Oceanographic and Acoustic Surveys Target Pelagic and Insular Habitats and Ecosystems

Cruise map for the oceanographic expedition indicating location of stations where various sampling gear was deployed along the 158° W meridian between 26° N and 36° N latitude.

PIFSC scientists recently completed a 4-week collaborative research expedition to survey North Pacific habitats critical to key Hawaii-based fisheries for tuna and bottomfish and determine the relative abundance and composition of fishes residing there. The cruise was directed by Chief Scientist RĂ©ka Domokos, oceanographer in the Ecosystem and Oceanography Division (EOD).

During the first 2-week leg of the cruise in March, data were collected to identify key biological and physical oceanographic characteristics to improve scientific understanding of the ecological regions occupied by bigeye tuna and swordfish, the primary species targeted by the Hawaii longline fishery.

The survey data also expanded 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.

To better understand this habitat, in situ measurements of conductivity, temperature, oxygen, and pigment concentration were taken from the ocean surface down to 1,000 meters depth at 37 sampling stations along a 158°W longitude transect line from 23°N to 36°N latitude (see cruise map). Additionally, water samples were collected at these stations using Niskin bottles for later laboratory analysis of chlorophyll a and chloropigments.

Bioacoustic backscatter (measured with sonar) and currents (measured with an Acoustic Doppler Current Profiler) were continuously monitored during the survey, enabling a comparison of observed changes in depths of the sound scattering layers, made up of micronektonic organisms, with changes in regional oceanography. The bioacoustics data were analyzed to derive preliminary estimates of the composition and relative biomass of organisms in the scattering layers that may serve as prey for bigeye tuna and swordfish.

Panels (top to bottom) show acoustic backscatter (derived from sonar) and temperature, salinity, dissolved oxygen, and chloropigments (derived from CTD deployments) versus depth (m) along the 158° W meridian.

Preliminary oceanographic results show that during the survey there appeared to be a larger distance between the physical (e.g., temperature) and biological fronts within the North Pacific Transition Zone than previously observed. During 2008 and 2009 surveys, the biological front, indicated by the sea surface contour line (isopleth) where water has a chlorophyll concentration of 0.2 mg m-3 Chl a, has been farther north than in previous years. Location of the fronts may affect the spatial patterns of swordfish that are associated with the temperature front and loggerhead sea turtles that are associated with the biological front. Conversely, the relative biomass and distribution of forage species seem to respond to the physical environment as opposed to the biological front. Data collected during the cruise will be analyzed and used along with other data to refine habitat predictions for swordfish and loggerhead turtles.

A trawl net was deployed 10 times over 4 stations within the sound scattering layers to collect specimens of organisms from within these layers, both at shallow depths during the night and deep depths during the day. Data from the trawl hauls will enable bioacoustic estimates of the composition and biomass of organisms. They will also be used to "ground truth" the estimates of biological composition derived directly from bioacoustics data and will provide a richer set of ecological information about these distinctive oceanographic areas.

During the second 2-week leg of the cruise, bioacoustics (sonar) data were collected while a BotCam (bottom camera) was deployed simultaneously on the seafloor to collect stereo video images of fish nearby. Joint use of acoustic and video sampling gear was part of an ongoing effort to develop a time-series of estimates of relative abundance of adult bottomfish near Penguin Bank, located off the southwest tip of Moloka'i. The survey focused on 6 species of bottomfish harvested there by Hawaii-based handline vessels. The abundance estimates will provide valuable information on changes in the multi-species bottomfish stock over time.

Penguin Bank is an important habitat for bottomfish and a favored fishing location in the main Hawaiian Islands. Joint use of bioacoustics and BotCam may be an effective way to assess the density and composition of the fish assemblage on the bank. 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. Further research using the sonar-BotCam combination is required on several fronts. For example, bait is frequently used in BotCam operations to attract fish and retain them within the camera's field of view. The effects of baiting on BotCam estimates of fish abundance and composition, including potential biases, is poorly understood. Acoustics data, collected simultaneously with the BotCam operations, can be used to help judge the effects of bait on behavior of the fish and BotCam estimates of their abundance.