and new moon. During February, the area was sampled every 2–5 days. About 17.3% of the ichthyoplanktonic community was composed of clupeiform larvae: 92.9% of these larvae were Sardinella aurita, whereas 4.7% and 2.4% were respectively Engraulis encrasicolus and Sardina pilchardus. Sardinella aurita larvae appeared during the whole year with two periods of maximum abundance: June to September and December to February. During the full moon their abundance was on average 38.5% (±6.8%) of their numbers during the new moon, showing a clear lunar periodicity. Engraulis encrasicolus larvae appeared from November to March, also coinciding with the new moon. Sardina pilchardus larvae only appeared during two short periods, both coinciding with filaments shed from the African coastal upwelling which reached the island. This fact confirms the transport of fish larvae from the upwelling area off northwest Africa to the Canary Islands, promoting a genetic flow among both sites.
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