AUTHOR=Dedman Simon , Aalto Emilius A. , Stokesbury Michael J. W. , Schallert Robert J. , Castleton Michael R. , Block Barbara A. 

TITLE=Assignment of tracks from tagged Atlantic bluefin tuna Thunnus thynnus to potential stocks using behavioural differences and habitat partitioning

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 10 - 2023

YEAR=2023

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1165910

DOI=10.3389/fmars.2023.1165910

ISSN=2296-7745

ABSTRACT=Atlantic bluefin tuna (ABT) are large, migratory pelagic predators of high economic importance. ABT are currently managed as two independent stocks assigned to discrete spawning areas (Gulf of Mexico, and Mediterranean); however, stock overlap outside spawning areas makes accurate assignment of catch to stock-of-origin difficult. Within this two-stock paradigm, we characterised stock-specific spatial distribution and behaviours by comparing habitat usage and vertical movement behaviours of 118 electronically tagged adult ABT assigned to the GOM and Med spawning grounds. These spatial and behavioural differences were used in tests to probabilistically assign unknown individuals (which did not visit the GOM/Med) to a stock. This new methodological approach to stock differentiation and designation using existing tag data be achieved quickly and cheaply, before genetic assignments.
We identified similar habitat utilization in the northwest Atlantic Ocean off the US east coast between November and April, but markedly different movement patterns May through October. For example, GOM-assigned ABT were never north of 50°N, rarely east of 55°W; Med-assigned ABT were rarely south of 30°N. Within the Gulf of St. Lawrence, Med-assigned ABT were mostly found at greater depths within the colder deeper central channel, while GOM-assigned ABT were typically inshore at shallower depths. In the northwest Atlantic, GOM ABT are near the surface thrice as often as Med ABT and at higher latitudes, whereas in the Central-West Atlantic, GOM ABT are in deeper waters. GOM ABT travel more directly while migrating to their spawning ground, staying offshore with more frequent vertical movements into the mesopelagic (greater daily mean and maximum depths); in contrast, Med ABT do not return to their spawning grounds most years, and either track the coast to over-winter near Cape Hatteras, or travel far offshore to the mid-Atlantic.
The probabilistic assignment approach had 97% in-bag testing accuracy, then assigned 190 individuals to a stock, doubling the number of assigned stock-of-origin tracks in our dataset, and potentially revealing novel movement and behaviour patterns among pre-spawning-age ABT. This approach can be easily adapted to other study species, more stocks, and different testing variables, hopefully serving as a useful addition to the fisheries management toolkit.