Frontier-Tanzania Environmental Research REPORT 103 Misali Island

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Misali Island: A detailed description of the subtidal regionsNB. Reef-associated fish species diversity was calculated with Shannon-Weiner andSimpson’s indices using the Species Diversity and Richness programme 1 . The reasonfor this is that the former is used as a measure of species richness whereas the lattertends towards species dominance. The diversity index values were calculated for eachsite. The diversity index results were then tested for significant differences in diversitybetween two sites. The test used for this comparison was a randomisation test asdescribed by Solow (1993).2.2 RUGOSITY MEASUREMENTSRugosity measurements were taken for each survey site. This was done by laying a 25-metre tape measure (transect line) as level and directly as possible across the substratecontour at approximate depths of 6m and 16m. A small-link, 25-metre chain was thenlaid along the transect but closely following the reef topography. The length at whichthe end of the chain met the transect line was recorded. A rugosity ratio was calculatedby dividing the transect length (variable) by the length of chain (25 metres). Eachtransect was conducted three times at each depth and site.All surveys described were conducted in the waters immediately surrounding MisaliIsland. Depths were recorded at the time and date of individual surveys. These depthswere not converted to chart datum levels relative to the regional tide levels (TanzaniaHarbour Authority 2001 & 2002) because of the maximum 4-metre tidal range. It wasconsidered that a 4-metre depth range within surveys conducted at the same site wouldstill give sufficient representation for that location. For example, a shallow surveycarried out at 6m may have translated to between 2 metres and 10 metres if convertedto chart datum levels. However, this was considered not to affect the results from thatarea, hence complicated chart datum calculations for each survey were not considered anecessity.2.3 DEPTH PROFILE TRANSECTSDepth profiles were measured using a NASA Marine Clipper Range Depth Sounderwith speed and distance logger mounted on the research vessel in order to visualise thedifferences between each site. Depths were recorded at certain time intervals, rangingfrom 2 – 5 seconds, dependant on the gradient and therefore extent of the site between4-metre and 20-metre chart datum depth.The research vessel was positioned shallower than the 4-metre chart datum depth ateach survey site, and this starting point was marked using GPS. The vessel then headedaway from the island in a direction perpendicular to the coastline, noting depths at theappropriate time intervals. The depth transect was terminated beyond the 20-metrechart datum depth, and the finish GPS location was marked. Each survey site depthprofile was repeated and the data corrected to chart datum levels using regional tidedata (Tanzania Harbour Authority 2001 & 2002). This was necessary to allow exactand direct comparison between sites. The distance of the transect was estimated usingthe start and finish GPS points, which allowed the depths to be plotted against distance.1 Species Diversity and Richness, Version 2.64. Designed by Dr P.A.Henderson and Dr R.M.H.Seaby. Pisces Conservation Ltd,IRC House, The Square, Pennington, Lymington, SO41 8GN, England.Frontier-Tanzania Environmental Research Report 103 12
Misali Island: A detailed description of the subtidal regions3 RESULTS3.1 SAMPLING INTENSITYTable 2 summarises the number of survey person dives to date at each survey site andfor each survey type.Table 2: Summary of the survey person dives at each survey site to date (January 2003)Survey Site FixNumber of survey person dives toJanuary 2003Survey Site no.Lat. (S)Long. (E)SubstratumReef FishInvertebratesCommercialFishTotals1 05º14’ 10.1 39º36’ 35.5 24 21 28 19 922 05º14’ 25.2 39º36’ 50.8 38 23 22 20 1033 05º15’ 07.3 39º36’ 50.0 54 22 24 22 1224 05º15’ 28.6 39º35’ 24.6 42 17 24 18 1015 05º14’ 42.6 39º35’ 32.1 34 32 20 23 1096 05º14’ 26.6 39º35’ 41.5 34 22 22 20 987 05º14’ 16.0 39º35’ 47.1 22 18 22 18 808 05º14’ 02 8 39º36’ 04.9 36 18 18 22 94Totals 284 173 180 162 799NB Total number of survey person dives vary for each site as access to these is weather dependent3.2 BENTHIC SUBSTRATA MAPPING3.2.1 Site ReportsRugosity measurements and comments for each survey site are summarised inTable 4. Each site is summarised with respect to substratum composition and bioticcover of ‘hard’ 2 corals, seagrass and coralline algae. Table 3 summarises thesubtidal regions within the north, east, south and west regions around Misali Island.Categories of coral morphological forms are described in Appendix 5. All seawater depths are approximate as they are not corrected to chart datum.2 NB. Hard coral is the term used to describe Scleractinian corals but also solid-structured, reef-building, non-Scleractinian'corals' such as Tubiporidae ('organ pipe' coral), Heloporidae ('blue' coral), Milleporidae ('fire coral') and Stylasteridae.Frontier-Tanzania Environmental Research Report 103 13
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Frontier-Tanzania Environmental Research REPORT 103 Misali Island

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