The Desert Star Systems SeaTag-LOT was used in this study. The SeaTag-LOT is powered by a solar-charged capacitor rather than a battery and does not contain a pressure (depth) sensor. Once a tag is fully charged, which takes approximately 30 min in full sunlight, enough solar power can be stored so that the tag will continue to record and archive data for up to three days in complete darkness. While the tag records light and temperature data at 4-min intervals, the SeaTag-LOT only archives and transmits daily summary data for four light- and temperature-related measurements: a) capacitor voltage (daily average); b) solar panel voltage (daily average); c) temperature (daily minimum, maximum, and average); and d) maximum daily change in temperature per minute (ΔT min−1, calculated by dividing the maximum change in temperature between measurements by four). In addition, the tag transmits day length and local apparent noon time for each day of deployment. The reduced quantity of data archived, lack of pressure sensor, and use of solar power rather than a battery contribute to the relatively low per-unit cost of this PSAT ($899 for quantities of less than 50, or roughly one quarter the price of other commercially available PSATs).
Because the SeaTag-LOT only transmits daily summary data, it was necessary to consider how the tag should be configured to detect post-release mortality specifically for bluefin tuna off the US east coast. Tag configuration included the development of thresholds, under which the tag would pop off prior to the scheduled deployment date, for three mortality scenarios: (1) a fish dies and sinks to the bottom in shallow water (i.e., on the continental shelf); (2) the fish/tag is eaten (scavenging or predation); or (3) a fish dies and sinks in water deeper than the tag’s 1200 m service depth.
For scenario 1, the maximum ΔT min−1 recorded by the tag was used as an indicator of whether a fish was alive and moving vertically in the water column. High-resolution (~ 5 min) depth and temperature data transmitted from Microwave Telemetry High-Rate X-Tags previously deployed on school-size Atlantic bluefin tuna and white marlin (Kajikia albida) (J. Graves, unpubl.) were examined to determine the minimum ΔT min−1 typically exhibited by a living fish moving vertically in the water column, which could distinguish it from a dead fish resting at a constant depth on the sea floor (or a shed tag floating on the surface). Surviving fish generally exhibited a daily maximum ΔT min−1 well in excess of 0.2 °C min−1; data from tags deployed on school-size bluefin tuna, for example, indicated typical daily maximum ΔT min−1 values between 1 and 2 °C. Tags deployed on white marlin that subsequently died and rested on the bottom for several days, meanwhile, indicated maximum ΔT min−1 values of less than 0.05 °C. The release threshold was thus set for 72 h with a maximum ΔT min−1 of less than 0.2 °C. If a tag were to pop off due to exceeding this threshold, an examination of temperature data during the low ΔT min−1 interval could be examined to infer that the fish was dead and resting on the bottom in cool waters versus alive and maintaining a very stable depth distribution higher in the water column. A shed tag floating on the surface could be differentiated from a tag that popped off a dead fish because the former, when floating on the surface, would begin transmitting in an “On Fish,” rather than “Reporting,” status.
For scenario 2, a tag’s remaining in complete darkness for a certain minimum amount of time was considered an appropriate indicator of predation or scavenging. Ingestion of PSATs (and presumably the fish to which they were attached) by predators or scavengers is well-documented [8, 9, 18, 23], and tags generally remain inside the consumer’s stomach for at least several days before being egested, floating to the surface, and transmitting data. Given these findings, tags were programmed to release if maintained in complete darkness for 48 h.
For scenario 3, a low-temperature threshold at which the tag would pop off of the fish before sinking below the tag service depth was determined through inspecting depth-temperature data collected off the coast of North Carolina’s Outer Banks via the World Ocean Database . Depth-temperature data indicated that temperatures at 1000 m depth were typically in the vicinity of 4.5 °C. While Atlantic bluefin tuna have a broad thermal range and have been recorded in temperatures as low as 3 °C , we judged it preferable to keep the low-temperature threshold conservative to minimize the risk of a tag pressure housing failure. In addition, previous PSAT tag research along the east coast of North America has suggested that bluefin tuna in this region rarely encounter temperatures below 8 °C [20, 26]. It is possible that a deep-diving, surviving fish could swim below a conservative low-temperature threshold and cause tag pop-off, erroneously indicating a mortality; in such a case, however, the tag would provide information (e.g., daily temperature ranges prior to pop-off) from which survival could be inferred. The low-temperature threshold for pop-off was thus set at 4.5 °C. In addition to examining whether tags popped off due to exceeding the thresholds described above, daily summary data of light level and temperature were visually examined to infer whether a fish survived the deployment duration.
