Application of GAMs and multinomial models to assess the spawning pattern of fishes with daily spawning synchronicity: A case study in the European anchovy (Engraulis encrasicolus) in the central Mediterranean Sea

Information on the breeding behaviour of fish such as spatial segregation phenomena during spawning events is important both for understanding their reproductive ecology but also for correctly designing sampling schemes in stock assessment. The daily spawning pattern of the European anchovy, Engraulis encrasicolus, was assessed using a novel methodology that included the analysis of the hourly distribution of imminent and recent spawning stages using a combination of generalised additive models (GAMs) and of multinomial models for ordinal categorical data. To do so we used data from nine years of daily egg production method, DEPM, surveys, carried out in the Strait of Sicily. The spatial distribution and occurrence of actively spawning individuals were also analysed. Results showed that the actively spawning phase period (ASPP), i.e., the period from the onset of oocyte hydration to spawning ranged approximately 4.5 h between ca. 18:30 and 23:00 while a daily spawning peak was identified at 22:30. The sex ratio was skewed during the ASPP such that females exceeded males by ca. 20% at mid ASPP. During the ASPP the proportion of Day-0 females (i.e., females that spawned or would spawn the day of sampling) decreases with increasing bottom depth while outside the ASPP Day-0 spawners did not show any particular bathymetric pattern. In the study area, anchovies exhibit segregative spawning behaviour which drives spawners to select shallower waters, similar to other anchovy populations in the Mediterranean while in the Atlantic and in other clupeoid populations the opposite bathymetric trend has been observed.