Acoustic Surveys Study Abundance and Movements of Bigeye Tuna and Their Micronektonic Forage on the Cross Seamount

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Figure 1. Echograms at 3 sonar frequencies (top to bottom panels) show density and depth (vertical scale) of micronekton in shallow (SSL) and deep (DSL) scattering layers, as well as fish, as the Sette passes over the seamount plateau and flanks during early morning hours (transition period between night and day; left to right). Backscatter properties vary by frequency. Micronekton were not observed in the 200-400 m depth range away from the seamount flanks.

Juvenile and subadult bigeye tuna preferentially aggregate at Cross Seamount, located 250 km south of Oahu, and are targeted there by the Hawaii longline fishery and handline fishery. The concentrated fishing raises questions about the effects of the fishery on the local stock, and to a lesser extent its effects on the wider population of bigeye tuna in the Pacific that supports a large multinational fishery. To assess impacts on the local stock, it would be helpful to have a reliable measure of bigeye abundance at the seamount. A current research project in the PIFSC Ecosystems and Oceanography Division (EOD) is aimed at developing a fisheries independent method of bigeye tuna biomass estimation using active acoustics. Further, since populations of bigeye tuna depend on the biological and physical environment, the study is aimed at investigating the movement patterns and distribution of bigeye tuna and its forage base (micronekton), and estimating the composition of micronekton in the unique seamount environment compared with the pelagic environments nearby.

As part of this project, a research cruise by the NOAA Ship Oscar Elton Sette was conducted to Cross Seamount in 2008. The ship was equipped with a hull-mounted, split-beam Simrad EK bioacoustic system, operating at 38, 70, and 120 kHz frequencies. Survey strategy and design were optimized for by using data from the literature, experimental and commercial fishing data, and data obtained during earlier cruises conduced in 2005 and 2007. In addition to acoustic surveys to study bigeye tuna and micronekton, environmental data were collected by an Acoustic Doppler Current Profiler (ADCP), operating at 75 kHz frequency, while temperature, salinity, dissolved oxygen, and chloropigments were measured with a CTD rosette.

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Figure 2. LEFT IMAGE: Loose aggregations of bigeye tuna appeared to be foraging on descending layers of micronekton after dawn at the upcurrent flank of the seamount. RIGHT IMAGE: A V-shaped aggregation of fish was detected during afternoon with acoustic target strength values indicative of bigeye tuna of the expected size.

Results from the 2008 survey and the two earlier ones consistently show higher micronekton biomass at the seamount relative to the nearby open ocean environment. Higher biomass of tuna over the seamount was confirmed during the 2008 cruise, consistent with expectations. Further, results show that the seamount has an effect on the vertical distribution of micronekton over the seamount's plateau and its flanks. Over the plateau (~ 10 km wide, 400 m deep), several layers of micronekton occupy the depths of 200-400 m (Figure 1); this depth range is devoid of organisms away from the plateau. The deep scattering layer of organisms is thicker and extends vertically at the flanks. The influence of the seamount on micronekton seems to be limited to a maximum of 2-5 km away from the flanks. Micronektonic organisms are observed to be actively swimming against the currents during their diel vertical migratory periods and their composition is different at the seamount than away from it. These facts indicate that the seamount is likely to be occupied by some resident species of micronekton.

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Figure 3. At night, large aggregations of fish were observed on the seamount plateau. They are thought to be lustrous pomfret, one of the species caught by handline gear and marketed as monchong. At dawn, they descend along the seamount flanks to depths of the deep scattering layer, generally at the upcurrent side. The fish are seen in both low (upper panel) and mid-frequency (lower panel) images,

While the effects of the seamount on micronekton extend to slopes at depths of 800-1000 m, bigeye tuna are tightly associated with the plateau or slopes not deeper than about 500 m. As opposed to Fish Aggregating Devices (FADs) or natural floating objects, where bigeye tuna have been observed to be aggregating but not foraging, bigeye seem to aggregate over Cross Seamount at least partially to feed. Bigeye tuna appear at dawn at the upcurrent, southwest edge of the plateau to feed on specific micronekton layers which are migrating downward from the shallow scattering layer (Figure 2, left image). At this time, bigeye occupy a very small area of the plateau, are highly mobile, and form very loose aggregations. During the early morning hours, bigeye spread and occupy the southwest end of the plateau, still feeding. By late morning, they are spread over the plateau area south of the summit. During the afternoon and early evening, bigeye tuna occupy the entire area of the plateau and tend to form thicker aggregations (Figure 2, right image). At around sunset, bigeye start dispersing and seemingly leave the plateau, to appear next at dawn, again at the upcurrent edge. However, it is possible that they stay over the seamount as tight aggregations that are spread over the entire area of the plateau, in which case they would be much easier to miss during acoustic survey transects.

As opposed to bigeye tuna, large, thick aggregations of fish, likely to be lustrous pomfret (Eumegistus illustris), appear after sunset and occupy the entire area of the seamount plateau, not farther than 50 m off the seafloor (Figure 3). At sunrise they descend along the flanks, predominantly on the upcurrent side of the plateau, and occupy depths of the deep scattering layer at 500-750 m (Figure 1).

For more information contact: Réka Domokos