SCTB14 Working Paper

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M. Shiham AdamI, John Sibertl, David Itanol and Kim Holland2

Ipelagic Fisheries Program,University of

2Hawaiian Institute of Marine Biology, University oa Hawaii 14th Standing Committee on and Billfish Noumea, New Caledonia, 9 -16th August 2001 Yellowfin Research Group Size-specific tag attrition in bulk transfer models: Analysis of

Hawaii Tuna Tagging Project data M. Shiham AdamI, John SibertI, David Itanol and Kim Holland2

SCTB 14 -Presentation Summary

Introduction

The Hawaiian Islands are home to a mixture of recreationaVsubsistence and commercial fisheries for tuna, billfish and other pelagic species. There is a large mix of handline and troll vessels that seek tuna, billfish, wahoo (Acanthocybium solandri) and dolphinfish (Coryphaena hippurus) operating in the waters surrounding the main Hawaiian Islands and an offshore pelagic longline fishery. For the most part, all of these fisheries depend heavily on the tendency of their target species to aggregate in certain areas where they become more vulnerable to hook and line gear. This is especially true for the offshore handline fishery that concentrates on bigeye and found in aggregation with a productive offshore (Cross Seamount) and four offshore meteorological buoys that act as productive fish aggregation devices.

The Cross Seamount -Offshore Handline Fishery

Hawaii based longline vessels targeting medium and large have fished the Cross Seamount for decades using deep set tuna longline gear. Coastal handline boats began to fish the seamount and four offshore weather buoys in the late 1980s, concentrating on juvenile and sub-adult bigeye and yellowfin taken by a mix of shallow set handline and troll gears. This fishery quickly expanded to include fishing mixed tuna aggregations on offshore weather buoys, typically referred to as Buoys 1,2, 3 and 4. The high catch rates possible on these features contributed to a rapid expansion of the fishery during the early 1990s. The sudden appearance of significant landings of juvenile bigeye to local markets initially caused several problems, including low quality product due to poor catch handling and storage, market flooding, low price structure, roadside sales and some health concerns and wastage.

Since that time, the fishery has seen some stabilization and market development, currently landing about I million pounds of big eye and yellowfin tuna annually for local consumption.

User group conflicts

Since the beginning of the offshore handline fishery, various groups have expressed concern over the harvest of these juvenile sized . Direct gear interactions and conflicts have also occurred between longline and handline vessels operating on the limited fishing area over the summit of the Cross Seamount.

1 Pelagic Fisheries Research Program Joint Institute of Marine and Atmospheric Research University of Hawaii, 1000 Pope Road, MSB #312 Honlulu, Hawaii 96822, USA

2 Hawaiian Institute of Marine Biology, University of Hawaii PO Box 1346, Kaneohe, Hawaii 96744, USA

1 14th Standing Committee on Tuna and Billfish Noumea, New Caledonia, 9 -16th August 2001 Yellowfin Research Group

Inshore recreational and subsistence fishennen feel that the seamount fishery intercepts fish that would have later been available to them closer to the main islands. Inshore commercial handliners and offshore longline operators believe that the seamount fishery crops off too many young fish that would have later recruited to their fisheries as larger, more valuable sashimi grade tuna. Fish processors as well as fisheries that take larger sized bigeye and Yellowfin (longliners, charter vessels, commercial handliners, commercial trollers) see the offshore fishery as a waste of the resource at a low value. Conversely, the seamount fishennen suggest that longliners take too many large tuna, depleting local spawning stocks.

Research response

These debates have continued over the past decade through countless letters, meetings and discussions moderated by the based Western Pacific Regional Fishery Management Council. Since 1995, he Pelagic Fisheries Research Program of the University of Hawaii has supported tagging studies of yellowfin and bigeye tuna in Hawaii to address many of these issues; i.e. interaction, residence times, movement patterns, exploitation rates and aggregation effects.

