Effects of fishing on growth and reproduction of blackspot snapper, lutjanus ful viflamma (pisces: lutjanidae) in shallow waters of mafia island, Tanzania

The blackspot snapper, Lutjanus fulviflamma (Forssk&l 1775) is an important commercial reef fish in Tanzania. The reproductive biology, age, growth and mortality, food and feeding habits and abundance were studied in the Mafia Island. Monthly fish samples were collected from seine nets between May 1999 and April 2001. Contrasting populations in the Mafia Island Marine Park (MIMP) and those in the intensively fished areas were used to evaluate the effects of fishing on this species. Sex ratio was skewed in favour of males (0.9:1) and age and size at first maturity were higher for the populations in the intensively fished areas than the MIMP. Sex ratio was balanced at 1.03:1 (female:male) in the MIMP. Breeding occurred in the MIMP areas from September to March with no evidence for the breeding activity in the intensively fished areas, suggesting recruitment overfishing. Sectioned saccular otoliths (sagittae) showed that L. fulviflamma attained maximum age of 18 years. Ages of 2 - 4 years were predominant in the intensively fished areas with the MIMP showing majority of individuals in between 6-10 years. Fish in the MIMP averaged (± s.e.) at 211.4 ± 0.4 mm TL compared to 154.6 ± 0.3 mm TL in the intensively fished areas. The von Bertalanffy growth parameters were estimated at L*, = 290.3 mm TL, K = 0.15 y'1, and to = -2.7 y. Fishing mortality ranged from 0.2 to 1.4 y’1. An index of relative importance (IRI) showed brachyurans to be the main prey of L. fulviflamma accounting for 40% IRI. Teleosts accounted for 50% IRI in the diet of fish within size range of 270 - 300 mm TL. The underwater visual census (UVC) showed density and biomass of L. fulviflamma in the MIMP to be five and ten times respectively greater than that recorded for the intensively fished areas. Considering the possibility that reproduction could be impaired as a result of changes in population structure, the strategy of protection through marine parks and reserves is supported as a fair hedge against recruitment failure.