Aggregative and schooling behaviour of small pelagic fish schools through echo type characteristics
Résumé
In this work we compare the result obtained by in situ echosounder measurements on eastern Venezuelan and south Sénégal coastal shelves where Sardinella aurita was the predominant species occurred in fish school structure. The both stocks are known as genetically independent. Persistent school typology was already described by Petitgas and Levenez (1996) analysing Senegalese echogram database. We compiled and analyzed an acoustic database (7 surveys) of Venezuelan fish school populations. The echo type typology encountered in the both countries is similar over the year. On the others hand, two standardized acoustics surveys have been carried out in 1999, with the same research vessel and echosounder (38 kHz) in Sénégal and Venezuela. The 'echointegration by shoal' method was performed on the data collected on both stocks. The observed fish school descriptors (energetic, morphologic and spatial) showed that the differentiation between day and night fish schools was mainly due to morphologic variables (fractal dimension). In Venezuela the echo types were different between day and night as in Sénégal, but on the entire diel cycle the quantitative and qualitative characteristics of fish schools were similar on both ecosystems. Both acoustic fish school populations are similar; they are constituted by a same dominant species Sardinella aurita. Observation of small pelagic fish school during the night is not unusual. The schooling behaviour and the diel aggregative dynamics of small pelagic fish around the world are discussed according our result and literature review. School morphology appears to be common to large number of species, and aggregative dynamics, species dependent.