SCTB15 Working Paper BET-3 The role of oceanography in the aggregation and vulnerability of bigeye tuna in the Hawaii longline fishery from satellite, moored, and shipboard time series data: An ongoing study Jeffrey J. Polovina, Michael P. Seki, and Russell E. Brainard National Marine Fisheries Service (NMFS), Honolulu Laboratory, Hawaii
The role of oceanography in the aggregation and vulnerability of bigeye tuna in the Hawaii longline fishery from satellite, moored, and shipboard time series data: an ongoing study 1 Jeffrey J. Polovina, Michael P. Seki, and Russell E. Brainard National Marine Fisheries Service, NOAA Southwest Fisheries Science Center Honolulu Laboratory 2570 Dole Street Honolulu, HI 96822-2396 Introduction Bigeye tuna (Thunnus obesus) is the principal deepwater target species of the longline fishery in the Pacific Ocean yielding total annual catches exceeding 150,000 tons (Hampton et al., 1996; Hampton et al., 1998). In the Hawaii-based longline fishery, bigeye tuna have consistently been one of the dominant species in terms of landings and ex-vessel revenue (Ito and Machado, 1997). Although stock assessment of bigeye tuna has been conducted using indices of abundance, non-equilibrium production models, and yield-per-recruit analysis (Miyabe, 1995; IATTC 1997, Hampton et al., 1998), each of these methods requires the use of longline fishery catch-per-unit-effort (CPUE) as an index of bigeye abundance. Unfortunately, fishery-dependent CPUE does not necessarily represent the abundance of the stock, but rather the catchability of the stock (Hinton and Nakano, 1996). Catchability, in turn, is dependent to a considerable extent upon variable oceanographic conditions. Oceanographic variability can significantly affect the depth of the thermocline and the most likely depth of occurrence of bigeye tunas as well since the preferred foraging habitat of bigeye tunas is the 8-15 o C water at or near the base of the thermocline (Hanamoto, 1987; Holland et al., 1990; Boggs, 1992; Brill, 1994; Boggs et al., 1998; Musyl and Boggs, 1998). Evidence from a recently recovered NMFS archival tag from a bigeye tuna in Hawaiian waters revealed a repetitive pattern of remaining in the upper 10-90 m at night and diving to 350-500 m (8-10 o C) each day (Boggs et al., 1998). During the day, the tuna remained at depth, apparently foraging, until its body temperature dropped to about 17 o C (typically at depth for about 45 min). The fish then briefly migrated to warmer waters (50-150 m) before returning to depth. This behavior suggests that at times when the thermal structure is depressed, bigeye tunas may be generally deeper and less aggregated due to their extended vertical migrations. Although the stock abundance may be unchanged, catchability and CPUE are reduced. Conversely, when the thermal structure is elevated, the habitat is generally shallower and bigeye tunas may be more aggregated, resulting in increased catchability and CPUE. The velocity shear of the ocean currents also has a profound effect on catchability and CPUE by modifying the depth of penetration and performance of longline gear (Riggs, 1997; Mizuno et al., 1997, Sibert and Spalding, 1998). It is important to note that both vertical and horizontal velocity shears affect the performance of longline gear. Although strong current 1 A working document submitted to the 15 th Meeting of the Standing Committee on Tuna and Billfish, Honolulu, Hawaii, 22-27 July 2002.
