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Healthy Coral Populations Produce A Surprising Number of Offspring
Healthy coral populations can produce up to 200 times more juvenile corals than degraded coral populations nearby, according to a new study in Conservation Letters. By studying one of the Caribbean’s healthiest remaining coral reefs on the island of Curaçao, researchers found that healthy coral populations had a higher percentage of successful parents and each parent produced up to four times more offspring compared to corals in degraded populations. Combined with higher coral numbers overall, the healthy populations produced up 200 times more offspring per square meter of coral reef.
The conservation value of healthy coral reefs is therefore higher than previously thought because of their outsized contributions to coral reproduction and reef recovery. Traditionally, coral abundance was the most widely-used method for assessing reef health. This new study shows this measurement underestimates the hidden differences in reproduction between healthy and degraded reefs. Because coral offspring can swim and disperse to other reefs, the healthiest remaining coral reefs can help re-seed and regrow coral reefs on local and regional scales.
“Healthy reefs are critical nurseries for baby corals and they support the recovery of coral communities elsewhere,” said lead author Dr. Aaron Hartmann.
The reef area studied, known as Oostpunt, was recently ranked among the three healthiest coral reef ecosystems in the Caribbean by an IUCN study of the entire region. The Oostpunt area is undeveloped, yet the island of Curaçao is currently considering a plan to build over 2,000 hotel rooms, 18,000 residences, 3 golf courses, and their supporting infrastructure directly on the coastline. The new study shows that significant ecological damage is likely to occur island-wide due to the loss of high levels of coral reproduction from this thriving, undeveloped reef region.
“The coral reefs at Oostpunt represent a window into the past when Caribbean reefs were healthy and vibrant everywhere. We’ve identified an explanation for why Oostpunt’s reefs have remained this way – healthy parents there make lots of babies, while stressed parents on other reefs don’t,” said Hartmann.
The researchers also found that corals in healthy populations contain greater amounts of fats, which allow them to produce more offspring, and that corals in healthier populations make more babies per parent without reducing their investment in each offspring.
Coral reefs are worth US$1T per year in fisheries production, shoreline protection, job creation, tourism and drug discovery potential. The survival and recovery of these systems depends on the production and long-distance dispersal of coral larvae.
 Jackson JBC, Donovan MK, Cramer KL, Lam VV. Status and trends of Caribbean coral reefs. Gland: Global Coral Reef Monitoring Network, IUCN.
http://cmsdata.iucn.org/downloads/caribbean_coral_reefs___status_report_1970_2012. pdf. 2014.
“Corals in Healthy Populations Produce More Larvae Per Unit Cover”
Coral parents have more fats
Fats or lipids are critical sources of energy for corals, as they are for humans. When corals reproduce, their offspring are comprised almost entirely of fats that provide energy and buoyancy as they search for a location to settle on the reef. Fats in adult corals also become particularly important when corals bleach (i.e., lose their algal symbionts, which provide them most of their energy). Without its algal symbionts to provide it food, corals rely on their fat stores to survive. Once these stores are exhausted, they will likely perish. Thus, a coral with more fat is likely to survive longer after bleaching, providing additional time to reclaim its algal symbionts once environmental conditions return to normal.
Measuring coral cover misses important ecological values of reefs
Coral cover—the percentage of the seafloor covered in live corals—is one of the most common metrics used to determine the health of a coral reef. Its common use is understandable given that it is relatively easy and inexpensive to measure. Our data suggest, though, that comparing coral cover per se—such as 25% on one reef versus 50% on another) does not reflect a two-fold difference in the ecological value between these reefs. Instead this difference is many-fold higher. The unseen values are that on the higher cover reef substantially more offspring are produced and adult corals have greater energy stores. Thus, when calculating ecological values of reefs—such as when deciding on the amount of offset required when development harms the reef—there is a lot value that is not accounted for if value judgements are made from coral cover alone.
Small size shouldn’t be a disincentive for protecting coral reefs
Experts agree that much more of the ocean should be protected from resource extraction and damage if we are to maintain marine ecosystems into the future. While this argues for the creation of large protected areas, it should not disincentivize reef managers from protecting small “hotspots”—reefs with disproportionately high value, even if they are small. One such example are the reefs of Oostpunt in Curaçao, which produce exceptionally large numbers of coral offspring relative to other reefs nearby.
Read more: Open Access Paper Published in Conservation Letters by Aaron Hartmann, Kristen Marhaver, and Mark Vermeij
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