The ecology and exploitation of yellowfin tuna, Thunnus albacares (Bonnaterre 1788) in the Pemba Channel, Kenya
This thesis evaluates the utility of a sport fishery for yellowfin tuna, Thunnus albacares, in the Pemba Channel, Kenya, in providing ecological information relevant to commercial fishery assessment and management. Age, growth, reproductive status and diet are measured, together with an innovative assessment of the Indian Ocean yellowfin tuna stock. Catch/effort and weight data from the records of the sport fishery from 1963-1995 demonstrate a decline in tuna mean weight since the advent of purse seining in 1984. A periodicity of 5-6 years was detected in the Indian ocean longline fishery catches, and in the sport fishery catch/effort and mean weights. Samples were taken from tuna caught in the sport fishery from 1981-88. Sport-fished samples were more random in size, age and sex (ratio = 1: 1) than catches from any of the three major commercial fisheries. The total mortality rate of cohorts deconvoluted from weight-frequencies in the sport fishery catch over 6 years was used in a Caddy & Csirke analysis. Maximum Sustainable Yield estimates of the Indian Ocean stock were 113,000 tonnes (new method), 110-160,000 tonnes (Walters' method); compared with only 40-52,000 tonnes using the standard equilibrium method. Tuna (1653 over 7 years) were successfully aged using length frequency analysis and by a novel method based on clusters from a Principal Components Analysis of morphometric measures. Juvenile growth was fast, 3.1 cm/month, whereas adults grew at about 2 cm/month. Gonads (>1500) were staged for 5 years and histologically examined for 2 years. Female yellowfin mature earlier than males, but males become ripe earlier in the season. The spawning stock consists of females >120 cm FL. Diet was scored over 4 years and evaluated volumetrically over 2 years. Juvenile tuna eat fish, but adults opportunistically consume fish, cephalopods and crabs, depending on availability. Catchability by gear in both the sport and commercial fisheries is likely influenced by the currently abundant food.