Underwater tagging of the Atlantic bluefin tuna in the trap fishery of Sardinia (W Mediterranean)

Tagging bluefin tuna (BFT) has become an essential tool for fishery science that has improved the identification of growth parameters and age validation, population abundance estimates, movement and migration patterns and spatial and temporal population dynamics. Although innovative technologies and methodologies have been introduced, some unresolved issues remained regarding the possible alteration of fish behaviour and survival during post-tagging as consequence of capture and handling. Such issues have raised the question of whether underwater tagging might be less invasive and preferable to tagging fish on board. In the present study a framework to manage traditional trap gear "Tonnara" for underwater tagging and release purposes was developed for conventional tagging in the Sardinian traps. The general objective of the current study was to ameliorate the operational framework to determine best practices for the underwater tagging of BFT using pneumatic spearguns. Our specific objectives were: (1) to identify the proper size of pneumatic speargun and its operating pressure, (2) to identify the proper shooting distance for the aforementioned equipment, (3) to develop a tool for the indirect estimate of tuna size during tuna tagging operations and 4) to report the results of the tagging activities carried out with conventional tags in Sardinian traps during the 2014 season. The results of the penetration test showed that the shooting distance should be 1-3 m to be successful using a pneumatic speargun at 20 bars of pressure. The indirect length estimation of BFT size was more accurate when the lasers were exactly perpendicular to the animal. However, this method always underestimates the size of the fish, with an average relative error of about -30 cm. During tagging activities in the Sardinian trap in 2014, a total of 63 fish were tagged in 3.5 h, and only one fish died directly from tagging injuries. The trap represents an optimal system for tagging large numbers of BFT in confined waters when the main goal is to release the fish in the best possible condition. The fish can be confined for several hours in the death chamber, allowing determination of the survival of tagged fish and tag retention.