Surveys Underway to Help Improve Understanding of Food Web Supporting Ecologically and Commercially Important Species in the North Pacific Subtropical Frontal Region

Fig. 1: The solid black line shows the Sette's planned track through the subtropical frontal region. Satellite data shows the approximate locations of the temperature and chlorophyll fronts, where sampling efforts will be concentrated.

Fig. 3: The mid-water Cobb trawl net is spooled to the left. To the right, scientists strain the trawl catch through a net. The catch will then be brought inside the ship for analysis. (Photo courtesy of Jessica Chen)

Fig. 2: Water samples are collected with a suite of bottles, shown here. In the center of the bottles are a number of instruments that measure conductivity, temperature, depth, fluorescence, and dissolved oxygen.

Fig. 4: These computer monitors show the active acoustic returns at a range of frequencies. On the monitor on the lower left, several scattering layers composed of micronekton can be seen. (Photo courtesy of Jessica Chen)

April 3, 2015

The NOAA Ship Oscar Elton Sette is underway to survey the central North Pacific subtropical frontal zone. Scientists aboard from the Pacific Islands Fisheries Science Center and the University of Hawaii's Joint Institute for Marine and Atmospheric Research will spend 12 days at sea surveying the physical and biological ocean environment several hundred miles due north of Oahu (Fig.1).

The subtropical frontal region consists of both a temperature and a chlorophyll front. These fronts lie at the northern edge of the subtropical gyre and are used as a foraging ground and migratory corridor for a number of marine species. Commercially important species such as albacore tuna and swordfish forage along the frontal region. Additionally, loggerhead sea turtles migrate across the North Pacific in the frontal region and Laysan albatross and elephant seals forage in this area. Recent work shows that this ecologically important region may be experiencing long-term changes in position and intensity. The chlorophyll front in particular seems to be shifting farther north and chlorophyll concentrations in the vicinity of the front have been declining. The scientists aboard the Sette will be collecting a range of data to improve our understanding of this region and how these changes may be unfolding.

While underway, the Sette will be continuously monitoring sea surface temperature. This will help scientists locate the position of the temperature front. Additionally, scientists will also measure surface water chlorophyll concentrations to locate the chlorophyll front. Determining the locations of the fronts is important because these fronts migrate north and south seasonally and this migration can vary from year to year. Scientists will also be collecting water samples (Fig. 2) to measure nutrient concentrations and phytoplankton community composition in the vicinity of the fronts. Mid-water trawls (Fig. 3) will be conducted at stations spanning the frontal region. These trawls collect small (approximately 2 — 20 cm) fish and zooplankton called micronekton and will provide insight into how the ecosystem changes in response to the fronts. The trawl catches also help scientists understand what larger fish like tuna are eating in this area. In addition to collecting water and trawl samples, the scientists will be using active acoustics (Fig. 4) to study micronekton. The Sette's echosounder emits pulses of sound at distinct frequencies. These pulses of sound propagate through the water to a few hundred meters depth. When the sound reaches organisms like micronekton, it is reflected back to the ship. The time it takes for the sound to be reflected tells scientists how deep the micronekton are. The strength of the sound when it returns provides insight into how large the micronekton are. By comparing the acoustic data with the trawl samples, scientists can improve their analyses of the acoustic data.

The data collected by scientists aboard the Sette will help improve our understanding of the food web supporting ecologically and commercially important species in the North Pacific subtropical frontal region. When added to previous studies, the data from this cruise will lend insight into how this region is changing over time and whether these changes are part of natural climate variations or are linked to larger anthropogenic climate changes. This year's survey is particularly timely as we are experiencing both an El Niño and a very strong positive phase of the Pacific Decadal Oscillation. Both of these natural climate variations influence the transition zone. You can follow the scientists' work at sea on the PIFSC Blog.