The Seamount Tagging Project and the Hawaii Tuna Tagging Project (HTTP) has produced a tag release dataset of more than 17,000 bigeye and yellowfm tuna with an overall recapture rate of approximately 12.5% (Itano and Holland, 2000). One of the primary motivations for conducting the HTTP was to study the population dynamics of bigeye and yellowfin at the Cross Sea Seamount and FADs, and to investigate recruitment patterns from the Cross Seamount to the nearshore handline/troll fisheries and to the offshore longline fishery.

Earlier studies utilizing relatively small amounts of tag recapture data indicated that bigeye tuna are more persistent at the Cross Seamount in comparison to yellowfin (Holland, et.al 1999) and that transfer rates from offshore to inshore areas for both species were low (Sibert, et al 2000).

The analysis reported includes recaptures of tagged bigeye and yellowfin tuna released between January 1995 through June 2001 to the Hawaiian fishery between the geographic region 163°W to 152°W and 14°N to 24°N. Using release and recovery data up to June 2001, site and size-specific bulk transfer models were developed to estimate natural mortality, fishing mortality and transfer rates between various fishery components.

Results

The transfer rates from the Cross Seamount to the inshore fishery areas surrounding the main Hawaiian Islands were found to be low. Compared with this, the transfer rate from Cross Seamount and the offshore weather buoys to the offshore region were higher for both the species, indicating a considerable recruitment of Cross Seamount fish to the offshore longline fishery. The transfer rates from the seamount to Inshore, and vice versa, were about the same magnitude for bigeye where as for yellowfin, transfer rates from Inshore to the seamount were negligible.

The natural and fishing mortality rate was estimated over three size classes; for yellowfin 29- 45 cm, 46-55 cm and > 56 cm, and for bigeye 40-60 cm, 61-80 cm and >81 cm. For each species the estimates of natural mortality were higher for the smaller size class, decreased for

2 14th Standing Committee on Tuna and Billfish Noumea, New Caledonia, 9 -16th August 2001 Yellowfin Research Group the medium size class followed by an increase in the largest size class. For the Cross Seamount fishery, the fishing mortality rate of bigeye was found to be similar in the three size classes representing roughly 10% of the gross attrition in each size class.

Although the global attrition rate (losses from the Cross Seamount) for both species were nearly the same, the estimated residence times of bigeye tuna at Cross Seamount was twice that for yellowfin, reaffirming the findings of earlier studies. For reasons that are not yet clear, bigeye appear to be more persistent at the seamount while yellowfin more relatively transient. There is also a tendency for larger bigeye to persist longer, exposing them for longer period of vulnerability in the offshore handline fishery. For yellowfin their persistence rates were found to be similar across all size classes.

Fishing mortality rate estimates for both bigeye and yellowfin tuna at Cross Seamount are considered moderate at current levels of exploitation. For both species, emigration was found to be by far the greatest source of losses from the seamount. The current fishery does not appear to be adversely impacting the populations of bigeye and yellowfin at the Cross Seamount or their recruitment to inshore areas.

The size based estimates of natural and fishing mortality together with transfer rates and other ancillary information will be useful for a yield per recruit analysis to investigate various scenarios of increase/decrease in fishing effort and its effects on the fishery components. Additional strategic tag releases from all the fishery regions and components over broader size ranges should be carried out to further refine these issues of recruitment and interaction.

References

Holland, K. N., Kleiber, P., Kajiura, S. M. 1999. Different residence times of yellow fin tuna, Thunnus albacares, and bigeye tuna, J: obesus, found in mixed aggregations over a seamount. Fish. Bull. 97:392-395.

Itano, D. G. and K. N. Holland. 2000. Tags and FADs -Tuna movement and vulnerability of bigeye (Thunnus obesus) and yellowfin tuna (7: alba cares) in relation to FADs and natural aggregation points. Aquatic Living Resources. Aquat. Living Resource. 13: 1-11.

Sibert, J. R., K. Holland, and D. Itano 2000. Exchange rates of yellowfin and bigeye tunas and fishery interaction between Cross seamount and near-shore FADs in Hawai'i. Aquat. Living Resource. 13: 225-232.

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