Surfacing and vertical behaviour of Atlantic bluefin tuna (Thunnus thynnus) in the Mediterranean Sea: implications for aerial surveys

Abstract

Atlantic bluefin tuna (Thunnus thynnus) (ABFT) frequently engage in surface basking and foraging behaviour that makes them detectable from afar. This behaviour is utilized for the development of fisheries-independent abundance indices based on aerial surveys, although changes in the surface-feeding dynamics of ABFT are not yet accounted for. We investigated the daytime surfacing behaviour of ABFT at different temporal and vertical resolutions based on 24 individuals (117–158 cm fork length), tagged with pop-up archival tags in the Gulf of Lion, NW-Mediterranean Sea between 2015 and 2016. The results suggest that ABFT remain usually <2 min continuously within the visible surface (0–1 m) during daytime. ABFT presence in the 0–1 and 0–20 m layers varied over time and between individuals but showed a seasonal decline towards autumn with the breakdown of thermal stratification. Furthermore, the rate of surfacing events was highly correlated with the time spent in the 0–20 m layer. Geolocation estimates confirm a strong site fidelity of ABFT during the aerial survey period (August–October) in the Gulf of Lion. Our results support the choice of the survey region and period, but related indices should account for the seasonality of ABFT surface behaviour [i.e. the time spent in the 0–20 m layer.

Preliminary Insights on the Habitat Use and Vertical Movements of Pelagic Stingray (Pteroplatytrygon violacea) in the Western Mediterranean Sea

“…Atlantic bluefin tuna is a key commercially-important fish that is present in the GoL for most of the year (but less abundant during the spawning season (May to July) [45], with tagging studies showing site fidelity from August to October [46]. When the waters of the GoL are stratified during the summer months, bluefin tuna exploit the surface waters, where they are targeted by the longline fisheries, but as the stratification breaks down from autumn to winter, bluefin tuna move into deeper waters [46,47]. This study has shown that pelagic stingray surface daily, and data from one tag indicated that >40% of time was spent at depths of 0-2 m (Figure 3b).…”

Preliminary Insights on the Habitat Use and Vertical Movements of the Pelagic Stingray (Pteroplatytrygon violacea) in the Western Mediterranean Sea

“…Atlantic bluefin tuna is a key commercially important fish that is present in the GoL for most of the year but is less abundant during the spawning season (May to July) [47], with tagging studies showing site fidelity from August to October [48]. When the waters of the GoL are stratified during the summer months, bluefin tuna exploit the surface waters, where they are targeted by the longline fisheries, but bluefin tuna move into deeper waters as the stratification breaks down in late autumn and winter [48,49]. This study has shown that pelagic stingrays surface daily, and data from one tag indicated that >40% of time was spent at depths of 0-2 m (Figure 3b).…”