Studies Extend Knowledge of Habitat, Fish Stocks and Ecosystems Supporting Key Hawaii-based Fisheries

March 10, 2009
Samples of organisms from thick "scattering layers" are collected at various depths and 
    brought aboard the Oscar Elton Sette using the trawl net
Samples of organisms from thick "scattering layers" are collected at various depths and brought aboard the Oscar Elton Sette using the trawl net

Oceanographers from the NOAA Pacific Islands Fisheries Science Center (PIFSC) are embarked on a 4-week collaborative research expedition to survey ocean habitats and ecosystems critical to key Hawaii-based fisheries and determine the abundance and composition of fish stocks residing there. During the first 3 weeks, they will study fishing grounds for the Hawaii-based longline fleet north of the main Hawaiian Islands (MHI). Then they will spend a week surveying bottomfish habitat on Penguin Bank in the MHI.

During the first cruise leg, led by Chief Scientist Evan Howell of the PIFSC Ecosystems and Oceanography Division, the scientific party will identify key biological and physical oceanographic characteristics to improve scientific understanding of the ecological regions where bigeye tuna and swordfish, the primary species targeted by the longline fishery, reside. In waters of the North Pacific north of Hawaii, bigeye tuna and swordfish each prefer distinct regions with different oceanographic characteristics. Knowledge of the differences in physical and biological properties between these regions is essential to more completely understanding the ecology of bigeye tuna and swordfish.

In the study of tuna and swordfish habitat, the scientific team will include researchers from the PIFSC, the University of Hawai'i Joint Institute for Marine and Atmospheric Research (JIMAR), Hawaii Pacific University (HPU), and Scripps Institution of Oceanography. Starting in Pearl Harbor, Oahu, the team will work on board the NOAA Ship Oscar Elton Sette to conduct a 960 nautical mile survey transect northward through three distinct oceanographic regions. As they move along the transect, they will use several methods to assess hydrographic properties of the ocean and measure the abundance and composition of marine organisms within the "sonic scattering layers" in subsurface waters, thick layers of organisms which provide the food base for larger predators such as bigeye tuna and swordfish.

An example acoustics transect which shows a dense layer of organisms (shown in red) 
        migrating from shallow to deep waters during dawn.
An example acoustics transect which shows a dense layer of organisms (shown in red) migrating from shallow to deep waters during dawn. Click here or the image above for high resolution version.

To measure the abundance and composition of the scattering layers, the scientists will use a sonar instrument to collect bioacoustic data continuously throughout the duration of the survey transect. The sonar instrument sends very short bursts, or "pings", of sound waves from the ship toward the seafloor, and then measures the intensity of echoes reflected back up to the ship by larger fish in the path of the sound waves, such as bigeye tuna or swordfish, smaller prey species and other objects in the water column. The sound waves are sent out at three frequencies, each providing data over a different range of depths. Researchers can analyze the acoustics data to preliminarily identify species and estimate the composition of organisms in the scattering layers that serve as prey for the bigeye tuna and swordfish.

To positively identify organisms within the scattering layers, the research crew will deploy a trawl net to collect specimens of organisms from within the layers. Certain organisms within the scattering layers move from the sea surface to deep waters during the day, returning to the surface at night. To collect these migrating organisms, trawl operations will be conducted during both day and night, with the purpose of collecting species from appropriate depth ranges as identified from the acoustic signals. The organisms collected in the trawls will allow researchers to "ground truth" the estimates of biological composition derived from the bioacoustics data and provide a richer set of ecological information from these distinctive oceanographic areas.

In addition to the acoustics surveys and trawl sampling, scientists will collect a variety of oceanographic data to analyze changes in the ocean environment and assess whether these changes affect the variability of abundance, composition, or depth of organisms within the scattering layers. Oceanographers will collect data on ocean temperature, salt content, and concentrations of chlorophyll-a and oxygen from the ocean surface down to 1000 meters using a CTD rosette with a coupled oxygen sensor and fluorometer. Data on the speed and direction of the ocean currents from the surface to 700 meters depth will be continuously collected throughout the survey transect using an Acoustic Doppler Current Profiler (ADCP).

Scientists deploying a CTD rosette with Niskin bottles.
Scientists deploying a CTD rosette with Niskin bottles.

The primary focus of the 1-week second leg will be to study the abundance and distribution of bottomfish species at Penguin Bank, located off the southwest tip of the island of Molokai. Scientists from PIFSC, JIMAR, HPU, and the University of California at Santa Cruz will use the multi-frequency active acoustics system on board the Sette to study bottomfish abundance and distribution at Penguin Bank. The Hawaii-based handline fishery for bottomfish heavily targets adult fish there, particularly 6 species. The catches have raised concerns about the state of stocks and impacts of fishing on their sustainability and indicate a need for careful monitoring of stock changes on Penguin Bank.

Led by Chief Scientist Dr. Réka Domokos, the bottomfish study will be part of an ongoing effort to develop a time-series of adult abundance of the 6 bottomfish species, which will provide valuable information on changes of the stock over time. In addition to collecting acoustics data, at predetermined positions the research team will simultaneous deploy a "Botcam". The Botcam is a stereo video camera system which is baited to attract boffomfish, then sent down to the ocean floor to record information on the number of fish within range of the camera lens, their species composition, and sizes.

Use of bioacoustics and Botcam jointly has significant benefits for understanding bottomfish abundance, species makeup, and size composition. Baited Botcam operations can be used to estimate abundance of particular species of fish, but little is understood about spatiotemporal effects of bait on the fish under study, i.e., whether presence of the bait biases estimates of fish abundance. Acoustics data, collected simultaneously with the Botcam operations, will provide much needed information that can be used to judge the affects of bait on Botcam abundance estimates. At the same time, the data on fish size and species composition from Botcam video recordings will be used to help interpret the acoustic backscatter data and better separate acoustic signals that identify bottomfish species targeted by the fishery.

The data collected aboard the multi-faceted Sette expedition will provide valuable information on the ecological landscapes of bigeye tuna, swordfish, and bottomfish in the central North Pacific Ocean. Further, analysis of the data will help scientists understand the biological dynamics between these economically and ecologically important fish species and their prey, and how these biological relationships are affected by the surrounding physical environment.

The survey data will also expand knowledge of the habitat for loggerhead sea turtles, which frequent the longline fishing grounds for swordfish and, on rare occasions, are caught incidentally on hooks set for swordfish. The habitat information is used to find ways to reduce the likelihood of such interactions, providing benefits to both turtles and longline fishers.