Abstract

Oil and gas production in the northern Gulf of Mexico has expanded into deep waters of the outer continental shelf, where the deepwater petroleum structures (DPSs) act as fish aggregating devices (FADs) for tunas and other species. We initially studied yellowfin tuna (YFT) aggregations around a DPS by tracking acoustic-tagged fish from a research vessel (Edwards and Sulak, 2006) and more recently, around a large spar platform, Medusa, 69 km off the Mississippi delta moored in 678 m depth, by attaching automatic receivers to the platform at 9 m depth, and tagging 31YFT with surgically-implanted, coded acoustic transmitter tags in July, 2005. Most fish remained highly associated with the platform, with many present for relatively long, continuous periods without day-scale absences early in the monitoring period; many returning for periods of several days after day-scale absences; some returning more than one time; and several returning after long absences. The pattern of residence was dissimilar to other, disparate patterns reported in comparable studies of YFT at FADs and seamounts, showing that YFT aggregation, residence, movement, and migration patterns are complex and vary greatly from location to location. The patterns observed in this study suggest that DPSs greatly affect YFT presence, distribution, movement, catchability, and possibly population structure in the northern Gulf of Mexico. Because the impact is potentially large, a new study is being planned for 2007, in which all or most of the DPSs in the Mississippi Canyon area will be instrumented, and a large number of YFT (~150) will be tagged and monitored. Additionally, YFT movement to and from a natural bathymetric feature (Sackett Bank = Midnight Lump), an area of aggregation during winter supporting a large recreational fishery, will be assessed by tagging some fish there and by deploying monitoring instruments during winter 2007-2008

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