Estimating Natural Mortality of Atlantic Bluefin Tuna Using Acoustic Telemetry

Abstract

Atlantic bluefin tuna (Thunnus thynnus) are highly migratory fish with a contemporary range spanning the North Atlantic Ocean. Bluefin tuna populations have undergone severe decline and the status of the fish within each population remains uncertain. Improved biological knowledge, particularly of natural mortality and rates of mixing of the western (GOM) and eastern (Mediterranean) populations, is key to resolving the current status of the Atlantic bluefin tuna. We evaluated the potential for acoustic tags to yield empirical estimates of mortality and migration rates for long-lived, highly migratory species such as Atlantic bluefin tuna. Bluefin tuna tagged in the Gulf of St. Lawrence (GSL) foraging ground (2009-2016) exhibited high detection rates post release, with 91% crossing receiver lines one year post tagging, 61% detected after year two at large, with detections up to ~1700 days post deployment. Acoustic detections per individual fish ranged from 3 to 4759 receptions. A spatially-structured Bayesian mark recapture model was applied to the acoustic detection data for Atlantic bluefin tuna electronically tagged in the GSL to estimate the rate of instantaneous annual natural mortality. We report a median estimate of 0.10 yr^-1 for this experiment. Our results demonstrate that acoustic tags can provide vital fisheries independent estimates for life history parameters critical for improving stock assessment models.

Figure 1

External acoustic tag attachment for an Atlantic bluefin tuna with two titanium darts in the dorsal musculature.

Figure 2

Locations of acoustic detections in Canadian waters including Vemco receivers on moorings, and small detectors on electronically tagged Grey seals and wave gliders. This map was generated in ESRI ArcMap software (Version:10.3.1 & http://desktop.arcgis.com/en/arcmap/10.3/main/get-started/whats-new-in-arcgis-1031.htm).

Figure 3

Receiver locations of detected Atlantic bluefin tuna. (a) All receiver locations. (b) Graduated symbol detection count by region. This map was generated in ESRI ArcMap software (Version:10.3.1 & http://desktop.arcgis.com/en/arcmap/10.3/main/get-started/whats-new-in-arcgis-1031.htm).

Figure 4

Percentage of tags detected by year. Tags were initially programmed to last 2.5 years which accounts for the drop off in detection in year 3.

Figure 5

Detections of individual bluefin tuna with an acoustic tag. Eight consecutive years of deployments (black square) and subsequent acoustic detections for a fish from 2009–2016 deployments (diamonds are receiver detections colored by regions as indicated in the legend).

Figure 6

Number of acoustic detections on receivers along the Cabot Strait Line (top) and Halifax Line (bottom). Each bar represents an individual receiver.

Figure 7

Deployents (black square) and acoustic detections (colored diamonds) of Atlantic bluefin tuna released in March, 2013 off North Carolina.

Figure 8

Prior (dashed blue line) and posterior pdfs for annual natural mortality.

Figure 9

(a) Weekly detection frequencies by receiver array for acoustic tagged Atlantic bluefin tuna. (b) Posterior monthly movement rate estimates out of the Gulf of St. Lawrence (wide distributions e.g. in January and May reflect the prior). (c) Posterior monthly movement rate estimates into the Gulf of St. Lawrence.

Figure 10

Posterior estimates of acoustic detection probabilities by year. (a) Gulf of St. Lawrence (b) outside the Gulf of St. Lawrence.