Techniques development for the reestablishment of the long-spined sea urchin, Diadema antillarum, on two small patch reefs in the upper Florida Keys

 

Ken Nedimyer, Principal Investigator (sealife@terranova.net)

Martin A. Moe Jr., Associate Investigator (martin_moe@yahoo.com)

 

Abstract

 

A project funded through FKNMS was begun in the fall of 2001 offshore of the Upper Keys to explore the feasibility and ecological results of translocating juvenile long-spined sea urchins, Diadema antillarum, from areas with relatively high settlement and extensive winter mortality, the reef crest rubble zones, to nearby deeper water (about 25 feet, 7.5 m) patch reefs at densities approaching those on Florida reefs before the Diadema plague of the early 1980s. Four patch reefs: two experimental and two controls, varying in size from about 44 to 96 sq. m were selected for the study. During the period from September 2001 to December 2001, 434 juvenile long-spined urchins were placed on experimental reef # 1 (96 sq. m), a total potential density of 4.5/m2, and 262 were placed on experimental reef # 2 (88 sq. m), a potential density of 3.0/m2. An additional 16 urchins were placed on reef # 2 on 10/23/02 bringing the total urchins placed on reef # 2 to 278, a potential density of 3.2/m2. The translocated populations were evaluated for number and placement of surviving urchins 10 times on reef # 1, and 11 times on reef # 2 over various intervals during the period from September 8, 2001 to February 5, 2003.

 

Percent survival of the Diadema urchins was roughly similar on both experimental reefs from the first count on 09/08/01 through the final count on 02/05/03. Initial survival rates over the first three days of 80% and 90% dropped to about 40% to 45% on both reefs from 11/09/01 to 05/29/02, and then, on experimental reef # 1, survival remained at about 30% from 08/08/02 to the last count on 02/05/03. On experimental reef # 2, survival remained at 40% on 08/08/02, dropped to 30% on 10/08/02 and then dropped again to 17% at the count on 11/30/02. Survival was 20% on the final count on 02/05/03 due to placement of 16 urchins on this reef late in the study (10/23/02). The average density of urchins over the entire 17 months of the study was 1.6m/2 on experimental reef # 1 and 1.0/m2 on reef # 2. The highest density on reef # 1 (2.1/m2) was achieved on 02/26/02 and the highest density on reef # 2 (1.4/m2) occurred on 10/24/01 and on 02/26/02. The final density on 02/05/03 on reef # 1 was 1.2/m2 and on reef # 2 was 0.6/m2. Decline in survival and density on both reefs was generally gradual and stable at a similar rate of decline during the last 12 months of the study. Reef # 1 lost 87 urchins, a survival of 57% over the last 345 days of the study. The total loss in urchin density on reef # 1 over this period, 02/26/02 to 02/05/03, was 0.9/m2, which was a decline in density of 0.0026/m2 per day. Reef # 2 lost 67 urchins during this 345-day period, a survival of 45% and a loss in density of 0.8/m2; which was a decline in density of 0.0023/m2 per day (This data for reef # 2 includes 16 urchins released on reef # 2 on 10/23/02).

 

The gradual mortality over the term of the project indicated that predation was the main cause of population decline and not mortality due to storms or plague. Population counts before and after two instances of tropical storm conditions in the fall of 2001 indicated that these storms did not cause mortality in the translocated urchin populations on the experimental deep reefs, and no evidence of plague caused Diadema urchin death was observed.

 

Although evidence of some movement between reef quadrants and some concentration of urchins on the more rugged and complex areas of reef # 1 was evident, in general, urchins remained broadly distributed over all reef areas on each experimental reef.

 

NURC (NOAA’s National Undersea Research Center) conducted a rapid habitat assessment of the four project reefs on 08/31/01 and 09/01/01, before translocation of the urchins and again on 09/18/02, about one year after translocation of the urchins.

The benthic ecology of the experimental reefs changed considerably during the period of exposure to “normal” pre plague density of Diadema urchins. The results of the NURC assessment showed that the percent stony coral cover increased on the experimental reefs from 9.8% to 15.3% (+ 56% relative increase) and decreased on the control reefs from 9.1% to 6.8% (-26% relative decrease). Sponge cover decreased on the experimental reefs from a mean of 7.4% to 5.3% and increased on the control reefs from 5.3% to 6.0%. Algal turf cover decreased slightly on the experimental reefs from 28% to 24% (- 16.2% relative decrease) while algal turf increased on the control reefs from 23.4% to 27.8% (+18.7% relative increase). Crustose coralline algae exhibited the most significant change. Coralline algae cover increased on the experimental reefs from 7.5% to 19% (+ 153% relative increase) while coralline algae cover decreased on control site 1 (reef # 3) and slightly increased on control site 2 (reef # 4), a total change of 7.8% to 8.8% (+ 6.5% relative increase) on the control sites. The presence of crustose coralline algae has been shown to stimulate settlement of certain species of stony corals. Green calcareous algae (mostly Halimeda spp.) showed little change on the experimental reefs (a decline from 3.8% to 3.1%), but increased on the control sites (an increase of 1.8% to 3.8%). Brown foliose algae, mostly Dictyota spp., greatly declined on the experimental reefs a decrease of 10% to 5.1%, a – 48% relative decrease) and increased slightly on the control reefs (an increase of   4.5% to 5.9% increase, + 31% relative increase). Brown foliose algae declined on experimental reef # 1 to a remarkable extent (11% to 1.8%, a – 511.1% decrease), and also declined on control reef # 4 (which hosted a small population of Diadema urchins) from 3.0% down to 1.0%. Experimental reef # 2 showed a small decrease in brown foliose algae from 9.0 to 8.5%, while control site 1 (reef # 3) showed an increase in brown foliose algae from 6.0% to 10.8%. Brown foliose algae are important competitors with corals for space and sunlight, and reduction of these algae is critical to coral recovery. The density of juvenile corals increased on the experimental reefs from an average of 6.2 juveniles/m2 to 15.3 juveniles/m2, a relative increase of + 147%. Average (mean) densities also increased on the control sites (reefs # 3 and # 4) but to a lesser degree, 6.6 juveniles/m2 to 9.9 juveniles/m2, a relative increase of +51%.

 

These positive changes over the short term of one year show a marked reduction in algal prevalence and signify a return to a coral dominated ecology. These changes in the ecology of the experimental reefs are what was expected from a return of Diadema urchins to the reefs, and reflect the changes that have occurred on limited areas of Caribbean reefs where populations of Diadema have returned naturally. This study presents evidence that translocation of Diadema urchins from environments with high risk of mortality to deeper reef areas along the Florida Keys results in survival and population densities that can affect change in the ecology of coral reefs, moving reef areas from algal expansion back toward dominance of coral growth.