Using spatial indicators to investigate fish spawning strategies from ichthyoplankton surveys: A case study on co-occurring pelagic species from the North-East Aegean Sea

Abstract

Multispecies ichthyoplankton associations are considered adaptive and their objective definition is useful in designating species with similar or contrasting spawning strategies. In this case study, a suite of indicators, recently developed to characterize the spatial patterns of animal populations, are applied to egg abundance data of the summer-spawning European anchovy (Engraulis encrasicolus), round sardinella (Sardinella aurita) and chub mackerel (Scomber colias), in an effort to identify common or contrasting spawning patterns in the North-East Aegean Sea and their relation to the major oceanographic features of the area. There was a considerable increase in abundance and area of presence between the early phase (May) and advanced phase (July) of the 2010 spawning period. The identification of major spatial patches revealed a persistent multispecies spawning location associated with the outflow of Black Sea water. A second major patch was defined for anchovy and chub mackerel in July, clearly associated with the Samothraki gyre (SG). In contrast, round sardinella spawned in coastal waters outside the SG. Surprisingly, in May 2010, the spawning intensity of anchovy was very low in the SG, which was attributed to unfavourable conditions caused by the bloom of mucilage-producing phytoplankton. The levels of aggregation and occupation of space seemed to be adequately described by the indices of Positive, Equivalent and Spreading Area. Conclusively, many spatial indicators applied in this study are potentially important tools for studying egg distributions and the changing priorities of adult fish when selecting spawning sites.