Effect of external factors (environment and survey vessel) on fish school characteristics observed by echosounder and multibeam sonar in the Mediterranean Sea

Abstract

The size of pelagic fish schools depends on several parameters related to internal factors such as species, number of fish, fish swimming speed and physiological status and to external factors, such as hydrological factors and presence of predators. In order to better understand these relations, results coming from echosounder and multibeam sonar databases are analysed. Field data are collected during four acoustic surveys in the Mediterranean Sea in two different areas (Catalan and Adriatic Seas). The analysis shows differences between the two areas regarding size and position in the water column: schools are deeper and their mean size is lower in the Catalan Sea in comparison with Adriatic Sea. The differences in size of schools are mainly related to differences in school length. Moreover, the elongation of schools seen with the sonar is greater than one and half higher in the Adriatic Sea than in the Catalan Sea, whereas one would expect similar values for the two areas. The results are discussed in terms of environmental influence, avoidance reaction and acoustic capabilities of both tools. A hypothesis is proposed: the variation of school length and consecutively the variation of the correlated dimensions is first related to the strength of the avoidance reaction in front of the vessel and this effect can be reinforced depending on the environmental conditions. The model takes into account the effect of the boat, the vertical constraints undergone by the schools, and the internal requirements of the schools, such as the necessity for fish to keep visual contacts and the cohesion of the group.

Introduction

Spatial dynamics are of paramount importance to the understanding of the forces governing population dynamics. In the particular case of fish stocks, studying the factors inducing and maintaining the aggregation of fish within schools could be helpful to better understand the spatial heterogeneity of small pelagic fish populations and the mechanisms that lead to the particular mode of distribution of schools Pitcher et al., 1996, Mackinson, 1999, Fréon and Misund, 1999, Booth, 2000. Moreover, schooling behaviour is a powerful mechanism to sort fish by length Fréon, 1984, Fréon, 1985 an improved understanding of the mechanisms driving space-time variations in school size should greatly improve the estimation of demographic parameters. According to previous investigations, the size and the shape of pelagic fish schools depend on several parameters related to internal and external factors. Among the internal factors, the most commonly quoted are the species Partridge et al., 1980, Misund, 1993a, Maes and Ollevier, 2002 the swimming speed or the body length of fish Hara, 1987, Peukhuri et al., 1997, Dagorn et al., 1997, Hoare et al., 2000 the foraging behaviour Pitcher and Partridge, 1979, Pitcher and Parrish, 1993, Mackinson et al., 1999 or the physiological status Morgan, 1988, Robinson and Pitcher, 1989, Robinson, 1995. With regard to the external factors, related literature reports the influence of the stock abundance Misund, 1993b, Petitgas and Lévénez, 1996, Petitgas et al., 2001), the interactions between species like the presence of predators Fréon et al., 1992, Parrish, 1992, Krause and Godin, 1995, Pitcher et al., 1996 or the trophic competitors (Massé et al., 1996), the environmental conditions such as thermocline depth (Swartzman et al., 1994), the food availability and the food density (Nøttestad et al., 1996) or the specific disruptive events like strong gales (Scalabrin and Massé, 1993) or the arrival of a boat Olsen et al., 1983, Gerlotto and Fréon, 1992. Finally, an important phenomenon must be pointed out: the avoidance behaviour of the schools, which has an impact on factors such as catchability and abundance estimates. The recent development of 3D acoustic technology might lead to a better accuracy of school size estimations and to a better understanding of adaptation mechanisms of schools to local variations Gerlotto et al., 1999, Misund and Coetzee, 2000, Mackinson et al., 1999. In order to study this issue, we analysed the morphological and spatial characteristics of schools measured by echosounder and multibeam sonar during acoustic surveys in relation to the external conditions measured in the near field of these schools. We focus on both environmental changes occurring during the day in the vertical plane (thermocline, halocline and chlorophyll concentration) and local perturbations induced by the vessel. Data composed of two species (sardine and anchovy). Two surveys were conducted during successive years (1994, 1995), using the same research vessel in two different areas of the Mediterranean Sea showing different environmental conditions. Based on our database and on previous studies (Bahri and Fréon, 2000), the object of this paper is to compare the school characteristics in these two areas, measured with two acoustic devices, so as to test the hypothesis that the differences are related to environmental conditions and to the impact of the vessel.