PFRP
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Reproductive Biology of Yellowfin Tuna, Thunnus albacares, in Hawaiian Waters and the Western Tropical Pacific OceanThe reproductive biology of yellowfin tuna was studied to determine the seasonal and spatial variations in spawing. Samples of yellowfin were obtained from surface and longline fisheries throughout the region. Gonads were examined histologically to determine reproductive state and time since last spawing. Work was coordinated by Mr. David Itano in collabration with the South Pacific Commision, the Micronesian Maritime Authority and the National Research Institute of Far Seas Fisheries (Japan).
A final project report published as part of the
SOEST-JIMAR publication series:
Color images of figures 4 - 9 (yellowfin histological
sections) from this report may be viewed at Yellowfin
oocytes samples
Article
published in NMFS Tuna Newsletter: See Other Publications and the SOEST-JIMAR Publications Page for other articles and PFRP reports by PFRP investigators
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Investigators: |
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Mr. David
Itano
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Dr. E. Gordon Grau, Interim Director Hawaii Institute of Marine Biology (HIMB) University of Hawaii at Manoa Coconut Island P.O. Box 1346 Kaneohe, Hawaii 96744 Phone (808) 236-7406 FAX (808) 236-7443 |
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Mr. Richard Shomura Hawaii Institute of Marine Biology (HIMB) University of Hawaii at Manoa Coconut Island P.O. Box 1346 Kaneohe, Hawaii 96744 Phone (808) 236-7401 FAX (808) 236-7443 |
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Project SummaryFunding was received at the University in December 1993.
Project DescriptionA comprehensive understanding of the link between yellowfin tuna biology and fisheries is necessary for sound management of the species. The PFRP funded project 2060 to investigate the reproductive biology and spawning activity of yellowfin tuna in the central and western Pacific region in relation to fisheries vulnerability and interaction. The project was lead by David Itano with collaboration from the South Pacific Commission, the Micronesian Maritime Authority, the National Research Institute of Far Seas Fisheries, the National Marine Fisheries Service and the Forum Fisheries Agency.The study area encompassed the region of the main surface tuna fishery of the western Pacific; stretching from the highly productive waters of the southern Philippines and Indonesia in the west, to an area including the Line Islands of Kiribati in the east, as well as a large area surrounding the Hawaiian Islands. The reproductive parameters of size at maturity, spawning frequency, time of spawning and the determination of peak spawning areas and seasons were determined by examining over 10,000 yellowfin tuna ovary samples collected over a two year period from purse seine, longline, handline and troll vessels of several distant water and domestic fishing fleets. Sampling for the project ended the second quarter of 1996. Batch fecundity estimates were made from hydrated ovaries collected shortly before the fish would have spawned. Fishery and biological data, such as the set type, school association type, fishing gear configuration and school behavior were recorded for each sample to examine fishery interaction and gear vulnerability issues. Stomach contents of sampled fish were also examined to investigate the relationship between feeding activity and reproduction.
Results - Reproductive BiologyThe estimation of length at maturity for western Pacific yellowfin was made from a subset of the database of over 7500 tuna ovary samples collected from purse seine, longline and handline vessels within ten degrees of the Equator where warm sea surface temperatures are conducive to spawning activity throughout the year. Fish were classified as mature if fully yolked oocytes or the atresia (resorption) of fully yolked oocytes was observed in the ovary sample by microscopic observation. The minimum length at maturity for female yellowfin from the Equatorial sampling was for a 73 cm fish with fully yolked oocytes but exhibiting significant atresia of more than 50% of all yolked oocytes. However, slightly larger fish were found to be actively spawning in this region. Sexual maturity increased steadily to 104.6 cm when 50% of the fish sampled in these length classes were classified as sexually mature. A yellowfin tuna of this size corresponds approximately to a fish over two years in age and 50 pounds in weight.Yellowfin tuna, like other members of the tuna family are serial spawners, meaning that they will release several batches of eggs throughout the year in contrast to temperate water fishes that may release a few batches of eggs during a brief spawning season. Batch fecundity estimates from the equatorial region of the study area ranged from 0.55 to 4.06 million eggs per batch with a mean relative batch fecundity of 54.7 oocytes/gram body weight. Batch fecundity estimates from fish near Hawaii were more variable ranging from 0.43 to 10.61 million with a similar relative batch fecundity estimate of 54.1 oocytes/gram body weight. Ovary samples collected from purse seine, longline and handline gear along the Equator that were histologically classified as mature (including mature but non-spawning, "atretic" samples) revealed that 50% had spawned within 24 hours of capture indicating a spawning frequency on average of once every 2 days. If only ovary samples with fully yolked oocytes are examined, the spawning frequency estimates from all gear types are very similar and indicate near daily spawning frequencies (average spawning frequency of once every 1.18 days). The resulting conclusion is that the fish do not spawn every other day, but engage in periods of daily spawning interspersed with periods of reproductive inactivity. It is not known how long a particular fish can maintain a daily spawning rhythm and this is no doubt highly variable. However, observations of captive yellowfin in spawning condition suggest that they can maintain near daily spawning activity for lengthy periods of several weeks to months at a time. Careful examination of fine structures within the ovaries and the oocyte condition leading up to ovulation in relation to the time of capture can result in accurate estimates of the time of spawning. This study and previous studies on the reproductive biology of the species conclude that yellowfin tuna spawn primarily in the evening to early morning hours and it is thought that spawning takes place at or near the surface. The eggs are close to one mm in diameter and supplied with a single oil droplet that suspends the eggs at the surface until hatching. Fertilization is external from mature males that predominate in the populations with rapid larval development and juvenile growth rates.
