Dyamics of dema trap finfish catches in the Coastal waters of Zanzibar, Tanzania.

Dynamics of traditional dema traps finfish catches from coralline substrate, sand flats and sea grass beds in the coastal waters of Zanzibar were studied during the period between May 1994 to April 1995. The catch comprised 24 families, 44 genera and 92 species. A total of 1401 fish were trapped during 34 sampling days and the total weight of the sample was 186.8 kg with catch per unit effort of 3.4 fish trap-1 day-1. The first dominant group from the catch was goatfish, Parupeneus barberinus Lacepede (Mullidae) which constituted 14.1% of the catch. The snapper, Lu jafrus johnii Bloch (Lutjanidae) was a principal representative of the second dominant group and contributed 10.1% of the total catch.The mean daily catch rate of dema traps was 0.195 kg trap-1 and mean daily catch was 6.1 kg with peaks in November and April. Estimated annual yield was 2.23 tones. Fish abundance differed relative to nature of substratum (p<0.001). Tukey's test for multisamplR comparisons showed differences between coralline/sand substrates (0.05<p<0.025) and sand/sea grass substrates (0.05<p<0.025) and there was no significant difference in fish abundance between coralline/sea grass habitats (p>0.1). Species composition varied with type of habitat and seasons. Species abundance was not different between coralline/ sea grass bed (0.2<p<0.01). A difference was observed between coralline/sand habitats (0.02<p<0.01) and sand/sea grass bed (p<0.001). Species were abundant on coral reefs during Southwest monsoon while species number was high on sand and sea grass habitats during Northeast monsoon. A total of 337 and 352 male and female P. barberinus Lac. and 210 and 216 male and female L. johnii Bioch respectively were examined. Length-weight relationship revealed an allometric growth in both species. Length-weight relationship of male and female P. barbernnirrus Lac. and L. johnii Bloch showed no significant difference. Sex ratio was 0.96:1 for P. barberinus Lac. and 0.97:1 for L. johnii Bloch in favour of females of both species. However, these ratios were not significantly different from 1:1. Females of both species were found to be preponderant at larger sizes than males. Length at first maturity was 21.9 cm and 22.1 cm for male and female P. barberinus Lac., respectively, while length at first maturity was 18.4 cm and 18.8 cm for male and female L. johnii Bloch, respectively. Both species were found to be reproductively active throughout the year.