Effect of external factors (environment and survey vessel) on fish school characteristics observed by echosounder and multibeam sonar in the Mediterranean Sea
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.
Section snippets
Survey designs
The surveys were carried out during 1994 and 1995 in Catalan Sea (39–41°N/0–2°E) during spring, and in Northern Adriatic Sea (43°30'–45°30'N/12°15'–13°30'E) at the end of summer (Fig. 1). In Catalan Sea, the studied area is characterized by a wide continental shelf (30–40 nmi wide by 90 nmi long) and receives in its northern part freshwater from Ebra River. Two back-to-back coverages of the zone were performed during each of the two cruises. In the North Adriatic, the area includes the plume of
Survey characteristics
General survey characteristics are indicated (Table 1). First, we noted a difference between the density of schools recorded by the sonar in Adriatic Sea in 1994 and 1995 (respectively, 1.1 and 2 school nmi–1). This difference can be partly due to the change in the image smoothing rate (from 4 to 2). Nevertheless, this interpretation is minimized by the fact that similar results are obtained with the echosounder (0.3 and 0.5 school nmi–1, respectively). Since the sonar has a greater sampling
Discussion and conclusion
Few restrictions must be done regarding our database. We did not use the algorithm proposed by Diner (2001) in order to “correct” school length, because as stressed by this author, schools resulting from complex shapes and varying internal densities do not fill the conditions for the use of the algorithm, and this was the case for most of the schools of our database. Besides, as usual in fisheries acoustics, the change of settings may have a dramatic effect on the results, mostly by changing
Acknowledgements
This work was funded by a grant from the European Union (T-ECHO, project AIR1-CT9200314). We are grateful to our colleagues for their help in data collecting and processing and to J. Rucabado, A. Castillón and P. Schneider, deck officers, and crew from the Instituto de Ciencas del Mar of Barcelona for the excellent cooperation on board RV “Garcia del Cid”. Comments and constructive criticism from two anonymous referees are acknowledged.
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