Research Expedition Compares Methods for Surveying Deepwater Bottomfish in the Maui Triangle Region

April 15, 2013
NOAA Ship Oscar Elton Sette. NOAA photo by Benjamin Richards.
NOAA Ship Oscar Elton Sette. NOAA photo by Benjamin Richards.

Researchers from the NOAA Pacific Islands Fisheries Science Center (PIFSC) are leading an expedition to develop fishery-independent methods of assessing the abundance of deepwater bottomfish around the main Hawaiian Islands. The bottomfish population supports important commercial and recreational fisheries in Hawaii.

The research expedition is being carried out by a team of PIFSC scientists on the NOAA Ship Oscar Elton Sette in collaboration with colleagues from the Northwest Fisheries Science Center, University of Hawaii at Manoa (UHM), Joint Institute for Marine and Atmospheric Research at the University of Hawaii at Manoa, Pacific Islands Fisheries Group (PIFG), and the NOAA Teacher at Sea program. The Sette departed her home port at Ford Island, Pearl Harbor, on April 15, 2013, for the study area in waters of the Maui Triangle, an ocean region delineated by the islands of Maui, Molokai, Lanai, and Kahoolawe. Leading the 15-day mission is Dr. Donald R. Kobayashi from the PIFSC Ecosystems and Oceanography Division. Cooperating with the Sette will be the chartered research vessel Huki Pono and 3 chartered bottomfish fishing vessels of the PIFG: Imua, Naomi K, and Hokuloa. Working together, the vessels will collect information on the abundance of bottomfish in the study area using hook-and-line fishing, hydroacoustics, and underwater video.

The expedition has 2 objectives. First, the vessels will accomplish a near-simultaneous survey of deepwater bottomfish in the Maui Triangle region using 4 methods, or gears: a Simrad EK60 echosounder (operating from the Sette), an autonomous underwater vehicle (AUV) and remotely-operated vehicle (ROV) camera system including a BlueView imaging sonar unit (deployed from the Sette), baited underwater stereo video camera systems (or BotCam, deployed by a UHM team aboard the Huki Pono), and hook-and-line fishing from the fishing vessels. Second, the Sette will conduct acoustic target ground-truthing experiments using ROV cameras and BlueView in conjunction with Simrad EK60 active acoustics.

In the first portion of the research project, the 4 survey gears will be intensively deployed within 10 selected survey grid areas in the Maui Triangle region, each 500 m x 500 m. The 10 survey grids will be chosen from a larger set of candidate grids (see map) shortly before the project mobilizes to best take advantage of prevailing weather conditions, proximity to port, and patterns of fish abundance, and to mitigate impacts of the fishing operations on activities of local fishermen and management regions. Each survey method will provide information helping to measure bottomfish density in the survey area. Within each grid area, the various gears will be used repeatedly over a 7-8 day period, generating data that will enable researchers to better understand variability in the measures of bottomfish abundance both within each gear and between the different gears.

The results of this portion of the project will be useful in development of an operational, non-extractive, non-lethal survey methodology which can supplement or eventually replace the standard method presently used for measuring changes in bottomfish abundance, catch-per-unit-of-fishing-effort (CPUE) derived from commercial bottomfish hook-and-line fishery data. The CPUE index of abundance is used as a key data input for quantitative assessments of the Hawaii deepwater bottomfish stock. However, the fishery CPUE method has many limitations: it requires an active fishery with good data recording; it only applies to open fishing areas and seasons, so cannot provide measures of abundance for time periods or areas not open to fishing, or not fished by bottomfish vessels for other reasons (e.g., distance from port, weather, etc.); and it provides information only on species which are caught on hook-and-line fishing gear and therefore does not provide a broad view of the fish community needed for monitoring ecosystem health. Furthermore, the CPUE is notoriously problematic as an index of abundance in a multi-species context, particularly in fisheries which require a great deal of fisherman skill such as Hawaii deepwater bottomfish and where fish behavior is thought to play a large role in catch rates. Most of these concerns can be alleviated by optical or acoustical stock assessment methods, but the various approaches need to be carefully compared and tuned. The current research expedition, the 4th in a series of deepwater bottomfish calibration cruises in the Maui Triangle region, is the first to deploy all 4 survey methods at the same time.

The ground-truthing study will take place after the methods comparison operations. Acoustical survey approaches are typically hindered by uncertainties in the identification of species that generate the acoustic signals. This is particularly the case in areas with high fish diversity such as Hawaii. To some extent, discrimination of fish species can be accomplished by examining the differential acoustical returns from the Simrad EK60 device operating at different frequencies; however, ground-truthing against optically-identified fish targets is required to assign particular acoustical signatures to particular species and sizes of fish. A paired approach using acoustics and optical systems together is needed. In this part of the expedition, the ROV aboard the Sette will be quickly deployed to locate and identify acoustical targets acquired by the Simrad EK60 acoustical device. The ROV will be equipped with stereo cameras as well as a BlueView imaging sonar unit. Data gathered in the ground-truthing operations will contribute to a library of species-specific and size-specific acoustical signatures which can then be applied towards existing, current, and future acoustical data to estimate bottomfish biomass.

Operational area of the multi-vessel study of deepwater bottomfish in the Maui Triangle region. Potential 500 m X 500 m survey grids 
                 are outlined in bright green. Actual operations will take place in a selected subset of these candidate grids. Grid selection will 
                 depend on weather and other considerations.
Operational area of the multi-vessel study of deepwater bottomfish in the Maui Triangle region. Potential 500 m X 500 m survey grids are outlined in bright green. Actual operations will take place in a selected subset of these candidate grids. Grid selection will depend on weather and other considerations.