An evaluation of resource partitioning between two billfish, Tetrapturus belone and Xiphias gladius, in the central Mediterranean Sea

Abstract

The present study attempts to give information on the resource partitioning between the Mediterranean spearfish (Tetrapturus belone) and the swordfish (Xiphias gladius). The contents of 53 T. belone and 95 X. gladius non-empty stomachs were analysed from specimens caught in the central Mediterranean Sea (Strait of Messina), from 2004 to 2006, by the harpoon fishery. Daily catches (expressed as number of fish) showed the contemporary occurrence of both species in the studied area then allowing direct comparison of diets. Epipelagic fish were the dominant prey (%IRI = 99.1) of T. belone. Eight families were identified among them, with the dominance of Belonidae and Clupeidae, which represented 40.9% and 36.8%, respectively, of the total preyed items in terms of %IRI and were mostly composed of Sardinella aurita and Belone belone.Xiphias gladius preyed mainly on teleosts and cephalopods, which represented 59% and 39.1%, respectively, of the total preyed items in terms of %IRI. Eleven teleost and five cephalopod families were recognized among them with the dominance of Trichiuridae (IRI% = 30.5) and Ommastrephidae (IRI% = 27.6). The first was represented only by Lepidopus caudatus (IRI% = 30.5), while the latter by the squid Todarodes sagittatus (IRI% = 21.1) and Illex coindettii (IRI% = 6.5). Results of a multivariate statistical analysis demonstrated that the dietary compositions differed significantly between swordfish and spearfish. Diet overlap analysed with the Schoener and Horn indices showed low values (0.23 and 0.21 for the two indices) underlining a food partitioning that prevents competitive exclusion. Our results highlight a feeding strategy that is more related to the habitat of the species than to the food availability. In fact, migration patterns of the two predators are quite different. Swordfish show vertical migrations from 0 to 800 m while spearfish are characterized by limited migrations, ranging between 0 and 200 m depths. The observation of specific prey items in the stomach content of both billfish confirmed the bathymetric range of their migrations.

Pelagic cephalopods of the central Mediterranean Sea determined by the analysis of the stomach content of large fish predators

“…While these three species are usually able to explore a large part of the water column, the Mediterranean spearfish does not seem to dive deeper than the thermocline (Nakamura 1985), as reported in studies on its feeding Romeo et al 2009). …”

“…In this way, it is possible to describe the occurrence of pelagic cephalopods in an area and to obtain precious information on the ecology and behavior of cephalopods (Bello 1996;Tsuchiya et al 1998;Cherel et al 2004;Lansdell and Young 2007). Although several studies underlined the significant presence of cephalopod prey in the diet of large Mediterranean pelagic fishes (Bello 1991;Bello 1999;Salman 2004;Sinopoli et al 2004;Peristeraki et al 2005;Sarà and Sarà 2007;Castriota et al 2008;Consoli et al 2008;Karakulak et al 2009;Salman and Karakulak 2009;Romeo et al 2009), data on the specific composition and distribution of pelagic cephalopod communities in the Mediterranean are still poor.…”

Relationships between otolith size and fish size in some mesopelagic and bathypelagic species from the Mediterranean Sea (Strait of Messina, Italy)

“…However, to this day, the lack of data in these area has not always permitted an appropriate quantification of their prey biomass and classification to species level in the diet of Mediterranean pelagic predators. In fact, quite a high percentage of unidentified mesopelagic fishes and otoliths in swordfish and tuna stomach contents have been recorded (Sinopoli et al, 2004;Falautano et al, 2007;Mostarda et al, 2007;Romeo et al, 2009). …”

Satellite telemetry reveals physical processes driving billfish behavior

“…Although not a target species for most fisheries, it is caught by harpoon in the Straits of Messina, where it is also known to reproduce [26]. Analysis of stomach contents suggests that pelagic fish and cephalopods comprise the diet of this species [109,110]. However, no reports of satellite-tagged Mediterranean spearfish were found, and there is little other information available about the movements and habitat preferences of this species.…”