ALBERT

Description: A study of recent publications on ichthyoneuston of subtropical seas showed that this fauna is neither regionally nor seasonally uniform. Available material of abundant and characteristic species off Northwest Africa was re-examined under this aspect. This material was collected during five German expeditions (1967-1973) between 20° and 36° N and between the coast and 30° W. It includes 366 hauls that could be utilized for qualitative analysis; out of these 333 hauls yielded quantitative information. Based upon variation in geographical and hydrographical factors, distribution of sampling locations and zoogeographical considerations, the study area was divided into five subregions (two neritic and three oceanic), each represented by a different number of samples. A clear faunistic boundary appears between neritic and oceanic subregions. This boundary is related to water depth and distance from shore. It is sharp for neritic species. It is also sharp for mesopelagic fishes as they do not occur in water shallower than the slope, whereas oceanic-epipelagic taxa are less affected by this boundary. Latitudinal zonation is also evident as it is a function of hydrographic factors, especially temperature, probably showing seasonal variations. The latitudinal boundary is less pronounced than the neriticoceanic one; simplified it is assumed as running through the Canary Islands region. In all subregions, ichthyoneuston abundances appear to be highest during winter months. Maximum of abundance is produced by spawning that is restricted in many species to this season. The five most common species constituted more than 80% of the total catch during winter. Their proportion decreased to less than 60% in warmer months due to the immigration of less common (largely tropical) species and the mortality and ontogenetic emigration of animals born during winter. Seasonal differences in diversity did not prove significant as species number in ichthyoneuston catches is generally high. Seasonal trends are conspicuous for typical species only and are discussed. The northwest African current system probably causes considerable drift in ichthyoplankton. This raises the question as to how stocks survive despite drift of early life stages. This problem is discussed for characteristic taxa. Depending on the species, several mechanisms acting either singly or in combination are possible: 1. Adults of tropical neustonic forms actively migrate into the study area in summer, passive southward transport of early stages being a necessary condition for survival during other seasons. 2. Southward transport of species performing diurnal vertical migrations may, in certain areas, be reduced if such species reach the poleward undercurrent and remain in it during part of the day. 3. South of Cape Blanc, inshore eddies and the temporary surface countercurrent may permit a longer stay or return transport. This mechanism is probably effective for neritic euneustonic animals.