Modelling small pelagic fish potential spawning habitats: eggs vs spawners and in situ vs satellite data

We combined generalised additive models and GIS to model and map anchovy potential spawning habitat using anchovy egg and adult presence/absence data from the North Aegean Sea along with in situ environmental information, collected during the beginning / peak of the spawning period. First, we compared the spawning habitat models constructed using different data subsets (egg or adult data for different months and areas). This analysis highlighted the potential effects of (a) temporal variability in environmental conditions during the transitional period of spring-early summer, in relation to survey timing and peak spawning; (b) stock contraction/expansion due to interannual changes in spawning biomass; and (c) increased area of egg vs adult presence due to fast egg dispersal on the estimated habitat model/map characteristics. We conclude that potential spawning habitat models should be constructed based on pooled data (preferably adult data) covering as much as possible the seasonal and interannual variability in spawners' distribution. Subsequently, we compared the pooled egg and adult models/habitat maps with those previously published for the same area using satellite environmental information. We demonstrate that the predicted habitats based on satellite vs in situ data were quite consistent, further supporting the use of readily available satellite data in essential habitat studies.