Research Cruise off the Kona Coast of the Island of Hawaii Supports the PIFSC Integrated Ecosystem Assessment Program
During July 1-13, 2011, PIFSC scientists completed a research cruise on the NOAA Ship Oscar Elton Sette (SE) to provide support for the Center's Integrated Ecosystem Assessment (IEA) Program in the Kona region of the island of Hawaii (the Kona region is also known as West Hawaii). The main goal of the cruise was to learn about the characteristics and functioning of the marine ecosystem, including physical and chemical features of the ocean waters and the biology of marine organisms living in the area. The scientific field party included researchers from PIFSC, the University of Hawaii (UH), the Joint Institute for Marine and Atmospheric Research (JIMAR), and the American Museum of Natural History. An additional goal of the research cruise was to provide experience and training for future scientists. The field party included an undergraduate student from UH, a graduate student from the UH School of Ocean and Earth Science and Technology, an undergraduate PIFSC summer intern, two undergraduate recipients of the NOAA Ernest F. Hollings scholarships, and a NOAA Teacher-at-Sea.
The mission of the cruise was to better understand spatial ecosystem differences off the West Hawaii coastline. Ecosystem information collected during the survey will help identify north-south and onshore-offshore differences that may be present in the environment. Researchers surveyed the "Kohala Shelf" area between the islands of Hawaii and Maui, which appears to be a center of activity for some cetacean species.
During the 13-day cruise the scientific party conducted field operations continuously and at selected point locations (stations) along a survey grid (Fig. 1). They gathered data on the physical, chemical, and biological characteristics of the ocean using several techniques. Vertical profiles of physical and biological data, including ocean conductivity (salinity), temperature, oxygen, and fluorescence, were documented at each station to depths of 1000 meters. During most conductivity-temperature-depth (CTD) profile operations, water samples were collected at 10 standard depths using 10-liter Niskin bottles. Samples were filtered at sea to measure biological properties or stored for later analysis in the laboratory. Stored samples were measured for chlorophyll-a during the cruise and for nutrient concentrations post-cruise at the University of Hawaii in Hilo. Preliminary results from the CTD data collected during the cruise appear to show gradients in physical properties from the south to north, with the northern stations featuring a shallower thermocline, pycnocline, and subsurface chlorophyll-a maximum.
Bioacoustics data collected during 71 acoustic transects conducted on and between cruise stations showed that the shallow scattering layer (SSL) is typically located in the upper 200-230 m, while the deep scattering layer (DSL) is located between 400-800 m. The two layers seem to be very different from each other in their composition of organisms. In the SSL, organisms typically register stronger backscatter at the 38 kHz frequency while in the DSL, backscatter from the 70 kHz channel is stronger. Backscatter data indicate that there are onshore-offshore and north-south differences in the composition and biomass of both SSL and DSL. Backscatter shoals and increases to the north at all frequencies, indicating a possible effect of changes in water characteristics and a likely increase in biomass. The strongest increase is found at the 70 kHz frequency, resulting in a change in the relative strength of backscatter at the different frequencies. This change in relative backscatter is indicative of changes in biological composition of the scattering layers. The most pronounced composition change in backscatter is seen between stations onshore and offshore.
Two replicate trawling operations were completed at 11 stations. Trawl catch compositions are shown in the accompanying graph with the pie charts scaled by total catch volume. Catch composition clearly has strong spatial variation, with both north/south and offshore/onshore patterns apparent.
On a preliminary examination of trawl data, some interesting patterns have emerged. First, larvae and juveniles of shorefishes are more abundant in the offshore stations than the onshore stations. Second, trawl catches at stations on inshore portions of the Kohala Shelf (off the northwestern shore of Hawaii) yielded a higher biomass than those at stations off the shelf and stations to the south. For example, the mid-station in the north had the highest catch volume as well as the highest fluorescence recorded, indicating that the Kohala Shelf is a high productivity area relative to more southern locations along the coast. This finding is consistent with observations indicating this area is favored by cetaceans. It is also plausible that the shelf serves to concentrate organisms since the most offshore station in the northern area, just off the shelf, had the lowest catch volumes. Third, volume ratios among the six ecosystem components (organism categories) were relatively constant among stations, with the exception of the northeasternmost site, which incidentally had the shallowest bottom depth. At this site, crustaceans were the dominant category by volume, whereas typically myctophid fish made up half the catch.
During the cruise, marine mammal specialists visually surveyed the sea surface for marine mammals, logging 70 field hours of survey effort covering over 900 km of transect. There were 20 marine mammal sightings, including a repeat sighting of an individual pygmy killer whale (accompanying photos), recognizable by its distinctive dorsal fin. Additionally, more than 3000 identification photographs of marine mammals were taken during the cruise. Of particular interest was the sighting of false killer whales, the highest priority species of the cruise at the southern edge of the Kohala Shelf around sunset on July 11.
By analyzing all the measurements and observations made during the cruise, PIFSC hopes to create a comprehensive ecosystem "snapshot" assessment of the West Hawaii oceanic region. The various methods used have provided information on each component of the ecosystem, and the repetition of surveys at stations along the Kohala Shelf and the coast of West Hawaii should provide enough information to spatially define, compare and contrast important ecosystem features. The information collected during the cruise can be compared with previous ecosystem observations in the region and can serve as a baseline for future research projects off the Kona coast.