Spawning SeasonalityThe only area that indicated a strong and repeated seasonal pattern of peak spawning along the Equator during the sampling period was in the southern Philippines as determined from samples collected in the Moro Gulf and Celebes Sea. Decreases in spawning activity among mature fish were noted to coincide with slight drops in sea surface temperature for this region between February and May. Previous authors have also indicated seasonality to spawning of yellowfin in the southern Philippines that they proposed was linked to the changes in monsoon weather patterns and local productivity.Different peak areas and seasons of yellowfin spawning have been proposed for the central and western Pacific Equatorial region. Peak spawning to the west (135 E - 165 E) has been suggested to occur during the fourth and first quarter and a central Pacific region of peak spawning (180 - 140 W) to occur during the second and third quarter. Findings from this PFRP study confirmed active spawning among mature fish sampled all along the equator throughout the year but some declines in the relative degree of spawning noted for some areas and months. Yellowfin spawning in this area appears to be highly variable and may be linked to shifting zones of productivity that vary within and between years. The spawning season for yellowfin tuna around the Hawaiian Islands occurs between April and October and peaks in June, July and August, when mature yellowfin become more vulnerable to all hook and line fisheries near the main islands. During the brief summer spawning peak, over 85% of mature yellowfin sampled were in a near daily spawning mode. During the winter season, mature yellowfin caught in this area exhibit complete atresia of yolked oocytes and completely shut down reproductive activity.
Fishery data and fishing methodsThe tuna fishermen of the western Pacific utilize fishing techniques that take advantage of the behavior of the fish and tuna schools. Purse seine vessels operating in the tropical western Pacific generally employ two different fishing methods to capture large tuna schools. The majority of the catch consists of juvenile tuna caught in association with drifting objects, such as natural logs, jettisoned cargo or man-made rafts set out to attract tuna schools. These "log sets" as they are commonly referred to can be accomplished before dawn when the tuna can not see and avoid the nets.So called "free schools", or unassociated schools are also set on which produce the majority of mature sized yellowfin taken by western Pacific purse seine fleets. These schools are not truly "unassociated", but are actively feeding schools aggregated to concentrations of small baitfish. These schools are called "boilers" or "foamers" by the fishermen due to the foaming white appearance of the schools as they whip the surface to a froth in pursuit of baitfish (Figure 1). Tuna become so pre-occupied in these feeding frenzies that they fall easy prey to purse seine gear. Under normal conditions, it is very difficult to seine large yellowfin tuna in the western Pacific due to the elusive nature of the fish in the clear water and the depth of the thermocline which is normally deeper than the purse seine nets. Figure 1. US purse seine vessel (1200 ton class) operating in the western Pacific with a 200+ ton "foamer" school of tuna in the net. Longline fleets of the western Pacific employ one of two basic styles of longline fishing. Japanese longline vessels sampled by the project set 15 to 20 hooks between deep floatlines and set with a line shooter to throw additional line in the set to sink the gear very deep in the water column. Their gear is normally fished during daylight hours targeting large bigeye that frequent deep waters during the day close to the thermocline. In sharp contrast, the Taiwanese and mainland Chinese longline fleets set very shallow gear with only 5 hooks per basket hung from short floatlines and soak the gear at night. These vessels also hope to catch bigeye, but close to the surface at night and concentrate effort during full moon periods. They also catch a high proportion of large yellowfin, billfish and sharks (Figure 2). Figure 2. Taiwanese longline vessel unloading iced yellowfin in Apra Harbor, Guam. Handline fishermen of the Philippines and Indonesia also take advantage of the tendency of tuna to aggregate around floating objects by dropping baited handlines near anchored fish aggregation devices (FADs) for large yellowfin and bigeye tuna (Figure 3). Lines are often baited with juvenile yellowfin as the adults are highly cannibalistic. Figure 3. Handline caught yellowfin tuna were sampled at Lion Beach, Mindanao in the southern Philippines.
