Research Cruises Explore Fishery-Independent Assessment of Bottomfish Density and Oceanography of the Subtropical Front

PIFSC scientists recently completed a research cruise on the NOAA Ship Oscar Elton Sette with 2 key objectives: (1) to evaluate acoustic surveys as a fishery-independent way to assess bottomfish stocks in the main Hawaiian islands, and (2) expand knowledge of pelagic fishery habitat in waters north of Hawaii. Both objectives were met.

The bottomfish resource in Hawaii, consisting of several species of snapper and a grouper, is important to both commercial and recreational fishers. Currently, the bottomfish species complex is assessed using standard fishery-dependent methods: models of stock abundance based on catch and effort data. Researchers at PIFSC are exploring ways to improve assessments by developing fishery-independent methods of measuring stock abundance. A possible alternative fishery-independent approach is an acoustic survey.

Typical acoustic signatures of bottomfish, mostly snappers and approximately 30-60 cm long, at 70 kHz 
                 frequency during day and nighttime surveys.  The composition of fish schools seen in the acoustic data 
                 was confirmed by BotCam (daytime) and experimental fishing (nighttime).  Figure by Réka Domokos.
Typical acoustic signatures of bottomfish, mostly snappers and approximately 30-60 cm long, at 70 kHz frequency during day and nighttime surveys. The composition of fish schools seen in the acoustic data was confirmed by BotCam (daytime) and experimental fishing (nighttime). Figure by Réka Domokos.

During the first nine days of the Sette cruise, the scientific field party conducted an acoustic survey of bottomfish distribution and abundance in waters on the leeward side of Maui, in the main Hawaiian Islands. Acoustic signals identified as representing bottomfish, collected using the Sette’s sonar equipment, were ground-truthed by two independent methods: baited video cameras (BotCams) deployed near the ocean bottom by cooperating University of Hawaii scientists on the chartered vessel Hukipono and the Sette; and targeted handline fishing conducted by three cooperating fishing vessels from the local bottomfish fleet.

Multiple acoustic images of bottomfish were captured by the Sette’s sonar. In the images shown above, several aggregations of snappers are revealed near the seafloor in both day and night surveys. These acoustic signatures of fish were calibrated and validated by both BotCam video footage and fish catch data collected concurrently at the survey site. A full analysis of the survey data will get underway soon. But the preliminary results are promising, indicating that such coordinated surveys can lead to improved, fishery-independent stock assessment procedures.

In the second phase of the research cruise, PIFSC oceanographers gathered data to assess the physical, chemical, and biological oceanography of the fishing grounds in the North Pacific Transition Zone. This pelagic habitat is frequently visited by the Hawaii-based longline fleet in pursuit of tuna and swordfish.

Vertical profiles of ocean conductivity (salinity), temperature, dissolved oxygen, and fluorescence were documented from the sea surface down to 1,000 m at every quarter degree of latitude from 26° - 36°N along the 158°W survey transect. Water samples were also collected at ten discrete depths between 200 m and the sea surface to measure concentrations of total chlorophyll, chlorophyll-a, nutrients, and chloropigments. The profile data reveal the location of temperature and chlorophyll fronts that characterize the North Pacific Transition Zone. As shown in the first figure below, these fronts were located near 31°N and 33.5°N, respectively, during the cruise. The latitude of these fronts varies during the year and interannually in response to phenomena such as seasonal changes and changing phases of the El Niño-Southern Oscillation.

Several key species in the pelagic ecosystem are known to migrate and forage along the Transition Zone fronts, including tuna, swordfish, and sea turtles. Acoustic data were collected throughout the Sette oceanographic survey to gain an understanding of the micronekton (small fishes, crustaceans, and other invertebrate marine life) that these species prey upon. Micronekton are found in several sound scattering layers in the water column. As shown in the second figure below, the acoustic data collected during the cruise reveal changes in the density and depth of these layers along the survey transect. North of the Transition Zone fronts, particularly the chlorophyll front, the deep scattering layer shoals. Changes in the shallow scattering layer with latitude can also be seen.

Temperature (°C), salinity, fluorescence (mg chlorophyll m<sup>-3</sup>), and dissolved oxygen 
                 (ml l<sup>-1</sup>) profiles versus depth (pressure) from 26° - 36°N along a 158°W survey 
                 transect. The temperature front can be seen near 31°N, and the chlorophyll front near 33.5°N.
Temperature (°C), salinity, fluorescence (mg chlorophyll m-3), and dissolved oxygen (ml l-1) profiles versus depth (pressure) from 26° - 36°N along a 158°W survey transect. The temperature front can be seen near 31°N, and the chlorophyll front near 33.5°N.


Acoustic backscatter created by micronekton scattering layers in the water column along 158°W 
                 revealed at 38, 70, and 120 kHz frequencies.  The alternating strong and weak backscatter in the upper 
                 200 m is due to the vertical migratory behavior of most micronekton that move to deeper water during 
                 daytime.  Note that the deep scattering layer, between 400-800 m, has a much weaker diel pattern.  
                 Figure by Amy Comer and Réka Domokos
Acoustic backscatter created by micronekton scattering layers in the water column along 158°W revealed at 38, 70, and 120 kHz frequencies. The alternating strong and weak backscatter in the upper 200 m is due to the vertical migratory behavior of most micronekton that move to deeper water during daytime. Note that the deep scattering layer, between 400-800 m, has a much weaker diel pattern. Figure by Amy Comer and Réka Domokos

The data gathered on this cruise will help scientists better understand the habitats and forage base of the commercially and ecologically valuable tuna and swordfish. And combined with fishery data, the oceanographic data will help them find ways to reduce the likelihood of undesirable interactions between the longline fishery and sea turtles that also frequent the Subtropical Frontal Zone. Finally, information gathered during the oceanographic survey, combined with similar data from previous cruises, will expand the capacity to measure changes in the pelagic habitat that studies suggest may occur in this region due to climate change, including ocean warming.