Extensive movements of bluefin tuna have been reported for decades as marked fish from the east and west Atlantic have been caught on opposite sides of the ocean from where they were tagged. However, the extent of this mixing was always assumed to be minimal, usually less than 5-10% annually. This reported mixing rate is important because bluefin are managed by ICCAT as two separate stocks divided by 45 degree longitude. Different quotas are allocated to each side based on the best estimates of abundance by managers. Extensive mixing could lead to problems when trying to estimate abundance for two "separate" stocks. Traditional mark-recapture was also very limited because the fish needed to be recaptured in order to determine if it crossed the boundary line. Simple mark and recapture tagging does not tell scientists where the fish traveled between the release and subsequent capture points.

Recent advances in tagging technology within the last decade have produced pop-up satellite archival tags (see picture below). These small minicomputers are externally attached to blue fin (see picture right) and record location, depth and external temperature. At a pre-programmed time the tags will jettison from the fish and "pop-up" to the surface. Data is then transferred from the tag to an orbiting Argos satellite and e-mailed back the researcher.

TECHNOLOGY- Pop-up Satellite-transmitting Archival Tags (PSATs) have been developed and refined over the last decade, and offer a unique way of monitoring the long range movements of pelagic species. As bluefin tuna rarely breach the surface and radio signals do not transmitted through salt water, these data loggers rely on a jettison mechanism to release the tag from the fish which then floats to the surface. Once there, they transmit information through orbiting ARGOS satellites. The actual data is therefore retrieved after a deployment time of up to a year.

The tags deployed by the Large Pelagics Lab are manufactured by Microwave Telemetry, Inc. and typically record time series of temperature and pressure with a 15 minute resolution.Ambient light levels are also monitored to provide estimates of sunrise and sunset times, and thus an estimate of the daily position of the fish. Recent advances have allowed for the smaller "X-tag" to be used in juvenile bluefin tuna tagging as well, rather than standard-sized PSAT tags previously used for adult bluefin only.

RESEARCH ON PSATS AT THE LPRL- PSATs document movements with a sufficient accuracy for studying basin-scale processes, yielding important information about the stock structure and migratory habits of bluefin. However, monitoring behavior over smaller (regional) scales is necessary for understanding how fish interact with their oceanographic habitat, and how seasonal distributions evolve over time, possibly revealing shifts linked to climate variability.

The behavior of the fish (i.e. the depths visited at sunset and sunrise) and water clarity play a major role in the final accuracy of the geolocation. Additionally, this light-based method is limited during the equinoxes as latitude becomes indeterminate (i.e. day length is almost constant at all latitudes). Errors are usually of the order of 1 degree in longitude and 2 to 3 degrees or more in latitude. Biases can be also expected, as the light sensor can be degraded or fouled over time.

To uncover behavior and patterns over smaller spatial scales, it is necessary to improve the geolocation capability of PSATs. This is achieved by post-processing data here at the lab.

OVERVIEW OF TECHNIQUE-We developed a method to improve tag positioning, by making better use of the information on temperature and depth. This method relies on an Ensemble Kalman Filter, a time series tool used in engineering and oceanography: using this, one can reduce variability in the observed data, and thus smooth the original trajectory according to a number of constraints. Specifically, we use here daily sea surface temperatures and maximum depth in conjunction with temperature maps and high-resolution bathymetry. The temperature maps are built from weekly composites of AVHRR or MODIS 4km products, with an interpolation step for cloud filling.

EXAMPLE OF RESULT- This technique allows interpolation of the trajectory when data is missing (in case of transmission problems), and reduces the uncertainty in location at crucial times of the year. The constraint on bathymetry proves very helpful in the vicinity of well-defined features, such as the shelf-break or the channels of the Gulf of Maine . High temperature gradients typical of the Gulf Stream area also help greatly for locating the fish.

The resulting trajectories are a significant improvement over the raw geolocations provided by the manufacturer. This opens the door to new studies, such as the effect of the Gulf Stream features on seasonal distribution of bluefin tuna.

Since 1997, the LPRL has focused on PSAT tagging of bluefin tuna and has deployed over 350 tags in the Gulf of Maine, North Carolina, and Nova Scotia, Canada. Through collaborative efforts, PSATs have recently been deployed on bigeye tuna (Thunnus obesus), blue sharks (Prionace glauca), ocean sunfish (Mola mola), bigeye thresher sharks (Alopias supersiliocus) and blue marlin (Makaira nigricans).

Data returned from these tagging efforts enhances our knowledge of how large pelagic species utilize their environment and provides crucial insight for fisheries managers.

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