Townsend Cromwell
TC-99-05 Swordfish Oceanography
TC-99-05 Swordfish OceanographyApril 26-May 9, 1999
May 7, 1999 We completed our southerly 725 nautical mile transit through 8-10 foot seas and are now performing CTDs (right) (500 meter casts) along two transects. On the chart these transects cross to form a huge "X", 90 miles high and 40 miles wide. At the center of this "X" is where the Honolulu NMFS lab plans to deploy a deep water (2500 Fathoms = 15,000 feet = 2.8 miles) mooring containing oceanographic instruments which will collect data year around. In addition to the CTDs we're performing, we're graphing the bottom topography to ensure the mooring site is of the proper depth and that the bottom is relatively flat. Accompanying us on this cruise is Dr. Robert Bidigare, Professor at University of Hawaii's Oceanography Department. He has written the following description of the first leg of the cruise where we performed CTDs along the 158W longitude line. He also provides a description of the data collection techniques.
"On this cruise, physical (temperature and salinity), chemical (oxygen and nutrients), and biological (chlorophyll and chlorophyll fluorescence) parameters were measured in the upper 200 m (1 meter = 3.28 feet) at 15 nautical mile intervals along the 158W longitude line. Our cruise track began at 22.75N and ended at 33N latitude (a distance of 625 nautical miles). Temperature, salinity and oxygen were detected using electronic sensors mounted on a Conductivity-Temperature-Depth (CTD) instrument. The CTD system was also equipped with Niskin bottles for collecting seawater samples at pre-determined depths in the water column. For this study, we collected samples at 0, 20, 35,
50, 65, 80, 100, 125, 150, and 200 meters. Once the CTD was brought back aboard the TOWNSEND CROMWELL, seawater was dispensed into smaller sample bottles for determining nutrient and chlorophyll concentrations. The nutrient samples were frozen for shore-based analyses of nitrogen and phosphorus concentrations. Chlorophyll samples were filtered through glass-fiber filters aboard the ship and placed in acetone to remove the pigments from the phytoplankton. After allowing the filters to extract for 24 hours, the chlorophyll concentration for each sample was measured using a fluorometer. The data was then loaded into a computer and plotted using MATLAB software."
"Contour plots for temperature (above left) (EC) and chlorophyll (right) (mg per cubic meter) are shown in the included pictures. The temperature data are used to determine what water type we sampled and the chlorophyll data are used to map phytoplankton distributions. In the water temperature figure left above, warm and cold temperatures are indicated as red and blue shading, respectively. In general, cold seawater is always found at deeper depths and warm water is found closer to the sea surface. Warm temperatures were measured in the surface waters from 22.75N to 30N. The temperatures dropped dramatically at 32N, indicating that we had crossed into the "Winter Frontal Zone" (WFZ). The chlorophyll map (right) shows that phytoplankton were concentrated at 80-140 m between 22.75N to 30N. At the WFZ, however, the phytoplankton concentrations were found higher and closer to the sea surface (60-80 m). The increase in phytoplankton results from the input of nutrients from the cold waters below (see the cold water feature at 32N in the temperature map). The high phytoplankton productivity in this region yields higher concentrations of zooplankton. The elevated zooplankton, in turn, causes an increase in other "food chain" animals, including squid and swordfish." From this study, we hope to gain an understanding how phenomena, such as El Nino, influence the oceanographic features of an area and how that influence effects fishery yields, in this case swordfish. Related web sites:
- NOAA Ship Ka'imimoana web site for more pictures and ship activities.
- NOAA Hawai'i CoastWatch web site for satellite data.