Searching for Atlantic bluefin tuna larvae and more in the Slope Sea

The NOAA Vessel Gordon Gunter departed on June 10 from Newport, Rhode Island, and immediately headed off the continental shelf to water deeper than 1,000 meters (about 3,300 feet) known as the Slope Sea.  The Slope Sea is an area of the ocean that is bounded to the north and west by the northeast United States Continental Shelf and to the south by the Gulf Stream, whose dynamic currents provide a strong influence over the area.  The Gulf Stream is constantly shifting position and pathways, like a giant water snake slithering through the ocean, and will frequently shed pockets of warm water, called Warm Core Rings, into the Slope Sea.  The dynamic nature of the Gulf Stream creates a mosaic of habitats that are used by a wide range of species.  However, very little of the Slope Sea has been explored, and little is known about who calls it home.

GU1702_10June2017_NE slope sea graphic

Sea surface temperature image from NOAA-19 satellite for the Slope Sea blue fin tuna cruise the day of departure.  The water temperature is shown in the colors, with blue cold temperatures and red warm.  The Gulf Stream shows up as the darkest red.  The black line represents the proposed cruise track for the northeast portion of the cruise.

The cruise now underway is targeting a different set of objectives from the standard Ecosystem Monitoring (EcoMon) cruises run by the Northeast Fisheries Science Center (NEFSC)’s Oceans and Climate Branch.  As described in a previous post, the EcoMon cruises are designed to monitor lower trophic levels, oceanographic conditions, and the distribution of fish larvae and eggs across a wide area of the continental shelf.  While some changes have been made over time, this area has been sampled with the same general sampling approach since the 1970s.

The current cruise stands in sharp contrast to the EcoMon program in that it is focused on a rarely sampled area, the Slope Sea, and much more prominently on a single species, Atlantic bluefin tuna.

Atlantic bluefin tuna are one of the iconic marine fish species.  Over the course of their seasonal migrations, individual bluefin tuna can occupy both nearshore and oceanic waters, swim across the ocean, and cross through international waters and the exclusive economic zones of multiple nations.  These migrations expose bluefin tuna to a wide variety of fisheries, a factor that presents a notable challenge to managing this species.

In recent decades, the common view of Atlantic bluefin tuna was that they spawned only in two places, the Mediterranean Sea in the Eastern Atlantic and the Gulf of Mexico in the western Atlantic.  However, in the summer of 2013 two cruises sampled the Slope Sea, both of them achieving noteworthy catch rates of early-stage bluefin tuna larvae.  These collections were consistent with a hypothesis first put forward in the 1950s that the Slope Sea was a third spawning ground for this species.  Follow up sampling in 2016 again achieved notable catch rates of bluefin tuna larvae.

Researchers aboard the NOAA Ship Gunter are evaluating the distribution and abundance of bluefin tuna larvae in the Slope Sea spawning ground.  This cruise is sampling earlier in the season than previous Slope Sea cruises to provide us information on the regional start of the spawning season for this species.

The scientists aboard also include two sea bird observers who will be conducting visual observations during the daytime transits between stations.  In addition to sea birds, they will be looking for whales and dolphins, sea turtles, large fish, and jellyfish.

While the focus of this cruise is on Atlantic bluefin tuna, one other exciting aspect of the project is that we are not certain of the full suite of other species we might collect or observe during visual surveys.  The Slope Sea has long been hypothesized to be important in the life history of other species, besides Atlantic bluefin tuna, that support economically valuable fisheries off the northeast United States.  Given the limited sampling that has been done in this region, it is quite possible that a new discovery will result from this cruise.  Follow along  for the next 2 weeks, as we travel through the Slope Sea and down the United States Atlantic coast to Cape Canaveral, Florida, as we further describe our mission and our initial discoveries.

Scientists and Crew of the NOAA Ship Gordon Gunter, GU1702
Harvey Walsh, Chief Scientist

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