Diel variation in the vertical distribution and schooling behaviour of sardine (Sardina pilchardus) off Portugal

Abstract

Zwolinski, J., Morais, A., Marques, V., Stratoudakis, Y., and Fernandes, P. G. 2007. Diel variation in the vertical distribution and schooling behaviour of sardine (Sardina pilchardus) off Portugal. – ICES Journal of Marine Science, 64: 963–972. Diel patterns in the schooling behaviour and vertical distribution of pelagic fish schools were studied by examining their echotraces from repeated acoustic survey transects at three inshore sites off the Portuguese coast. At two sites, sardine was the dominant pelagic species, and echotrace characteristics of fish schools were similar to those reported in the literature. At the third site, where there was a multispecies pelagic assemblage that included sardine, there was more variability in several of the school descriptors. At all sites, fish schools expanded after sunset, enlarging their cross-sectional area along the horizontal plane and reducing their mean internal acoustic density, while maintaining their overall mean abundance. Downward migration was rapid (within 1 h) after sunset and simultaneous with school expansion. School-like aggregations with total backscattering similar to daytime schools were present throughout the night, although the proportion of small schools and scattered fish appeared to increase at that time. At dawn, sardine rose back up the water column and rapidly reformed into the typical daytime schools. This pattern of diel vertical migration is opposite to that described for most clupeoids worldwide. The implications of this behaviour on abundance estimation by acoustic monitoring surveys for small pelagic fish are discussed.

Bathymetric segregation of spawning stages in the Atlantic sardine Sardina pilchardus

“…Such formations presuppose collective behavior patterns that are mainly based on the interactions of individuals, which in turn depend on a complex sum of sensory stimuli (Weissburg & Browman 2005). Such collective behavior during spawning was shown by means of egg and acoustic sampling data for the Atlantic sardine stock by Zwolinski et al (2006Zwolinski et al ( , 2007. Therefore, even if buoyancy is af fected by pregnancy and spawning individuals are forced to move offshore and downwards to spawn, the formation of massive and functional social structures that ensure spawning success through enhancing fertilization rates and progeny survival would also require intra-and intersexual interaction among individuals.…”

“…The same authors found, using acoustic sampling, that the mature sardines that contributed to the spawning of these egg cohorts were located at 60 m depth. Zwolinski et al (2007) suggested that large shoals of actively spawning sardines are formed in close association to the seabed around the hours of spawning.…”

Catch of pelagic hauls in Mediterranean acoustic surveys: Is it the same between day and night?

“…Time of sampling is among the main factors that could cause bias in the haul catch. The bias could be even more pronounced in small pelagics, mainly because they are known to show diel vertical migration, form dense schools during daytime, and disperse at loose aggregations close to the surface during night-time (Woodhead 1966, Massè 1996, Giannoulaki et al 1999, Zwolinski et al 2007, Tsagarakis et al 2012. Time of fishing might also be related to trawl selectivity, increasing the possibility for the smaller individuals to escape from the net during the daytime because of the optical contact with the trawl and the large mesh size of the net at the front of the gear, while the absence of this contact during the night-time supports "blind", random selection sampling.…”

“…The effect of the time of day on acoustic and biological data sampling is an issue that has raised several contradictions regarding the introduction of error in abundance estimates (Iglesias et al 2003, Zwolinski et al 2007. The latter largely depends on parameters related to species behaviour.…”

“…Small pelagics are known to exhibit diel vertical migrations, form dense schools during daytime composed of individuals of the same swimming ability (i.e. the same size), and disperse forming loose aggregations during night-time (Woodhead 1966, Azzali et al 1985, Massè 1996, Freon and Misund 1999Giannoulaki et al 1999, Szcucka 2000, Zwolinski et al 2007, Tsagarakis et al 2012. This diurnal behaviour of fish schools makes fish either unavailable to the acoustic apparatus or difficult to distinguish acoustically from other scatterers, which could generate a degree of bias in the echo density used for abundance estimates (ICES 2005).…”