PSATs were deployed on large school- and small medium-size Atlantic bluefin tuna caught onboard recreational charter vessels using typical light-tackle methods during the 2015 and 2016 fishing seasons off the coasts of Massachusetts and North Carolina. The majority of tagged fish were caught using spinning tackle and braided line with a rated breaking strength of 36–45 kg; one tagged fish was caught on a conventional (revolving-spool) vertical jigging rod and reel with 36-kg breaking strength braided line. Artificial lures used to catch bluefin tuna included soft plastic lures rigged with single J-hooks, hard-bodied lures rigged with either treble or single hooks, and metal jigs rigged with single “assist” hooks. In addition, on a few occasions fish were caught by casting live Atlantic mackerel (Scomber scombrus), rigged with a single J-hook, into a school of actively feeding Atlantic bluefin tuna using spinning tackle.
Atlantic bluefin tuna were fought, handled, and released in the manner typically practiced by each fishing vessel, with no input from the tagging researcher. Bluefin tuna were tagged regardless of condition, and following the method of Marcek and Graves , multiple fish were not tagged if hooked within 30 min of one another in order to avoid sampling from the same school of fish and maintain a random sample to the extent practicable. Methods of securing fish for unhooking and tagging included lip-gaffing (either maintaining the fish in the water or sliding it onboard through the vessel’s tuna door [a door in the transom to facilitate the landing of large fish]) or holding the fish under the operculum while supporting it against the vessel’s gunwale. Gear type, fight time (hooking to capture), total time (hooking to release), hooking location in the fish, fish length (CFL), sea surface temperature, release location, and other relevant factors were recorded for each fish. In addition, the condition of each fish was assessed using a modified version of the “ACESS” condition scale developed by Kerstetter et al. . Each fish’s condition was rated from 0 to 8 by evaluating four characteristics on a scale of 0 (poor) to 2 (good): overall activity, color, body positioning, and bleeding (i.e., a score of 2 means little/no bleeding).
The PSATs used in this study were programmed to record light level and water temperature every 4 min over the course of 31 days (or 30 full daily summaries), after which they were to detach from the fish via an ignition release, float to the surface, and transmit data. Tags were light-activated and maintained in sunlight for at least 30 min prior to deployment to ensure a full solar charge. The PSATs were rigged with 16 cm of 91-kg test monofilament fishing line attached to a hydroscopic nylon intramuscular tag anchor, following Marcek and Graves . Each tag anchor was inserted to a depth of approximately 10 cm into the fish’s dorsal musculature 10 cm posterior to the origin of the first dorsal fin and 5 cm ventral to the base of the first dorsal fin, where it was able to interlock with the pterygiophores supporting the dorsal fin . After tagging, at the discretion of the fishing crew, some fish were revived boat-side prior to release using a lip-gaff while slowly moving the vessel forward at about 2 kt.
PSATs will sometimes release from fish prior to the scheduled release date (i.e., are shed), which could occur during routine swimming (for example, if the tag anchor pulls out of the dorsal musculature), or due to other reasons, such as a predation event in which the tag, rather than being ingested by the predator, is dislodged and floats to the surface. It is important to establish a threshold deployment duration to determine which prematurely released PSATs should be included in the post-release mortality estimate [18, 29]. While previous post-release mortality studies have indicated that most capture-induced mortalities tend to occur within 48 h of release [6, 19, 30, 31], the 5 days following release has often been used as the interval during which mortalities would be considered angling-induced (as opposed to natural mortalities) [8, 18]. As a result, to avoid misinterpreting the fate of surviving fish from tags that released prematurely, only tags from fish that remained attached for 5 days or longer and whose summary data for the first 5 days were consistent with survival were included as survivors in the estimate of post-release mortality.
To determine the effect of sample size on the 95% confidence interval for the post-release mortality estimate, bootstrapping simulations based on 10,000 bootstrapped samples were performed using software developed by Goodyear . For the purposes of bootstrapping, natural mortality M was assumed to be 0.14 year−1 and age-independent, an assumption similar to that used in the 2014 stock assessment for western Atlantic bluefin tuna conducted by the International Commission for the Conservation of Atlantic Tunas (ICCAT) Standing Committee on Research and Statistics . The post-release mortality estimate for the light-tackle fishery was statistically compared with Marcek and Graves’  estimate for school-size Atlantic bluefin tuna caught in the troll fishery, as well as with Tracey et al.’s  estimate for southern bluefin tuna caught in the troll and drift fishery, using Fisher’s exact tests.
Net displacement for tagged fish was calculated as the first high-quality pop-off position estimate (Argos location code 1, 2, or 3). In some cases, a high-quality location was not transmitted in the period immediately (~ 8 h) after pop-off, in which case the first reasonable location estimate received (Argos location code 0, A, or B) was used to calculate net displacement. The straight-line distance between tag deployment location and pop-off location was calculated using ArcGIS version 10.2.2 (ESRI, Redlands, California).