Gear interaction and fisheries vulnerabilityInteresting differences in reproductive activity were noted between sexually mature yellowfin taken from different types of tuna schools or by different gear types. In general, purse seine caught fish were more reproductively active (spawning) compared to longline caught fish. Mature yellowfin from foaming or boiling schools (actively feeding) were typically in a daily spawning mode and more reproductively active compared to fish caught in associationwith drifting objects that had a higher proportion of atretic, or mature fish in a "resting" phase. Fishing strategy and the relative importance of log or school set types vary between purse seine fleets which highlights the importance of a broad sampling program for a biological study of this kind.Fishing strategies and the behavior of the fish determine interaction and vulnerability rates. The importance of forage fish became apparent during the course of study, and the stomach contents of sampled yellowfin were noted and preserved for analysis. In most cases, successful purse seine operations on free schools of large yellowfin in the western Pacific depend on concentrations of a single species of baitfish; the ocean anchovy (Encrasicholina punctifer). The distribution of the baitfish is apparently patchy, but when large concentrations occur the vulnerability of large yellowfin to purse seine gear can increase dramatically. Increased catch rates of mature sized yellowfin by purse seine gear has an added effect of increasing direct interaction between purse seine and longline fleets for the same large size fish operating in the same areas. The Hawaii based fisheries for large yellowfin depend to a large degree on reproductively active fish caught during the brief summer season. The surface troll fishery for ahi, or yellowfin tuna over 100 pounds, and the ika shibi handline fishery are both based on spawning aggregations of large fish close to the main Hawaiian islands. Yellowfin landings by Hawaii based longline vessels also peak during the summer season on spawning adults. However, direct gear interaction problems between the small gears and longline vessels have been minimized by the implementation of a closed zone for longline gear within 50 to 75 miles of the main Hawaiian islands during the summer months. Prior to the implementation of the longline exclusion zones, summer longline fishing for yellowfin tuna was very productive in near shore waters.
SummaryYellowfin tuna can be characterized as having a rapid growth rate that brings the fish into sexual maturity after their second year when mass spawning close to the surface commences. The females are capable of generating millions of eggs per day for extended periods of time. Such production requires a great deal of energy, possibly limiting large spawning events to particular seasons or productive areas with adequate forage resources. The rapidly developing larvae would subsequently benefit from beginning life in a region of high productivity.It is likely that a regular and repeated seasonal pattern to tuna reproduction in the equatorial central and western Pacific does not exist but varies depending on productivity and environmental conditions peculiar to a given year. Discrete areas of high productivity in this region appear to shift east and west from month to month. It is proposed that these areas produce concentrations of tuna forage, in particular the ocean anchovy. It is further proposed that abundant food resources can help to bring juvenile tuna into maturity or provide the fuel to resume spawning in mature but reproductively inactive (resting stage) tuna and subsequently maintain daily spawning rates with high batch fecundities. It is possible that highly productive areas, if they persist for a long enough period could result in massive spawning production, in effect creating a peak spawning "season" for that region of the western Pacific for that year. At other times, the tuna apparently shut down their reproductive processes, absorb yolked oocytes and enter a resting stage until their next major spawning event. Feeding and spawning aggregations and fish aggregation devices (FADs) dramatically increase the vulnerability of adult yellowfin to purse seine, longline, troll and handline gear resulting in increased catch and interaction rates. Direct gear interaction can be mitigated by geographically separating gear types, but it is possible that temporally or spatially remote fishery interactions could still be influencing other fisheries.
This page updated August 14, 2006 |
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PDF file of Itano's yellowfin biology report
