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Management of Atlantic bluefin tuna (Thunnus thynnus) is currently based on the premise of two principal zones of spawning and juvenile production, which occur in the Mediterranean Sea and Gulf of Mexico. Although trans-Atlantic migration of members from both production zones is well documented through conventional and electronic tags, some degree of residency to spawning/nursery grounds is assumed, justifying International Commission for the Conservation of Atlantic Tunas’ (ICCAT) separate assessments and regulations for “eastern” and “western” stocks. This assumption has been challenged in recent times due to the seasonal progression of Atlantic bluefin tuna across the 45°W meridian management boundary, as observed through recent landings data and electronic tagging results. As a consequence, there is a clear need for empirical methods to directly estimate the contributions of recruits originating from eastern (Mediterranean) and western (Gulf of Mexico) nurseries to the fisheries that depend upon these recruits. PFCP scientists and Dr. David Secor (University of Maryland) are using natural tracers in otoliths (ear bones) of Atlantic bluefin tuna to predict nursery origin, and use these natural markers to estimates mixing rates |
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of sub-adult and adult tuna. |
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Recent stock assessments of Atlantic blue marlin (Makaira nigricans) by ICCAT reported that stocks are well below the level to support maximum sustainable yield (MSY). Blue marlin appear to be comprised of single stocks in the Atlantic Ocean, and although blue marlin are managed under ‘single-stock’ paradigm, major uncertainties in stock structure exist because the nature and extent of movement is poorly understood. Through support of The McDaniel Charitable Foundation, PFCP scientists are currently using pop-up satellite archival tags to assess the stock structure of blue marlin in the Gulf of Mexico and assist in the identification of site fidelity. Pop-up tags are designed to track large-scale movements of pelagic fishes, and during deployment these tags collect detailed depth, temperature and light-level data. Data readings are provided in four types of summarized data: depth distribution (diving behavior), depth-temperature profiles, migration path during period of deployment, and GPS data |
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on pop-up location. |
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Adult blue marlin, white marlin, and sailfish are commonly taken by recreational and commercial anglers (bycatch from US longline fleet) during late spring to early fall, which happen to coincide with presumed spawning periods of all three species. Moreover, larval billfishes are commonly reported from the Gulf, possibly indicating the region represents an important spawning and nursery area of billfish. In response, PFCP scientists are currently examining the early life ecology of billfishes in the northern Gulf of Mexico (GOM). Our working hypothesis is that the NW GOM represents essential habitat (spawning & nursery habitat) of blue marlin, white marlin, and sailfish. The goal of this work is to identify spawning/nursery grounds and describe oceanographic features (upwelling zones, Loop Current and associated fronts, warm/cold core eddies) that favor the production, retention, and survival of larvae and juveniles. Using samples collected with net gears (neuston, larval purse seine) from our sampling corridor in the NW GOM, we are examining basic life history parameters (age, hatch-date, growth rate) of billfishes, and assessing the habitat quality through the use of biochemical condition (RNA:DNA) and growth (otolith microstructure) |
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measures. |
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PFCP scientists are currently evaluating the “ecological value” of Sargassum (free-floating brown algae) with respect to fisheries production. Survey work indicates that several pelagic species (e.g. amberjack, cobia, dolphin, tuna) utilize Sargassum mats, particularly during early life, and survival and recruitment success of these taxa may be linked to distribution and abundance of Sargassum. Efforts to date by PFCP scientists have focused on documenting patterns of habitat use by pelagic fishes and estimating rates of production within the Sargassum complex. In addition, we are in the process of examining the source(s) of organic matter supporting pelagic fisheries and delineating pathways of energy flow through the pelagic food web using natural |
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tracers (stable isotopes, fatty acids). |
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PFCP scientists are currently in the process of evaluating the mercury (Hg) concentration in the tissue of pelagic fishes from the NW Gulf of Mexico, examining spatial variation in concentration of Hg in the tissue of pelagic fishes and developing predictive models relating fish size and feeding histories (based on natural dietary markers-fatty acids) to Hg profile in pelagic fishes. Current work centers on relating patterns of movement and site fidelity (pop-up satellite tags) to fish tissue Hg, and developing a predictive model to evaluate risk factors associated with consumption of pelagic fishes with varying levels of fish tissue Hg. |
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Copyright 2006, Pelagic
Fisheries Conservation Program |
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