There is a general lack of information on the life history of tropical tunas. Basic attributes like age and stock composition, spawning dynamics, migratory routes, and habitat utilization remain poorly understood. For example, yellowfin and bigeye tuna are managed as a single, Atlantic-wide stock, yet there is little to no data that can be used to support those hypotheses. The Gulf of Guinea is known to be the major spawning ground for these tropical tunas, but histological evidence suggests additional spawning grounds in other regions of the Atlantic. We know little about which fish spawn in which areas, if they visit more than one spawning ground throughout their lives, and what the overall exchange of fish across the ocean basin might be. What we do know from limited conventional tagging is that the Atlantic may be unique compared to other oceans. It appears yellowfin tuna in the Atlantic express basin-scale migrations that can exceed 6,000 miles — migrations which seem to be restricted in other ocean basins. Explanations for why this occurs in the Atlantic, or if it even still occurs, remain elusive.
Non-conventional techniques used to identify stock structure in other tunas (e.g., otolith chemistry, genetic analyses), unfortunately, don’t work well for tropical tunas, so determining if they cross from one side of the ocean to the other requires more effort. Therefore, we rely on conventional and electronic tagging methods to gain understanding into how these tropical tunas use the Atlantic basin. Using information gained from those methods, we can assess and manage the stock more appropriately.
This project uses a large network of commercial and recreational fishermen who deploy conventional and electronic tags on skipjack, yellowfin and bigeye tuna throughout the northwest Atlantic. Conventional tags are simple. A printed unique number is placed on a small plastic tube attached to a nylon leader, and when fishermen catch one of these tunas, they put the tag in the fish and record appropriate data like fish length, species, and location. If the fish is recaptured and the tag turned in, we get the recapture location. While the sophistication of these tags is pretty minimal, they can tell us a fair amount about large-scale movements across ocean basins.
Electronic tags are a bit more sophisticated. They can record data for up to ten years and may be placed on the tuna externally or internally. Some require recapturing the fish to retrieve data, but other fishery independent tags possess the technology to report their data to an orbiting satellite network without having to recapture the fish. Electronic tags provide higher level data resolution, and also collect information on water temperature, depth, and location. They are more versatile and useful than conventional tags, as they can provide answers to more questions about habitat uses, migrations, and stock structures — but they are more expensive, and so are used less frequently.
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