Yellowfin tuna (Thunnus albacares) is a cosmopolitan species distributed mainly in the tropical and subtropical oceanic waters of the three major oceans, where it forms large schools. The sizes exploited in the Indian Ocean range from 30 cm to 180 cm fork length. Smaller fish (juveniles) form mixed schools with skipjack and juvenile bigeye tuna and are mainly limited to surface tropical waters, while larger fish are found in surface and sub-surface waters. Intermediate age yellowfin are seldom taken in the industrial fisheries, but are abundant in some artisanal fisheries, mainly in the Arabian Sea.

Spawning occurs mainly from December to March in the equatorial area (0-10°S), with the main spawning grounds west of 75°E. Secondary spawning grounds exist off and the Mozambique Channel and in the eastern Indian Ocean off . Yellowfin size at first maturity has been estimated at around 100 cm, and recruitment occurs predominantly in July. Newly recruited fish are primarily caught by the purse seine fishery on floating objects. Males are predominant in the catches of larger fish at sizes than 150 cm (this is also the case in other oceans).

Tag-recovery data, recent age readings of otoliths and modal progressions provide support to a multi-stanza growth pattern for yellowfin but more work is needed to accurately model this complex growth pattern so it can be used in stock assessments.

Direct estimates of natural mortality (M) have been estimated for juvenile (40 cm to 100 cm long) yellowfin in the Indian Ocean using the data from the RTTP-IO. The current estimates (0.8 for 0 to 1 year old fish and 0.4 for fish 2 years and over) are much lower that previously assumed levels.

Feeding behaviour of yellowfin has been extensively studied and it is largely opportunistic, with a variety of prey species being consumed, including large concentrations of crustaceans that have occurred recently in the tropical areas and small meso-pelagic fishes which are abundant in the Arabian Sea. It has also been observed that large yellowfin can feed on very small prey, thus increasing the availability of food for this species. Archival tagging of yellowfin has shown that yellowfin can dive very deep (over 1000m) probably to feed on meso-pelagic prey.

Longline catch data indicates that yellowfin are distributed continuously throughout the entire tropical Indian Ocean, but some more detailed analysis of fisheries data suggests that the stock structure may be more complex. A study of stock structure using DNA was unable to detect whether there were subpopulations of yellowfin tuna in the Indian Ocean.

The tag recoveries of the RTTP-IO provide evidence of large movements of yellowfin tuna, thus supporting the assumption of a single stock for the Indian Ocean. The average minimum distance travelled by yellowfin between being tagged and recovered is 525 nautical miles. Both RTTP-IO and fisheries data indicate that medium sized yellowfin concentrate for feeding in the Arabian Sea.

Fisheries

Catches by gear, and per year from 1959 to 2008 are in Figure 1. Contrary to the situation in other oceans, the artisanal fishery component in the Indian Ocean is substantial, taking approximately 20-25 % of the total catch.

The geographical distribution of yellowfin tuna catches in the Indian Ocean in recent years by the main gear types is shown in Figure 2. Most yellowfin tuna are caught in Indian Ocean north of 12°S and in the Mozambique Channel (north of 25°S).

Although some Japanese purse seiners have fished in the Indian Ocean since 1977, the purse seine fishery developed rapidly with the arrival of European vessels between 1982 and 1984. Since then, there has been an increasing number of yellowfin tuna caught although a larger proportion of the catches is made of adult fish, when compared to the case of the bigeye tuna purse-seine catch. Purse seiners typically take fish ranging from 40 to 140 cm fork length and smaller fish are more common in the catches taken north of the equator. Catches of yellowfin increased rapidly to around 128,000 t in 1993. Subsequently, they fluctuated around that level, until 2003 and 2004 when they were substantially higher (224,200 t and 228,600 t, respectively). In recent years, catches appear to be higher in the first quarter of the year. The amount of effort exerted by the EU purse seine vessels (fishing for yellowfin and other tunas) varies seasonally and from year to year. Since 2000 between 800 and 1200 boat days per month were fished annually.

The purse seine fishery is characterized by the use of two different fishing modes: the fishery on floating objects (FADs), which catches large numbers of small yellowfin in association with skipjack and juvenile bigeye, and a fishery on free swimming schools, which catches larger yellowfin on mixed or pure sets. Between 1995 and 2003, the FAD component of the purse seine fishery represented 48-66 % of the sets undertaken (60-80 % of the positive sets) and took 36-63 % of the yellowfin catch by weight (59-76 % of the total catch). Since 1997, the proportion of log sets has steadily decreased from 66 % to 48 %.

The longline fishery started in the beginning of the 1950’s and expanded rapidly over the whole Indian Ocean. It catches mainly large fish, from 80 to 160 cm fork length, although smaller fish in the size range 60 cm – 100 cm have been taken by longliners from , since 1989 in the Arabian Sea. The longline fishery targets several tuna species in different parts of the Indian Ocean, with yellowfin and bigeye being the main target species in tropical waters. The longline fishery can be subdivided into an industrial component (deep-freezing longliners operating on the high seas from , and ,) and an artisanal component (fresh tuna longliners). The total longline catch of yellowfin reached a maximum in 1993 (196,000 t). Since then, catches have typically fluctuated between 80,000 t and 123,000 t.

Artisanal catches, taken by bait boat, gillnet, troll, hand line and other gears have increased steadily since the 1980s. In recent years the total artisanal yellowfin catch has been around 130,000-140,000 t, with the catch by gillnets (the dominant artisanal gear) at around 80,000 t to 90,000 t.

Yellowfin catches in the Indian Ocean during 2003, 2004, 2005 and 2006 were much higher than in previous years but have returned to a lower level in 2007 and 2008, while bigeye catches remained at their average levels. Purse seiners currently take the bulk of the yellowfin catch, mostly from the western Indian Oceana around . In 2003, 2004, 2005 and 2006, purse seine total catches made in this area were 224,200 t, 228,600 t, 194,500 t, 159,800 t, respectively. Similarly, artisanal yellowfin catches have been near their highest levels and longliners have reported higher than normal catches in the tropical western Indian Ocean during this period. In 2007, purse seine catches decreased to their lowest levels since 1990 with a total catch of 97,800 t and were of 117,000 t in 2008.

After an initial decline, mean weights in the whole fishery remained quite stable from the 1970s to the early 1990s. Since 1993, mean weights in the catches in the industrial fisheries have declined. Prior to 2003, although total catch in biomass has been stable for several years, catches in numbers have continued to increase, as there has been more fishing effort directed towards smaller fish. As described above, this situation changed during 2003 and 2004; where most of the very large catches were obtained from fish of larger sizes.

Figure 1 Yearly catches (tonnes x 1000) of yellowfin by gear from 1958 to 2008.

Figure 2 Mean of annual total catches of yellowfin tuna by gear operating in the Indian Ocean over the period 1990-1999, 2000-2007, 2003-2006 and 2008. GILL = gillnet, LL = longline, PS = purse seine. Data as of October 2009 (note that catches of some artisanal fisheries, eg. Gillnet, where allocated according to the landing place rather than the actual area fished, as time-area catches from these fisheries are not available).

Stock size and fishing pressure are considered to be close to their value at MSY.