Scientists on the NOAA Ship Oscar Elton Sette are conducting a bioacoustics survey to learn about the distribution and abundance of bigeye tuna and its prey at Cross Seamount

April 21, 2007
Samples of organisms from thick "scattering layers" are collected at various depths and brought aboard the Oscar Elton Sette using a large trawl net

Scientists on the NOAA Ship Oscar Elton Sette are working in waters over Cross Seamount, southwest of the island of Hawaii (Big Island) using bioacoustics survey methods to develop a fisheries independent method of estimating the local biomass of bigeye tuna. The studies also hope to determine the effects of the seamount environment on both bigeye tuna and its prey. In addition to the primary research on Cross Seamount, the Sette will survey selected areas of Penguin Bank, an important habitat for bottomfish in the Main Hawaiian Islands, to see if bioacoustics methods can be used to assess distribution and abundance of the bottomfish resource.

Cross Seamount is an important bigeye tuna fishing ground for Hawaii-based longline and handline vessels and also an important habitat for other marine species including cetaceans. The 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, 400 m deep (1300 feet) and 10 km (5 nmi) across, over which both bigeye tuna and its prey, such as small fish, squid, and shrimp aggregate.

The Sette cruise is led by Chief Scientist Reka Domokos of the NOAA Pacific Islands Fisheries Science Center's Ecosystems and Oceanography Division. Funding support for the survey was provided by 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. The field party includes a large contingent of collaborating scientists from the University of Hawaii and other universities. This is the second bioacoustics survey at Cross Seamount; a similar cruise was conducted in 2005.

Organisms collected in the trawl net 500 m (1700 feet) below the sea surface are sorted and identified by scientists on the Oscar Elton Sette.

During the cruise, NOAA scientists and collaborators are conducting a series of survey transects over the seamount during which sonar instruments on the ship will emit short sound pulses ("pings") of various frequencies towards the seafloor and record the intensity of return signals ("echoes" or "backscattering") reflected back to the ship by the seafloor and marine life in the water column. Echo data will be analyzed to locate and assess bigeye tuna in the water column and smaller organisms that are important prey of bigeye tuna. The Sette is equipped with 2 hull-mounted split-beam transducers emitting sound pulses at frequencies of 38 kHz and 120 kHz, enabling measurements of organisms down to minimum depths of 1000 m (3300 feet) and 300 m (1000 feet), respectively. Differences in sound reflecting characteristics of marine organisms at the two frequencies help identify the organisms and estimate the composition of "scattering layers", thick concentrations of smaller organisms which provide the food base for larger predators such as bigeye tuna. The acoustic data will enable scientists to estimate the biomass of bigeye tuna and prey species across the seamount at various depths.

Acoustic backscatter data collected at Cross Seamount (left) and in the open ocean (right) show the aggregation of small organisms over the plateau of the seamount during both day (top panel) and night (bottom panel)

At intervals during the bioacoustics transects, the scientific team will also collect information on environmental parameters including water temperature, salinity, and the concentrations of dissolved oxygen and chloropigments. These measurements will be taken from the sea surface down to depths of 1000 m by deploying sensors on a "CTD rosette". Water samples will be collected at selected depths to ground-truth the chloropigment profiles. The speed and direction of ocean currents down to a depth of about 500 m (1640 feet) will be continuously monitored by a shipboard acoustic instrument. The crew will also deploy research trawl nets to collect samples of prey organisms at various depths. Scientists on the Sette will sort and identify the individual prey items. Information on prey composition will later be combined with the acoustic survey data to estimate the distribution and biomass of potential bigeye tuna prey species over the seamount.

Data collected during this research cruise and future expeditions will provide valuable information on bigeye tuna at Cross Seamount and the relationships between this commercially important species and the physical and biological characteristics of the seamount habitat.