Age, growth, and spawning of Yellowfin tuna, (Thunnus albacares) Bonnaterre 1788, in the Southern Philippines

Yellowfin tuna {Thunnus albacares) Bonnaterre 1788, were studied between February 1987 and March 1988 from the landings of two fisheries active in the Moro Gulf and Celebes Sea area of the southern Philippines. The primary objective of the study was to estimate the age and growth of juvenile yellowfin tuna by microstructural growth increments in the sagittal otoliths. Other objectives were to examine seasonal effects on the spawning, recruitment, and growth of juvenile yellowfin tuna, and for adult yellowfin, to determine if the relationship between weight and fork length differed between sexes and between years, to estimate age and growth by length frequency analysis and to examine sex ratios in the landings. Juvenile yellowfin landings from the General Santos ringnet were sampled for fork lengths and otoliths. Acid etched otoliths viewed under a scanning electron microscope showed incremental and discontinuous growth zones analogous to those observed in Central and Eastern Pacific yellowfin otoliths. The daily increment formation rate for yellowfin tuna between 25 and 40 centimetres in fork length was validated by an oxytetracycline marking experiment carried out at the Kewalo Basic laboratory of the NMFS, SWFC/NOAA. Two hundred and seven otoliths were prepared for ageing by acid (EDTA) etching and whole mounting or by embedding in fibreglass resin, sectioning, polishing, acid etching, and mounting. Sectioning was done in a frontal plane containing both the primordium and post-rostral tip. Increments were counted along a straight line path between the primordium and the post-rostral tip using a phase contrast light microscope at 400X magnification. Growth in fork length and weight was best described by two linear stanzas. The growth rate of yellowfin between 15 and 35 cm in fork length was 2.50 mm/day and between 35 and 79 cm was 0.96 mm/day. The growth rate of yellowfin between 50 and 2350 g in whole wet weight was 9.15 g/day and between 2350 and 9200 g was 23.68 g/day. Philippine yellowfin attain a fork length of 57.46 cm and a weight of 3726 g in a year. Transition points between growth stanzas may indicate ontogenetic changes in the early life history of yellowfin tuna. Estimates of age and growth were verified by length frequency analysis and back calculations done using daily increment measurements along the counting path of sectioned otoliths. Results showed that ageing by increments in otoliths is a precise and accurate technique. A reversed Lee's Phenomenon was apparent in back calculated lengths at previous ages, indicating that size selective mortality is adversely effecting the smaller fish of an age group. Consistent increment patterns were apparent in otoliths. Eight to ten evenly spaced and narrow increments surround the primordium, the next thirty increments increase in width to a maximum then slowly decrease in width over the next 120 and increments beyond 120 remain at a steady increment width. Monsoon seasons occur at five and seven month periodicity in the southern Philippines. The southwest monsoon begins in May and continues until September. The northeast monsoon begins in October and continues until April. Transitional tradewind seasons occur during April -May and September - October. Yellowfin spawning peaks estimated by trends in a condition factor, gonadosomatic, and hepatosomatic indices, were apparent from April through June and October through November. Relative recruitment frequencies estimated from back dating age converted length frequencies showed recruitment peaks in May through June and October through November. Condition indices and growth estimated by otolith ageing, for the juvenile yellowfin, reach a minimum in April. Recruitment and growth for the juvenile yellowfin appears to be enhanced with the onset of the monsoonal winds. Using four years of weight and length data for adult yellowfin significant differences in weight-length relationships between sexes were found in 1982, 1983, 1984, and 1987. Significant differences were also found between years for females and between years for males. From the length frequency analyses, females were estimated to be 127 cm and males 136 cm at age three. Growth rates were estimated to be 2.90 cm/mo and 3.27 cm/mo for females and males respectively. The growth rate decreases at 0.07 cm/mo and 0.10 cm/mo for females and males every month after age three. Sex ratios showed a preponderance of males over all fork length intervals above 135 cm, between 30 to 44 months of age, and in every month of the year except November.Science, Faculty ofResources, Environment and Sustainability (IRES), Institute forGraduat