MEPS 269:249-263 (2004)  -  doi:10.3354/meps269249

Association between bluefin tuna schools and oceanic features in the western Mediterranean

F. Royer1,2,*, J. M. Fromentin2, P. Gaspar1

1Collecte Localisation Satellites, Division Océanographie Spatiale, 8-10 rue Hermès, 31526 Ramonville St. Agne, France
2IFREMER, Centre de Recherche Halieutique Méditerranéen et Tropical, avenue Jean Monnet, BP 171, 34203 Sète cedex, France

ABSTRACT: We present an analysis of the distribution of bluefin tuna Thunnus thynnus schools spotted during aerial surveys in the Gulf of Lions, in relation to oceanographic features. Bio-optical and thermal properties of the sea surface derived from high-resolution sensors (AVHRR and SeaWiFS) were studied on a daily basis, and an edge-detection technique was applied to detect frontal zones. Geostatistics and point-process analyses were used to evaluate the role of the environment in structuring the spatial pattern of bluefin tuna (BFT). The distribution of schools spotted was strongly non-stationary both in space and time; this is believed to be an effect of the survey design (transect sampling) and the influence of transient oceanographic structures (surface fronts and eddies). The empirical variograms indicated a spatial range of the BFT schools at around 40 km, with substantial daily variability. Ripley¹s K statistic, as well as autocorrelation plots, revealed that the fish schools were clustered over a wider range of scales (from 10 to 80 km), indicating more spatial structure than would be expected from a random process. Finally, BFT school distributions appeared well determined by the oceanic features, except at very small scales (<10 km), where over-aggregation occurred, and at the largest scales of our study (>40 km), where over-spreading was detected. Dynamical ecological processes, such as foraging, are likely to induce this complex spatial pattern. Possible reasons for the association of tuna with fronts are presented.

KEY WORDS: Thunnus thynnus · Gulf of Lions · Point process analysis · Front detection · Sea surface temperature · Ocean colour · Aggregation

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