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Synchronous Assessment of Bigeye Tuna (Thunnus obesus) and Micronekton Biomass, Distribution, and Movement Patterns at Cross Seamount, and the Effects of the Seamount Environment

Progress Reports (PDF): FY 2010, FY 2009, FY 2008, FY 2007

Globally, seamounts play an important role in shaping the distribution of pelagic species — such as tunas and sharks — and are heavily targeted by commercial fisheries (Holland et al., in press). In Hawaii, the commercial offshore handline fishery targets seamount populations of bigeye and yellowfin tunas, species that are known to aggregate at these environments. The local handline fishery frequently concentrates its efforts at Cross seamount, located approximately 290 km south of the island of Oahu. This fishery takes mostly juvenile and subadult fish, while adult bigeye and yellowfin tunas around the Hawaiian Islands are an important target species for offshore longline fisheries. Concerns have been raised about the effects of the handline fishery removing too many juveniles that could otherwise recruit to waters away from Cross seamount and help maintain adult populations in Hawaiian waters. Current estimates from tag release and recapture methods point to moderate exploitation rates (10 - 30%, Adam et al. 2003), indicating that the biomass of bigeye and yellowfin tunas aggregated at Cross seamount should be closely monitored.

Insights into the underlying biological advantage to tunas of aggregating at seamounts are being gained through analysis of gut contents (Holland et al., in press) but much is yet to be learned about the behavioral interactions between the tuna and the mesopelagic micronekton community that comprises the dominant portion of seamount-associated tuna prey. Further, little is known about the interaction between prey abundance and tuna abundance at seamounts and whether or not fluctuations in prey abundance influence tuna residence times. Bigeye and yellowfin tuna populations at Cross seamount have already been studied using tag release and recapture methods that have provided information on characteristics such as size composition, dispersal patterns, residence times and natural and fishing related mortality rates (e.g., Adam et al., 2003; Sibert et al., 2000). While these methods provide useful information on the residence times and dispersal patterns of individuals, they do not provide reliable biomass estimates and tell us nothing about the behavioral interactions between predator and prey or how seamounts influence these interactions.

In this study, a fishery independent method will be developed to estimate the biomass, distribution and behavior of bigeye at Cross seamount and the interaction of bigeye tuna and their prey species. We will use active bioacoustic surveys in tandem with synchronous acoustic tracks of individual bigeye tuna. Similar methods have already been successfully applied to observe the movement patterns and distribution of tuna and their prey at FADs (e.g., Josse et al., 1998). The acoustic surveys will use a dual frequency hull-mounted Simrad acoustics system on board the NOAA ship Oscar Elton Sette. In addition to biomass, the distribution and abundance of bigeye tuna will be compared to the distribution and abundance of micronekton, as well as to the physical environment. During some of these acoustic transects, synchronous active acoustic tracking of individual tuna will be conducted from a collaborating tracking vessel thereby linking individual behavior to the assemblage distribution patterns observed by the acoustic surveys. Besides being fishery independent, the bioacoustic survey technique is able to provide simultaneous information on the abundance and distribution of both nekton (such as tuna) and micronekton by rapidly yielding large amounts of data on a controlled grid with relatively lower demands on resources. To help in the identification of acoustic tuna signatures, acoustic tracking and longlining methods will be utilized. Samples of organisms from the strong acoustic scattering layers will be taken by net trawls to ground-truth the acoustics data and help in micronekton composition and biomass estimates. A bioacoustician with experience in the use of acoustic surveys for tuna biomass estimation will be participating in the project to aid in the development of the technique.

1. Estimate bigeye tuna and micronekton biomass at Cross seamount
2. Study the distribution, composition, and movement patterns of micronekton specific to the seamount environment
3. Study the distribution and movement patterns of bigeye tuna aggregations and individual bigeye behavior relative to that of the physical dynamics and micronekton distribution

Proposed activities for this project will consist of two oceanographic cruises at Cross seamount on board the NOAA ship Oscar Elton Sette, taking place during two consecutive years. During the proposed cruises, evenly spaced acoustic transects will be conducted over a predetermined grid covering the plateau and flanks of Cross seamount, with physical and biological sampling stations spaced evenly along the transect lines. During some of the surveys, individual bigeye tuna, equipped with acoustic tags, will be actively followed by Opah, a tracking vessel provided by the Hawaii Institute of Marine Biology (HIMB).

Year 1 funding for this 2-year project estimated to be available mid-2006.

Adam, M.S., Sibert, J., Itano, D. and Holland, K. (2003) Dynamics of bigeye (Thunnus obesus) and yellowfin (T. albacares) tuna in Hawaii’s pelagic fisheries: analysis of tagging data with a bulk transfer model incorporating size-specific attrition. Fish. Bull. 101:215–228.
Holland, K.N & Grubbs, R.D. (I
n press) Tunas and Seamounts. In: Seamounts: Biology, fisheries and management T. Pitcher and T. Morato (eds) Blackwell Press.
Josse, E., Bach, P. & Dagorn, L. (1998) Simultaneous observations of tuna movements and their prey by sonic tracking and acoustic surveys. Hydrobiologia 371/372:61–69.
Sibert, J., Holland, K. & Itano, D. (2000) Exchange rates of yellowfin and bigeye tunas and fishery interaction between Cross seamount and near-shore FADs in Hawaii. Aquat. Liv. Res. 13:225– 232.

Project Investigators:
Dr. Reka Domokos
National Marine Fisheries Service
Pacific Island Fisheries Science Center
2570 Dole Street
Honolulu, Hawaii 96822 USA
Phone (808) 983-5368
FAX (808) 983-2902
email: Reka.Domokos@noaa.gov

Dr. Kim. Holland
Hawaii Institute of Marine Biology (HIMB)
University of Hawaii at Manoa
Coconut Island
P.O. Box 1346
Kaneohe, Hawaii 96744 USA
Phone (808) 236-7410/533-4110
FAX (808) 236-7443
email: kholland@hawaii.edu

Dr. Jeffrey Polovina
National Marine Fisheries Service
Pacific Island Fisheries Science Center
2570 Dole Street
Honolulu, Hawaii 96822 USA
Phone (808) 983-5390
FAX (808) 983-2902
email: Jeffrey.Polovina@noaa.gov


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This page updated September 28, 2010