Landings, Seasonality, Catch per Unit Effort, and Tag-Recapture Results of Yellowfin Tuna and Blackfin Tuna at Bermuda
Brian E. Luckhurst, Tammy Trott, and Sarah Manuel
Abstract.—Commercial landings of yellowfin tuna Thunnus albacares for the period 1975–1998 showed an overall increasing trend while blackfin tuna T. atlanticus landings did not. The landings of yellowfin tuna from 1992 to 1998 were significantly higher (mean = 51.7 mt) than in the previous five year period when there was apparently less directed effort. Maximum landings (67.3 mt) were recorded in 1996. Blackfin tuna landings over this same time period were generally less than 10 mt with a peak of 16 mt in 1986. The seasonality of commercial landings by quarter is presented for a 12 year period (1987–1998). For yellowfin tuna, the second and third quarters consistently had the highest landings while the first quarter was lowest. The second and third quarters combined accounted for greater than 70% of annual landings in every year. The seasonality in landings of blackfin tuna was more pronounced with the third quarter predominating in every year but one. Third quarter landings comprised 42–72% of the annual values while the first quarter was typically less than 5%. An analysis of catch per unit effort for both species over the same period (1987–1998) was conducted using an index developed from the detailed catch records of ten commercial fishers who target pelagic species. Mean values for yellowfin tuna increased significantly from 1992 onward and have oscillated in a small range (2.0–3.0 Kg per hour fishing) through 1998. The values for blackfin tuna were uniformly low and showed little variation. A long term recreational tagging program has resulted in high recapture rates for both species (yellowfin tuna 17.3%, blackfin tuna 10.8%) which may be associated with stable migratory routes or resident populations. An analysis of days at liberty for tag-recaptured fish indicated that 52.6% of all recaptured yellowfin tuna ( N = 78) were taken within 30 days of release. The corresponding figure for all recaptured blackfin tuna ( N = 46) was 13%. There is a significantly higher proportion of long term recaptures (up to four years) in blackfin tuna. An examination of the size-frequency distribution for yellowfin tuna indicates that a broad size range is caught in Bermuda waters, however, the catch is dominated by relatively small size classes (modal size = 79 cm FL).