Mesozooplankton and ichthyoplankton distribution around Gran Canaria, an oceanic island in the NE Atlantic

Abstract

In October 1991, mesozooplankton biomass and ichthyoplankton were studied in the waters surrounding the island of Gran Canaria (Canary Islands). The average dry weight obtained for mesozooplankton biomass (4.5 mg m −3) is typical of the area. Average fish egg abundance (94 eggs per 10 m 2) was similar to that found previously for the Canary Current. However, the average fish larva abundance (904 individuals per 10 m 2) was higher than previously recorded for the Canary Current and similar regions. The horizontal distributions of the planktonic components studied appear strongly related to the mesoscale oceanographic structures in the area. These included an area of weak flow around the stagnation point upstream of the island, where higher concentrations of neritic ichthyoplankton occurred, a warm lee region downstream, where mesozooplankton biomass and neritic ichthyoplankton were increased, particularly on the convergent anticyclonic boundary, and the offshore boundary of an upwelling filament from the NW African coast, which acted as a barrier for neritic ichthyoplankton. These concentrations suggest that the stagnation point and the lee are areas of retention for neritic fish eggs and larvae. Simple Lagrangian simulations of particle trajectories in the observed field of flow demonstrate the potential for retention of organisms, both passive and with limited swimming ability, in these areas. On the flanks of the island and in the filament, the simulation suggests even swimming organisms will be largely swept away. The various oceanographic structures, by increasing the planktonic production, are partially responsible for the relatively high values of abundance obtained for fish larvae.