Using spatial indicators to investigate fish spawning strategies from ichthyoplankton surveys: A case study on co-occurring pelagic species in the NE 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.

Ovarian dynamics, batch fecundity and spawning phenology of the lessepsian migrant Etrumeus golanii DiBattista, Randall & Bowen, 2012 (Clupeidae: Dussumieriinae)

“…A similar peak has been observed for the same species in the Egyptian Mediterranean Sea (Farrag et al, 2014). It may be attributed to the cessation of spawning activity, combined with an increased consumption of high energy prey such as fish larvae, especially anchovy Engraulis encrasicolus and round sardinella Sardinella aurita larvae, which are particularly abundant in the eastern Mediterranean during July (Tsoukali et al, 2019;and references therein). Indeed, Osman et al (2013) observed that the diet of E. golanii included high quantities of fish larvae in summer, which these authors attributed to their increased abundance at that time of the year.…”

Can we actually monitor the spatial distribution of small pelagic fish based on Sentinel-3 data? An example from the North Aegean Sea (Eastern Mediterranean Sea)

“…Instead, potential adult anchovy habitats have been correlated to bottom depth and sea level anomaly (Giannoulaki et al, 2013) because they are linked to zooplankton aggregations. Furthermore, the significance of depth and temperature could be related to spawning, which for anchovy typically occurs between late spring to early summer in the Mediterranean and the Aegean (Tsoukali et al, 2019;Basilone et al, 2020).…”

“…The Samothraki gyre acts as an entraining mechanism for mesozooplankton and larvae with high ecological value (Somarakis and Nikolioudakis, 2007). Due to high concentrations of food, anchovy tends to accumulate at the gyre's periphery, while sardine on the other hand usually is not associated with the gyre and is found closer to shore (Tsoukali et al, 2019). Furthermore, Reese et al, 2011 showed that the distribution of anchovy and sardine populations around thermal fronts differs significantly from the random distribution and that species tend to be attracted to them.…”

“…For example, Sentinel-3's OLCI and ocean color sensors in general calculate the CHL concentration at the penetration depth, which usually corresponds to the first few meters below the sea surface (Moutzouris-Sidiris and Topouzelis, 2021). Examples of 3D small pelagic fish spatial distribution modeling in the North Aegean are present in the bibliography, but their implementation require additional data in the form of survey transect measurements, oceanic circulation modeling and/or 3D satellite models (Tsoukali et al, 2019;Gkanasos et al, 2021). In the case of the first two, continuous monitoring will not be possible due to the high cost and analysis time and the latter option lacks the spatial resolution of the proposed methodology.…”

“…Higher resolution remote sensing data also contribute in finer oceanic circulation mapping, whose importance in fish spatial distribution modeling has been widely stated in both regional studies (Somarakis and Nikolioudakis, 2007;Tsoukali et al, 2019) and international ones (Godø et al, 2012;Arur et al, 2014;Kürten et al, 2019). Until now, fish spatial distribution studies and methodologies that use remote sensing data utilize lower resolution datasets (> 1km) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) (Zhang et al, 2017;Fauziyah et al, 2022).…”