Oceanographic and Acoustic Surveys Conducted in Waters of CNMI, Guam, and Micronesia

In mid-March, oceanographers from the PIFSC Ecosystems and Oceanography Division (EOD) embarked on a 4-week collaborative research expedition to survey the habitat and ecosystem in waters of the Commonwealth of the Northern Mariana Islands (CNMI), Guam, and Micronesia, focusing on a region critical to local fisheries. During the cruise of the NOAA Ship Oscar Elton Sette, data were collected to identify key physical and biological oceanographic characteristics to improve understanding of this economically important area, which is habitat for a variety of species targeted by fishermen.

To better understand this habitat, conductivity-temperature-depth (CTD), oxygen, and fluorometer measurements were taken from the ocean surface down to 1,000 meters depth at stations along three meridional transects at 143°48'E ("Western Transect"), 145°15'E ("Central Transect"), and 146°40'E ("Eastern Transect"), spanning the latitudes 10°-17°N, 10°-13°30'N, and 10°-17°N, respectively (see cruise track below). CTD casts were spaced every 0.25° of latitude along the transects. In addition, Niskin bottles were deployed with the CTD rosette to collect water samples for measurement of nutrients and chlorophyll-a, (at every degree of latitude) and chloropigments (every 0.5° of latitude). Discrete chlorophyll-a samples were filtered at sea and chloropigment samples were stored in liquid nitrogen for later laboratory HPLC analysis.

While the oceanographic transects were being executed, echosounders were used to continuously monitor bioacoustic backscatter, caused by organisms in the scattering layer of the water column, and an acoustic Doppler current profiler was used to simultaneously monitor currents. These operations enabled an analysis of relationships between oceanographic conditions and observed spatiotemporal changes in currents and backscatter. In addition to the three meridional transects, bioacoustics and current data were collected along two zonal transects at 10°N ("Southern Transect") and 17°N ("Northern Transect") between the Western and Eastern Transects.

Survey transects of the Sette oceanographic cruise SE-10-03 in the Mariana Archipelago.
Survey transects of the Sette oceanographic cruise SE-10-03 in the Mariana Archipelago.

The bioacoustics data were preliminarily analyzed onboard to estimate the composition and relative biomass of the scattering layer organisms that serve as prey for larger fauna, such as economically important fish. An Isaacs Kidd mid-water trawl (IKMT) was used to determine the composition of zooplankton, at the trophic level below micronekton, the main component of the scattering layers. Zooplanktonic organisms were sampled every 1° along the three meridional transects. All samples were collected in the upper 200 m during the night, when zooplankton (and micronekton) biomass is high as diel migrators move to near-surface waters to feed. During the oceanographic and bioacoustic operations, the main focus of this project, two scientists on the Sette recorded visual sightings of marine mammals to estimate their distribution and composition in the area.

Temperature (top panel), salinity (middle panel), and chloropigment concentration (Chl-a, bottom 
            panel) along the Eastern Transect of the Sette cruise SE-10-03, from the sea surface to 1000 m depth.
Temperature (top panel), salinity (middle panel), and chloropigment concentration (Chl-a, bottom panel) along the Eastern Transect of the Sette cruise SE-10-03, from the sea surface to 1000 m depth.

Preliminary results obtained from CTD casts show a general shallowing of the mixed layer in the south part of the survey area (graph above). Salinity was maximum at 100-250 m and minimum at 300-600 m. The salinity minimum shallows and becomes vertically thinner from south to north. The salinity maximum reaches the surface and is wider in its vertical extent to the north. It becomes a subsurface feature to the south, presumably due to dynamics of the North Equatorial Current, representing North Pacific Tropical Water, which is formed to the north and advected southward along isopycnals and westward by the North Equatorial Current.

Fluorometry results show a very low background concentration of chlorophyll-a (Chl-a) (< 0.05 mg Chl-a m-3) apart from a subsurface deep Chl-a maximum, which is situated close to 150 m in depth. Surface Chl-a values measured during the Eastern Transect were over 200% higher (~0.04-0.05 mg Chl-a m-3) than those observed during the Western and Central Transects (~0.01-0.02 mg Chl-a m-3). While these are preliminary results, available spatial Chl-a maps from MODIS appear to support the occurrence of higher surface Chl-a in the east during the cruise.

Bioacoustic  backscatter from the 38 kHz sonar channel along the Western Transect (upper panel) and 
            Eastern Transect (lower panel) during the Sette cruise SE-10-03.
Bioacoustic backscatter from the 38 kHz sonar channel along the Western Transect (upper panel) and Eastern Transect (lower panel) during the Sette cruise SE-10-03.

Bioacoustics data show definite differences between the Eastern and Western transects, as well as zonal differences along each transect (graph above). Biomass of the shallow scattering layer (SSL, in the upper 200 m) increases to the north along the Western Transect and decreases to the north along the Eastern Transects, with the vertical extent of the scattering layer increasing to the north at both longitudes. In contrast to the SSL, biomass and depth of the deep scattering layer (DSL, at 400-800 m) both increase from south to north along the Eastern and Western Transects. However, biomass in the DSL is consistently higher along the Eastern Transect in comparison to the Western Transect. Note that this higher micronekton biomass to the east corresponds to the higher background Chl-a levels found by fluorometry in that region. Deeper SSL and DSL towards the north is probably the results of changes in water characteristics: that is, the shallowing of the thermocline and high oxygen concentrations towards the south. Differences in backscatter characteristics at the three operational sonar frequencies (38 kHz, 70 kHz, and 120 kHz) in the east, west, north and south imply differences in the composition of the SSL and DSL.

IKMT tow samples show changes in both composition and biomass of scattering layer organisms from south to north and from east to west. In general, over 75% of the biomass (by volume) consisted of gelatinous organisms and crustaceans (52.2% and 32.2%, respectively). Other organisms in significant numbers were fish (7.8%), shrimp (6.9%), and squid (0.9%). Zooplankton diversity and biomass in the tows significantly increased south of 12°N along the Eastern Transect, corresponding to the increase in micronekton biomass in the SSL along the same transect. Ichthyoplankton diversity and abundance were higher along the Western Transect relative to the east. No larvae of coral reef or coastal fish families were found in samples collected, except between 13°N and 15°30'N on the Western Transect. Some individual coral reef fish larvae were also encountered at 10°N on the Western Transect. The observed pattern of occurrence of coral reef fish larvae is consistent with the general east to west circulation in the area, with larvae of reef spawners being swept to the west of the Mariana Islands and some also transferred from the east by the North Equatorial Current.