Schooling behaviour of small pelagic fish: phenotypic expression of independent stimuli

Abstract

We compared echo sounder measurements of fish schools on Venezuelan and Senegalese coastal shelves. In both areas Sardinella aurita was the predominant aggregative species; the stocks are known to be genetically independent. Persistent school typology is described by analysing 7 acoustic surveys in Venezuela and comparing with pioneer observations in Senegal. The echo types encountered in both countries were comparable, with S. aurita and their associated species having comparable schooling behaviour. School morphology appears to be more related to species than to global characteristics of their ecosystems, as fish school descriptors were similar for both stocks. Nevertheless, S. aurita may adopt different school morphologies. Differentiation between day and night fish schools was mainly due to morphological variables, as determined by fractal dimension. Echo types were different between day and night in both Venezuela and Senegal. This analysis shows that several independent parameters can influence school morphology. However, although schooling behaviour is temporally highly variable, it shows several constant, typical 2D morphological structures, i.e. echo types. The observation of small pelagic fish schools occurring during the night is not unusual according to the literature, although a strict diurnal schooling behaviour, with scattering during the night, is often reported. Schooling behaviour appears to be a phenotypic expression depending on biotic and abiotic stimuli, intrinsic characteristics and motivation.

Classification of fish schools based on evaluation of acoustic descriptor characteristics

“…The schooling behavior typically characterizes each fish school in daylight, which is essential for the fish identification. However, during twilight and nighttime, fish schools scatter and overlap, which biases the fish identification in acoustic processing [4,20].…”

“…Acoustic surveys using quantitative scientific echo sounders commonly employed to determine the abundance and biomass of pelagic fish are becoming increasingly important for the management of pelagic fisheries [3]. Owing to the common aggregative behavior, small pelagic species appear in echograms as a mixture of diverse fish assemblages [4]. Echo integration is used to estimate fish quantity since the sampled volume contains overlapping target fish echoes [3].…”

Contrasted optimal environmental windows for both sardinella species in Senegalese waters

“…However, it is worth highlighting that the assessment of reliable abundance indices of small pelagic fish from artisanal fisheries landings remains complex (Ould Talib Sidi, ). It relies on parameters difficult to estimate such as the fishing effort (Mangel & Bede, ), particularly in the artisanal sector targeting the sardinella which are mainly targeted when occurring in fish school structures (Brehmer et al., ). The approach used in this study avoids misleading interpretation due to eventual bias in the catch per unit of effort (CPUE) estimation because the number of trips (outings) considered as fishing effort is not a proxy of the real effort.…”

“…However, it is worth highlighting that the assessment of reliable abundance indices of small pelagic fish from artisanal fisheries landings remains complex (Ould Talib Sidi, 2005). It relies on parameters difficult to estimate such as the fishing effort (Mangel & Bede, 1985), particularly in the artisanal sector targeting the sardinella which are mainly targeted when occurring in fish school structures (Brehmer et al, 2007).…”

Effect of environmental conditions on the seasonal and inter‐annual variability of small pelagic fish abundance off North‐West Africa: The case of both Senegalese sardinella

“…In Senegal, small pelagic fish mainly include, by order of importance in the average landings: sardinella ( Sardinella aurita or round sardinella, and Sardinella maderensis or flat sardinella), mackerel ( Scomber scombrus ), anchovy ( Engraulis encrasicolus ), ethmalosa ( Ethmalosa fimbriata ), horse mackerel ( Trachurus spp.) and sardines ( Sardina pilchardus ); they are all aggregative species occurring in a fish school (Brehmer et al., ). The sardinella accounts for more than 70% of the total landings that is mainly provided by the small‐scale fishery (CRODT, ).…”