Population changes of a sea urchin (Echinometra mathaei) on an exploited fringing reef
In: African Journal of Ecology. Wiley: Oxford,. ISSN 0141-6707; e-ISSN 1365-2028, more
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| Keywords |
Echinometra mathaei (Blainville, 1825) [WoRMS]
ISW, Kenya, Coast, Diani
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sea urchin, densities, size |
| Authors | | Top |
- Muthiga, N.A., more
- McClanahan, T. R.
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| Abstract |
A comparison of Echinometra mathaei densities and sizes was undertaken on an inner reef lagoon and an outer reef edge on a densely populated tourist beach at Diani, Kenya. Echinometra mathaei densities and average lengths were significantly higher (t-test, P <0.001) in the inner reef lagoon (density = 14.2±1.7 no. m-2, n= 90; lengths = 40.8 ±0.6 mm, n= 144) than on the outer reef edge (density = 1.7 ± 0.13 no. m-2, n= 60; lengths = 31.2 ± 0.8 mm, n= 68). A comparison with density and length data collected 15 years previously (Khamala, 1971), showed increases in the numbers and average lengths (t-test; P < 0.05) in the inner reef and a decrease in the average lengths (t-test; P < 0.05) on the outer reef edge. Using a correlation of the length and average weight of individual urchins (weight = 0.0021 length2.64, r= 0.96,n= 144) and the average length and density measurements (no. m-2), an increase of 424 g m-2 on the inner reef and a decrease of 81 g m-2 on the outer reef edge was found over the past 15 years. Quantities of sediment found in the gut contents of the sea urchins (69.5% by weight) were positively correlated with the weight of the sea urchins (F=67.5, P<0.001, r= 0.73), which suggests that reef substrate degradation rates are proportional to the urchin biomass. Therefore, we suggest that there has been an increase in reef substrate degradation rates on the inner reef lagoon but not on the outer reef edge. The distribution of E. mathaei on the inner reef was positively correlated (F= 82.2, P < 0.001, r= 0.69) with the percentage of hard substrate (dead coral and coral rubble) but not on the outer reef, where the availability of shelter appears to be of greater importance. We suggest that the population increase in the inner reef is due to ecological release of E. mathaei from competitors and predators due to increased fishing and shelling activities. On the outer reef the stressful physical environment may limit both the sea urchin populations and man's ability to fish and collect shells. |
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