shears generally tend to lift longline gear and prevent it from penetrating to the depth of the bigeye habitat, convergent flows can also result in deepening of the longline. As these cases have demonstrated, CPUE does not necessarily reflect abundance of the stock. Oceanographic features which affect the depth of the 8-15 o C layer or result in regions of high vertical or horizontal velocity shear could impact CPUE of bigeye tuna. These features undoubtedly include cyclonic and anticyclonic mesoscale eddies, westward-propagating Rossby waves, shear instability waves, and large-scale interannual and decadal shifts in the positions and strengths of the North Pacific gyre. These features have significant variability over time scales from diurnal to decadal around the Hawaiian Archipelago (Patzert, 1969; Polovina et al., 1995; Bingham and Lukas, 1996; Firing, 1996; Lukas et al., 1996; Munch, 1996; Qiu et al., 1997; Lumpkin, 1998, Seki et al. 2001). Likewise, the overall CPUE for bigeye tunas in the Hawaiibased longline fishery show significant variability over these time scales (Ito and Machado, 1997). For purposes of stock assessment and management it is necessary to have an index of the bigeye local abundance which is less biased by changes in fish aggregation or catchability of the gear associated with these oceanographic features. Research Objectives The goals of this research are: 1) to examine closely the relationships between bigeye tuna CPUE from the Hawaii-based longline fishery and oceanographic features observed using moored, shipboard, and satellite time series of the vertical and spatial structure of the upper ocean, and 2) to utilize those relationships to develop methods to improve stock assessment estimates based on standardized logbook CPUE using remotely-sensed observations of sea surface height (altimeter), sea surface temperature (AVHRR), ocean color (SeaWiFS), and surface winds (scatterometer). Activities and Status BIGEYE Moorings The first major component of this program is the deployment of an oceanographic mooring to provide a high resolution time series of the vertical structure of temperature and currents in an area with moderately high and temporally varying values of CPUE. With assistance and technical expertise provided by the TAO Project Office at NOAA s Pacific Marine Environmental Laboratory (PMEL) in Seattle, two mooring deployments and recoveries have been made. The first mooring, BIGEYE-1, was designed, built, and deployed on December 11, 1999 at 20 o 36.0'N, 161 o 24.2'W from the NOAA Ship Ka imimoana in 4,685 m of water. The site for BIGEYE-1 was selected based on bigeye tuna CPUE, variability of CPUE, proximity to TOPEX altimetric satellite crossover paths, variability of oceanographic structure, and linkages to other Pelagic Fisheries Research Program (PFRP) projects. BIGEYE-1 was instrumented with 8 Seabird temperature recorders at the depths 25, 75, 125, 250, 300, 400, 500 and 700 m and 5 Aanderaa current meters at the depths 50, 100, 150, 200 and 350 m. After a little more than one year in the water, BIGEYE-1 was successfully recovered on December 14, 2000. The temperature, velocity, conductivity and dissolved oxygen data from BIGEYE 1 clearly showed significant eddy variability at the mooring site. Similar fluctuations of each of 2
these properties was observed with each significant eddy feature. It is noteworthy that the depth penetration of eddy features extends from the surface to as deep as 700 m. Temperature fluctuations at thermocline depths (125 m) ranged from 16 C to 25 C. The BIGEYE 2 mooring was deployed at 20 o 36.3'N, 161 o 34.4'W on December 12, 2000 and successfully recovered on November 24, 2001. This second deployment of the mooring resulted in 11.3 month time series of vertical temperature structure from 9 SeaBird temperature recorders at depths of 75, 100 125, 200, 250, 300, 400, 500, and 700 m, current velocities over the upper 300 m from a pair of upward and downward looking SonTek ADP (Acoustic Doppler Profilers), and current velocity, temperature, conductivity, and dissolved oxygen at 350 m as recorded by an Aanderaa RCM9. Vertical resolution and the ability to compute vertical velocity shear were significantly improved for the BIGEYE 2 mooring by replacing the Aanderaa current meters with two Sontek acoustic Doppler current profilers (ADCP). These ADCPs were installed on the mooring to provide velocity measurements every 8 m from a depth of about 15 m to a depth of about 275 m. This improved vertical resolution will allow better determination of shear and estimates of the effects on longline gear performance. BIGEYE-3 is planned for deployment in November of 2002. The mooring will be instrumented with SeaBird temperature recorders, the Sontek ADP pair, and the 350m Aanderaa RCM9, with temperature, conductivity, and dissolved oxygen sensors, in the same configuration as BIGEYE-2. Research Cruises The second major component of the Bigeye Oceanography Program is a series of shipboard surveys to (1) expand the spatial representativeness