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.

中文翻译：

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地中海大西洋蓝鳍金枪鱼（Thunnus thynnus）的表层和垂直行为：对航空测量的影响

大西洋金枪鱼（Thunnus thynnus）（ABFT）经常从事表面晒晒和觅食的行为，使它们在远处即可被发现。尽管尚未考虑到ABFT的地面觅食动态的变化，但这种行为可用于根据航空调查开发与渔业无关的丰度指数。我们调查了2015年至2016年之间在24个体（叉长117–158 cm长）上以不同的时间和垂直分辨率对ABFT进行的白天表面堆焊行为，并用弹出式归档标签对其进行了标记。结果表明，白天白天，ABFT通常连续不到2分钟在可见表面（0-1m）内保持。ABFT在0–1和0–20 m层中的存在随时间和个体之间的变化而变化，但随着热分层的破坏，显示出秋季至秋季的季节性下降。此外，堆焊事件的发生率与0-20 m层所花费的时间高度相关。地理位置估计证实了在狮子湾的航测期间（8月至10月），ABFT的站点保真度很强。我们的结果支持调查区域和时期的选择，但相关指标应考虑ABFT表面行为的季节性[即在0-20 m层中所花费的时间。