Scientists using underwater acoustics to study bigeye tuna and their prey at Cross Seamount

April 15, 2008
Samples of organisms collected using a large trawl net lowered to depths of thick "scattering 
    layers" are sorted in the "wet-lab" of the Oscar Elton Sette.
Samples of organisms collected using a large trawl net lowered to depths of thick "scattering layers" are sorted in the "wet-lab" of the Oscar Elton Sette.

At Cross Seamount southeast of the Big Island of Hawaii, PIFSC has embarked on a study that will improve scientific understanding of bigeye tuna. A team of researchers on the NOAA Ship Oscar Elton Sette is investigating waters over the seamount, describing their physical and biological characteristics and measuring the abundance of bigeye tuna and their prey species that frequent the area.

Cross Seamount is a prominent fishing ground for Hawaii-based fishing fleets. The bigeye tuna found there are part of a Pacific-wide population that is subject to intense fishing by a multinational fleet of longline vessels and purse seiners. Effective conservation of bigeye tuna requires a good understanding of their biology and population dynamics not only on an ocean-wide basis, but also on a regional and local scale.

Located about 290 kilometers (160 nautical miles) south of Oahu, Cross Seamount rises steeply from the ocean floor at about 5800 m ( 19000 feet) to its summit 330 m (1100 feet) below the sea surface. The summit is surrounded by a relatively flat plateau about 400 m (1300 feet) in depth and 10 km (5 nmi) across. Bigeye tuna and their prey, including small fish, squid, and shrimp, preferentially aggregate there. The biomass of bigeye tuna at the seamount depends partly on conditions at the seamount and partly on physical and biological processes across the range of the tuna stock. Understanding effects of the seamount environment on the movement patterns, distribution, and abundance of bigeye tuna and forage species, and the interactions between the tuna and their prey, is crucial to sound management of the bigeye tuna stock and sustainability of the Cross Seamount fishery.

"Echogram", displaying a large aggregation of bigeye tuna recorded over the 
        plateau of Cross Seamount at the 38 kHz (upper panel) and the 120 kHz (lower panel) frequencies.  Note that the aggregation of tuna 
        extends from near the sea surface down to the seamount plateau at a depth of 400 m, shown as a dark red line at the bottom of the 
        38 kHz echogram.
"Echogram", displaying a large aggregation of bigeye tuna recorded over the plateau of Cross Seamount at the 38 kHz (upper panel) and the 120 kHz (lower panel) frequencies. Note that the aggregation of tuna extends from near the sea surface down to the seamount plateau at a depth of 400 m, shown as a dark red line at the bottom of the 38 kHz echogram. Click Image for higher resolution.

Led by oceanographer Dr. Reka Domokos of the PIFSC Ecosystem and Oceanography Division (EOD), scientists on the Sette will assess the abundance of bigeye tuna and their prey over the seamount directly using underwater acoustics methods in combination with observations of the physical environment. The bioacoustics method involves sending very short bursts ("pings") of sound waves from the ship downward toward the seafloor and then measuring the intensity of echoes reflected ("scattered") back up to the ship by tuna, prey organisms, and other objects in the water column. Acoustic data are collected on board the Sette, which is equipped with 3 hull-mounted split-beam transducers operating at 38, 70, and 120 kHz frequencies, each providing data over a different range of depths. Differences in sound scattering characteristics of organisms at the three frequencies help in identifying organisms and estimating the composition of "scattering layers", thick layers of organisms which provide the food base for larger predators such as bigeye tuna.

The current research cruise, from April 15 to May 10, 2008, is the third expedition to Cross Seamount. It is part of a collaborative program between EOD and the Pelagic Fisheries Research Program of the Joint Institute for Marine and Atmospheric Research (School of Ocean and Earth Science and Technology, University of Hawaii), which helped fund the study. The field party includes collaborating scientists from the University of Hawaii and other universities.

A CTD rosette, complete with bottles to bring up water samples from various depths, is being lowered into the water off the starboard side of the Oscar Elton Sette.

During the cruise, the scientists will collect a variety of oceanographic data to assess the effects of the physical and biological environment on the abundance and distribution of bigeye tuna and its forage species. In addition to the bioacoustics information, data on water temperature, salinity, the concentration of dissolved oxygen, and the concentration of chloropigments will be measured in the water column from the sea surface down to a depth of 1000 m by deploying sensors on a "CTD rosette". The speed and direction of the ocean current will be continuously monitored by a shipboard acoustics instrument down to a depth of about 700 m (2300 feet). The identity of small fish and other prey species in the scattering layers that are observed by their echoes in the bioacoustics surveys will be estimated by deploying trawl nets at the appropriate depths and collecting biological specimens. Specimens of bigeye tuna will also be sampled by handlining off the "fantail" of the Sette at times when bigeye aggregations are observed acoustically near the surface.

Data collected during the Sette expedition will provide valuable information on bigeye tuna biomass over the Cross Seamount and relationships between bigeye tuna, their prey species, and the physical and biological characteristics of the seamount waters. Methods to assess biomass through acoustic surveys may prove useful for monitoring changes in the stock and helping ensure sound management of this economically important species.