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Diadema antillarum on St. Croix, USVI: Current Status and Interactions with Herbivorous Fishes.
Onufryk JI
,
Ebersole JP
,
DeFilippo J
,
Beck G
.
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The 1983-84 mass mortality of the long-spined sea urchin, Diadema antillarum, intensified the loss of herbivory that was a major factor in the degradation of coral reefs in the wider Caribbean. As determined from recent surveys, densities of D. antillarum populations at back reef locations on St. Croix, US Virgin Islands, are higher than densities immediately following the die-off - but still about an order of magnitude below pre-die-off densities and patchy in both time and space. Comparisons to similar surveys during the past twelve years and to earlier historical records indicate that recovery on St. Croix continues at the very slow rate that typified the first decade after the mass mortality. Populations of herbivorous fishes on St. Croix surged following the D. antillarum die-off but have experienced heavy fishing pressure ever since then. Reciprocal densities of D. antillarum versus roaming grazers (parrotfishes and surgeonfishes) on survey transects and stationary point counts indicate that negative interaction between these two groups is present despite the reductions to their populations during the last 35 years.
Figure 1. Location of St. Croix U.S. Virgin Islands in the Caribbean Sea and our study sites on the eastern half of the island. CB = Cane Bay, CL = Columbus Landing, TB = Teague Bay, SC = Split Cove, TH = Turner Hole, RB = Rod Bay.
Figure 2. Densities (log scale) of Diadema antillarum at four sites on St. Croix, USVI from 1983 to present. Larger marker symbols indicate sites that continued to be surveyed after 2002. Note the very low levels of recovery and irregular fluctuations in population densities. The study sites we used are denoted [ Turner Hole ( ♢ ), Rod Bay ( ▵ ), Tague Bay ( □ ), Columbus Landing ( ○ )].
Figure 3. (a) Total number of roaming grazers versus number of D. antillarum and (b) Total number of territorial fish herbivores versus number of D. antillarum (b) on 42 transects (2m x 25m) at four sites on St. Croix, USVI. The study sites are denoted [ Split Cove ( ○ ), Turner Hole ( □ ), Rod Bay ( ♢ )]. Least squares regression performed on untransformed data indicates a low, but significant slope in 3a: (y = - 0.4163x + 31.82, R2 = 0.08, p = 0.033) and a slope that was not significant in 3b: (y = -0.0077x + 9.588, R2 = 0.0006, p = 0.86).
Figure 4. (a) Total number of roaming fish grazers versus number of D. antillarum and (b) total number of territorial fish herbivores versus number of D. antillarum (b) during 8 stationary point counts (1m radius, 15-minute observation) at Cane Bay on St. Croix, USVI. The point counts revealed a significant negative association between roaming herbivorous fishes and D. antillarum [least squares regression (4a); y = -0.61x + 27.30, R2 = 0.57, p = 0.030]. In addition, territorial herbivorous damselfishes were significantly associated with D. antillarum [least squares regression (4b); y = 1.12x + 7.66, R2 = 0.60, p = 0.024].
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