Résumé de thèse Comparative study of the trophic ecology of Auxis rochei(Risso, 1810), Euthynnus alletteratus(Rafinesque, 1810), Sarda sarda(Bloch, 1793) and of the young-of-the-yearof Thunnus thynnus(Linnaeus, 1758) (Perciformes, Scombri- dae) in the southern Tyrrhenian Sea,par Davide CAMPO(1). Ph.D. thesis in Evolutionary Biology and Marine Ecology, 2006, Università di Pisa (Italy), 153 p., 54 figs, 39 tabs, 179 refs. [in Italian] The aim of this research was to study the feeding strategy and were the clupeiforms S. aurita followed by the sardine S. the trophic partitioning between four abundant pelagic species pilchardusand the anchovy E.encrasicolus.The diet did not vary belonging to the family Scombridae: the bullet tuna Auxis rochei significantly throughout the period of study, focusing, in the differ- (Risso, 1810), the little tunny Euthynnus alletteratus(Rafinesque, ent months, on the same prey species. Asignificant quantitative 1810), the Atlantic bonito Sarda sarda (Bloch, 1793) and the change in the diet composition of bonitos of different sizes was young-of-the-year of bluefin tuna Thunnus thynnus (Linnaeus, demonstrated. The anchovy was the predominant prey for the 1758). All fishes were caught by means of a drifting net, on a smallest specimens, sardine for the medium sized fish and round monthly basis between August 2002 and May 2004. Stomach con- sardinella for the larger bonitos. As suggested by the measurement tent analysis was the method used to study the diet. of the prey, this ontogenetic diet shift could be related to the size of The bullet tuna showed the broader trophic spectrum and a gen- the prey. eralist diet based on a variety of planktonic invertebrates, small The simultaneous presence of the four scombrids in the area sized cephalopods and fish larvae. The use of multivariate analysis permitted to study their trophic partitioning. Food resource parti- methods allowed to highlight a temporal variability of the diet, in tioning was quantified by means of the Schoener overlap index and confirmation with the opportunistic feeding strategy of this species. multivariate ordination methods. The results highlighted a clear Moreover,the analysis showed significant differences between the resource partition pattern between A. rochei,E. alletteratusand S. diet of the individuals belonging to the different size classes, sug- sarda, which coexisted in the pelagic habitat year-round. The gesting that an ontogenetic diet shift occurs as A.rocheigrows. young-of-the-year of T. thynnus,caught from August to October, This behaviour seems to follow the traditional optimal foraging exhibited a less clear partition pattern, showing a different extent of theory which states that animals should maximise their net rate of the feeding overlap with the other species. The trophic relation- energy return when selecting prey. ships between the four species, investigated by means of Bray-Cur- The little tunny showed an opportunistic feeding behaviour and tis dissimilarity coefficients and the dendrogram constructed using apiscivorous diet based mainly on larval stages of the clupeiforms hierarchical agglomerative clustering, highlighted the trophic sepa- Sardinellaaurita,Sardina pilchardusand Engraulis encrasicolus, ration of S. sardafrom the other three species at a dissimilarity and on the mesopelagic species Maurolicus muelleri.The temporal value of 60%. In fact, the Atlantic bonito was the only species to diet variation observed was mainly due to a difference on the tem- show a specialised diet, focusing exclusively on clupeiforms. poral availability of the larval stages of these clupeiforms in the Moreover, these clupeiforms being pelagic coastal species indicat- study area. An ontogenetic diet shift was also evident. The smallest ed that S. sardanever approaches the seafloor to eat. The presence fishes showed a diet focused on larval stages of clupeiforms while of epipelagic and mesopelagic prey in the stomachs of A. rochei,E. the larger ones tended to select M. muelleri. alletteratusand T. thynnussuggests a more complex pattern of ver- The young-of-the-year bluefin tunas were caught in the study tical migration throughout the water column for these species. It area only during August, September and October. In fact, authors has to be noticed that the observed trophic relationships agree with refers that during the first months of their life bluefin tunas occur phylogenetic relationships of the four scombrids. Bullet tuna, little both inshore and offshore in the southern Tyrrhenian Sea from tunny and bluefin tuna belong to the tribe Thunnini, comprising summer to early autumn. They showed an opportunistic feeding endothermic tunas. The expansion of the thermal niche permits to strategy and a diet dominated by small sized fish (larvae and juve- these predators to forage over a wider temperature range and to niles) and cephalopods. Diet variations between the specimens increase foraging time in cold waters, so as to feed on vertically caught during the different months were highlighted. Differences migrating prey. On the contrary, S. sardabelongs to the tribe Sardi- were also observed between the diet of bluefin tunas of the third ni, the ectothermic sister group to the endothermic tunas. The and fourth month of life, mainly due to a major consumption of inability to maintain elevated body temperature does not allow this cephalopods. species to tolerate sudden thermal variations and to perform rapid The Atlantic bonito showed a narrow trophic niche and a diet vertical migrations throughout the water column. Consequently the dominated by adult fish, while other prey such as invertebrates Atlantic bonito evolved a more specialized diet focused on abun- were recorded occasionally. Among fish, the most important prey dant coastal species. Key words. - Scombridae - Auxis rochei - Euthynnus alletteratus - Sarda sarda - Thunnus thynnus-MED - Southern Tyrrhenian Sea - Stomach content analysis - Feeding habits - Trophic partition - Ontogenetic diet shift - Trophic relationships - Phylogenetic relationships. (1) ICRAM, Istituto Centrale per la Ricerca Scientifica e tecnologica Applicata al Mare, Lab. di Milazzo, Via dei Mille 44, 98057 Milazzo (ME), ITALY.[[email protected]] Cybium 2006, 30(3): 220.