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The yellowfin tunas form one of the major components of oceanic tuna catch along the Indian coast. They are fished both along the mainland as well as the Island systems with the total annual catch from the mainland varying from 10,307 t to 19,163 t during 2010-2012. Commercial fishing is mainly by small mechanized wooden crafts and non-mechanized traditional crafts. Mechanized crafts operated pole and line, long line and gillnets and non- mechanized crafts operated hand lines and troll lines.
Highly skilled fishermen of Andhra Pradesh State situated along the east coast of India use traditional catamarans fitted with sails to catch yellowfin tunas from deep waters by operating either the hand lines or the troll lines. Around 1500 such units operated along the coast with an average annual landings of 4,300 t during 2010-2012. Fishing is carried out for a day as the crafts do not have any storage facility. Peak landings are during October–January followed by May-July. The annual catch per unit at Visakhapatnam was 58 kg and during the peak fishing season it increased to 71 kg per unit.
The fork length of the yellowfin ranged from 20 to 185 cm with the mean at 130 cm. Fishes above 80 cm were found to be mature and the size at first maturity was estimated to be between 85 and 90 cm. Males were dominant with a male: female ratio of 1: 0.53. The length- weight relationship is W= 0.017077L 2.976. Feeding habit of yellowfin tuna indicated the fish to be a nonselective generalist feeder, foraging on micronektonic pelagic or benthic organisms available in the epipelagic waters. Teleost fish, crabs, squids and shrimps were the major food items.
Age and growth were estimated using length based methods. The von Bertalanffy growth parameters estimated were L∞ = 197.42 cm, annual K= 0.30 and t0= -0.1157. Mortality estimates were M= 0.48 and Z= 0.71 and F= 0.23 with the exploitation ratio E= 0.32. Growth was rapid during the initial years when the annual growth increments was as high as 36.6 cm during the first year then declined to as low as 3.3 cm in the tenth year. The fish attained a fork length of 56.2 cm at the end of one year. Size at maturity (87.5 cm) corresponded to an age of 1.7 years and the oldest individual in the sample was 9+ years (186 cm). The annual mean lengths varied from 80.6 cm to 115.3 cm with an average mean length of 101.9 cm. The fishery comprised of mostly adults with 64% comprising of fishes larger than size at first maturity.
