Mangrove Vegetation Structure and the Abundance of Shrimps in Chilaw Lagoon, Sri Lanka
J.W.K.K. JAYASUNDERA, M.D. AMARASINGHE and K.B. RANAWANA
The mangrove vegetation structure and its relationship to abundance of shrimps around Pambala area in Chilaw lagoon, Sri Lanka were investigated. Belt transects of 10 in width were laid across mangrove stands with apparent variation in their vegetation structure. Data on floristic composition, stand density, basal area, mean stand diameter, tree height, stand above-ground biomass were collected and relative density,dominance, frequency and importance value of constituent species as well as complexity index and Shannon-Wiener diversity index were calculated with the data
Abundance of shrimps in the lagoon adjacent to these transects were obtained using a cast net of 1 cm stretched mesh size. Cast net was operated from a canoe by an experienced cast net fisher and samples were taken from locations at the distances of 50m, 100m, 150m and 200m from the shoreline. Samples were taken from these sites during four times of a day, i e. 08.00 — 09 00 h, 09.00 — 10 00 h, 10.00 — 11.00 h and 11.00 — 12.00 h.
Structural properties of the mangrove stands in the three localities were different from each other. Stand density (2,487 — 12,400 plants/ ha) was related negatively to mean stand diameter (6.82 cm — 3.24 cm) and Shannon-Weiner diversity index (1.242 — 0.304). Average above ground biomass for Rhizophora mucronata (23.69 kg — 2.26 kg) also showed a negative trend with increasing density, indicating more disturbed the stand due to cutting etc , higher the density of small stems and lower the species diversity in the stand.
The highest abundance (53.2%) of shrimps (Penaeus indicus, P. semisulcatus, P. monodon and Metapenaeus dobsoni together) was observed close to the mangrove stands (at 50m from the shoreline), which showed a gradual decline towards the middle of the lagoon. The highest shrimp abundance was (68.6%) at the site adjacent to the structurally richest mangrove stand (diversity index 1.242; complexity index 42.36) and the lowest (10.3%) around the sparse mangrove stand (diversity index 0.304; complexity index 46.06). The effects of structural complexity and the distance from the shore on the abundance of shrimps in this part of Chilaw lagoon and their interactions were found to be significant (p<0.05)
The results of the present study indicate that the abundance of shrimps in the Pambala area of Chilaw lagoon shows a positive relationship with the structural complexity and maturity of mangrove stands.