Drag Studies Aimed at Improved Performance of Pop-up Satellite Archival Tags

Pop-up Satellite Archival Tag (PSAT).
Pop-up Satellite Archival Tag (PSAT).

Many varieties of tags are available for use in fishery biology, but pop-up satellite archival tags (PSATs) are increasingly used because they provide data on movements and habitats of fish without having to be physically recovered. Despite the widespread adoption of PSATs in marine studies, concerns remain about reporting rates of the tags, length of tag retention by the fish, and data return. The vast majority of PSATs are shed before their programmed pop-up date, with median retention times of only 9 days found in one study of bigeye tuna. Factors influencing retention time, however, are not well understood. For PSATs to be an effective tool they need to stay attached to the animal long enough to enable monitoring of long-term movements and to maximize benefits from experimental designs.

Researchers at PIFSC, in collaboration with mechanical engineers at the University of British Columbia, have been addressing the issue of short retention time of PSATs by characterizing the movement, drag force, and lift force of PSATs moving at various speeds and accelerations in a tow tank. In studies of pelagic fish, PSATs are usually attached by inserting a tag head into the fish's muscles at the base of the dorsal fin between the pterygiophores to achieve maximum "mechanical" resistance. The tag head and PSAT are joined by a tether. Probably one of the most common routes for tag shedding by fish is through continual movement of the tag head caused by movement of the PSAT at high velocity swimming. The movement of the PSAT may cause irritation of the tissue surrounding the tag head. When coupled with the forces of lift and drag on the PSAT, the tags prematurely shed.

In the tow tank studies, the drag force on the PSATs increased with speed and at speeds upward of 5 m s-1 the force measured > 8 N, equivalent to nearly 2 lbs of pull on the tag anchor. PSATs oscillated regularly with a frequency of about 9 Hz; and the frequency of oscillation was independent of speed and acceleration. Currently, commercial tags from several manufacturers have been tested through speeds of 0.5 m s-1 to 5 m s-1 and at accelerations ranging from 1.6 to 9 m s-2. The present data suggest that the burst speed and quick acceleration of pelagic fish may cause up to 36 kg of pull force on the anchor tag head leading to the early ejection of a PSAT. In the coming year, PIFSC researchers plan to test tags at burst speeds of > 20 m s-1 and accelerations > 30 m s-2 to verify the modeled forces at high speeds. Further tests will also include tags attached to tuna casts (simulated tunas). The turbulence and boundary layer associated with the fish body will affect the drag force and movements/rotations of the PSAT tags. Data from these experiments will enable improvements in tag placement and design that will inevitably increase retention time of PSAT tags attached to pelagic fish and enable the recording of valuable ecological and vital rate data.