Effects of exploitation on age, growth and mortality of the blackspot snapper, Lutjanus fulviflamma, at Mafia Island, Tanzania

Estimates of the growth parameters (L-infinity and K), mortality coefficients (Z, M and F) and exploitation rate (E) for the blackspot snapper, Lutjanus fulviflamma (Forsskal) from the Mafia Island Marine Park (MIMP) and adjacent intensively fished areas in Tanzania were determined. Sectioned otoliths showed that L. fulviflamma in the MIMP attained a maximum age of 18 years, with a high proportion of fish between 6 and 10 years old. The maximum age was 8 years in the intensively fished areas, with a preponderance of 2- and 4-year-old fish. The size structures of the populations in the MIMP and that in the intensively fished areas were markedly different, with the MIMP fish averaging (+/-SE) 211.4 +/- 0.38 mm TL, but 154.6 +/- 0.32 mm TL in the intensively fished areas. The von Bertalanffy growth parameters were L-infinity = 290.3 mm TL, K = 0.15 year(-1) and t(0) = -2.7 years. There was no significant difference in growth between the four populations (L-infinity: F-stat = 0.14, P = 1.000, and K: F-stat = 0.26, P = 0.992). Total mortality was 0.55 and 1.64 year(-1) in the MIMP and intensively fished areas, respectively, natural mortality 0.27 year(-1) and fishing mortality 0.18 and 1.37 year(-1) in the MIMP and intensively fished areas, respectively. The exploitation rate was 0.51 and 0.84 in the MIMP and intensively fished areas, respectively. The artisanal seine net fishery is directed mainly at younger fish in the intensively fished areas resulting in growth overfishing. The protracted life span, the slow growth and natural mortality rates imply that L. fulviflamma is vulnerable to overfishing and that the protection provided by the park, although limited, is vital for sustaining the fishery at Mafia Island.