of the mooring observations, (2) closely examine the vertical water column structure associated with oceanic variability; e.g., fronts, eddies and frontal meanders, and (3) obtain information of longline performance particularly in response to prevailing oceanographic conditions. Over the course of the project thus far, five research cruises (April 1999-2001 and November 1999-2000) aboard the NOAA ship Townsend Cromwell (TC) have been conducted and have focused on obtaining measurements of dynamic oceanographic variability and its influence on the biology; a sixth research cruise was postponed to July 2002. During these cruises, closely-spaced conductivelytemperature-depth (CTD) casts were conducted to observe water properties at very high vertical resolution. In addition to temperature and salinity, dissolved oxygen, nutrients, chlorophyll, and other accessory pigments were sampled. The initial April 1999 research cruise was also used to conduct bathymetric surveys of the proposed mooring site and to provide baseline descriptions of the oceanographic structure around this location. During the November 1999-2000 research cruises, distinct cyclonic eddies were traversed and studied in detail with regards to the physical and biological responses to mesoscale physical forcing. These features were also continually monitored using satellite observations of sea surface temperature, sea surface height, and ocean color by the Hawaii CoastWatch program. Observations suggest that the cyclonic eddies induce localized upwelling and upward nutrient flux which enhances primary productivity however, there were marked differences in the magnitude of the vertical displacement of isopleths within each eddy. Typical ocean currents around these eddies were observed to be very strong, with velocities as high as 80-100 cms -1 (~2 knots). 3
The April 2001 survey occupied a 900 nmi long transect from 22/N to 7/N latitudes allowing an extension of the assessment of the BIGEYE mooring spatial representation and to enable characterization of the dynamic region influenced by the North Equatorial Current (NEC) and North Equatorial Counter-Current (NECC). In recent years, considerable fishing effort by Hawaii-based longline fishing vessels has been focused in this region. During this time of year, this region has now been closed to longline fishing. In April 2002, we opted to participate on a 20-d commercial longline fishing trip aboard the F/V Tucana in lieu of the typical research survey. This venture enabled biological and gear sampling that otherwise would been less productive if attempted on the TC. A total of 13 longline sets were made on which two sections of longline on each set were instrumented with 3 to 6 time-depth-temperature recorders (TDRs) and hook timers. Thirty fish were caught on the instrumented sections, eight of which were bigeye tuna. Additionally, three large viable bigeye tuna (ca. 70-110 lbs.) were instrumented with Wildlife Computers popup satellite archival tags (PSATS) upon capture and released. All three PSATs have since released from the fish with times at liberty ranging from about 10 days to six weeks; these data are currently being analyzed. Information on core temperatures of bigeye tuna were also collected with internal temperature loggers inserted upon capture of twenty-nine fish. Upcoming, a research cruise (11-30 July 2002) aboard the TC will again survey the dynamic region influenced by the North Equatorial Current (NEC) and North Equatorial Counter-Current (NECC) region along a NE-SW Topex passover track and a second cooperative venture with a commercial longline fishing vessel is planned for November 2002. Satellite Remote Sensing The final major component of the Bigeye Oceanography Program involves examining relationships between surface features observed using satellite remote sensing and both the vertical structure of the upper ocean temperatures and currents and fishery-dependent CPUE of bigeye tunas. Early efforts focused on relating satellite observations of pronounced mesoscale features, such as the Loretta and Mikalele eddies, to shipboard observations of the vertical structure. These efforts have since moved on to refining the use of Topex altimetry to index changes in vertical thermal structure. Preliminary examination of concurrent in situ vertical temperature collected on research cruises and along track Topex satellite sea level height measurements indicates that there is a generally a good coherence between the two parameters and reinforces our optimism that algorithms can be developed enabling the linking of satellitesensed sea surface height to estimate vertical water column structure of the upper ocean.. Preliminary analyses of the locations and CPUEs of longline vessels in relation to these mesoscale features have also begun. During the periods observed to date, it appears that longline vessels may be avoiding the regions of strong currents and velocity shear associated with the cyclonic eddies. 4
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