Tag Archives: diving

Sewage Treatment, Coral Disease, and Koch’s Postulates

Coral reefs are in a tight spot these days. Increasing CO2 levels and rising ocean temperatures aren’t doing them much good, but their biggest problems are more direct. Overfishing is wiping out important predators, the aquarium trade picks off whatever looks pretty, agricultural and other runoff is clogging the filter-feeders, and some folks are even blowing them apart with dynamite.

Places with strict environmental regulations and protected marine preserves are generally doing a better job protecting their reefs, but even there we may be doing damage without realizing it. For example, what if coral reefs are catching human diseases?

That seems to be exactly what’s happening in Caribbean elkhorn coral (Acropora palmata), an iconic and structurally crucial reef species that’s been dying from a mysterious condition called white pox, or acropora serratiosis. The disease has been so deadly that the US EPA declared A. palmata an endangered species in 2006. Now, scientists have fingered human sewage as the source of the pathogen causing white pox.

A diver swims past an elkhorn coral colony. Image courtesy James Porter, University of Georgia.

A diver swims past a healthy elkhorn coral colony on Molasses Reef, near Key Largo, FL. Many other elkhorn colonies are dying from an infection that may be caused by a human pathogen. Image courtesy James W. Porter, University of Georgia.

Researchers at Rollins College and the University of Georgia described the finding yesterday in PLoS ONE:

Here we hypothesize that [Serratia marcescens] strain PDR60 isolated from two distinct environments, one terrestrial (human wastewater) and one marine (APS-affected A. palmata, apparently healthy Siderastrea siderea and Coralliophila abbreviata) causes APS [acroporid serratiosis] in A. palmata. To examine this hypothesis we conducted challenge experiments by inoculating eight isolates of Serratia marcescens representing three strains onto A. palmata fragments maintained in closed seawater aquaria. Our results confirm strain PDR60 as a coral pathogen through fulfillment of Koch’s postulates. These results are also consistent with the hypothesis that non-host corals and predatory snails may function as interepizootic reservoirs or vectors of the APS pathogen. Furthermore, we show that S. marcescens isolated from human wastewater causes APS in as little as four days, unequivocally verifying humans as a source of a marine invertebrate disease.

S. marcescens is a ubiquitous gram-negative bacterium. It’s in dirt, sewage, and probably your shower. If you haven’t noticed it, it’s because you have a working immune system. People who aren’t so fortunate – especially in hospitals – can get serious opportunistic S. marcescens infections. When this bug first turned up as a possible culprit in white pox, I figured it was probably an opportunist in the elkhorn corals as well. Perhaps the coral somehow got the anthozoan equivalent of a suppressed immune system, and Serratia took advantage of the situation.

The new work suggests otherwise, though I’m not sure it quite seals the case. The investigators experimentally infected elkhorn corals growing in tanks of purified saltwater, and found that a single inoculation with a pure S. marcescens strain cultured from sewage effluent was enough to give the corals a virulent case of white pox. This is called fulfilling Koch’s postulates, and it’s the Holy Grail of epidemiology. We can now say for sure that S. marcescens from human sewage causes white pox.

Or can we? I’m certainly convinced that the bacterial strain in sewage is capable of causing the distinctive pathogenesis of this disease, consisting of bleached white zones that spread across the coral colony. But we need to read the fine print.

The coral colonies these researchers used were harvested from “healthy” wild corals. That’s the only practical way to get experimentally useful amounts of this slow-growing creature. Unlike mice or guinea pigs, we can’t just breed up a bunch of stock from a well-characterized lab strain. However, picking apparently healthy corals from the wild doesn’t prove that they really are healthy. They’re presumably exposed to the same stresses and insults that afflict their white pox-infected neighbors, and could very well be on the brink of contracting the disease themselves. Maybe they’re already infected with the real underlying cause of the disease, and are just one wound or stressor away from getting the S. marcescens component that will finish them off. Because it’s currently impossible to characterize all aspects of the immunological and infectious status of a coral sample, we can’t know whether the bacterium alone is enough to cause disease.

Worse, we can state with certainty that the corals in these experiments were exposed to unusual stress. The scientists chipped off a piece of the colony (traumatic injury), sampled its mucoid coating (open wound), then carried it by boat to a laboratory tank (physiological stress).

That’s not to say I don’t believe the conclusions or the authors’ recommendations. Indeed, the measures they suggest include improved sewage treatment plants throughout the Caribbean, a step that’s clearly a good idea for a long list of reasons, whether or not it will save the elkhorn coral. Humans already get well-documented cases of sewage-borne diseases, and many Caribbean towns use inadequate treatment systems that raise the risk of these infections. That’s why Florida is already in the process of upgrading the treatment plants throughout the Keys. Of course, someone should also take a long, hard look at the waste treatment (or lack thereof) on cruise ships in international waters.

In the meantime, I hope researchers will continue studying white pox, with an eye toward preventing and perhaps treating it. As one of the authors points out in an accompanying press release, the stakes are high, even if the metaphors are a bit mixed:

“These bacteria do not come from the ocean, they come from us,” said Porter. Water-related activities in the Florida Keys generate more than $3 billion a year for Florida and the local economy. “We are killing the goose that lays the golden egg, and we’ve got the smoking gun to prove it,” [University of Georgia Ecology Professor James] Porter said.

1. Sutherland, K., Shaban, S., Joyner, J., Porter, J., & Lipp, E. (2011). Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023468

Lionfish Derbies vs. Groupers

I love both diving and fishing, so the continuing saga of Pacific lionfish invading the Caribbean has definitely caught my attention. The backstory is that Pterois volitans and its cousin Pterois miles probably escaped from home aquarists’ tanks in Florida sometime several years ago. It could have been from a hurricane flooding someone’s house and washing the fish out, or (more likely) some hobbyists discovered that these big, venomous fish were more than they could handle, and “returned” them to the sea. However they got out, these critters quickly adapted to their new environment and started following the standard invasive species script: without the predators and pathogens that keep them in check in their home seas, they’ve bred like crazy.

Lionfish in hand.

Lionfish, with spines removed. Image courtesy Serge Melki.

Fisheries biologists are concerned, but not quite panicking yet. Just being prickly and venomous isn’t anything special in the Caribbean, and top predators such as reef sharks can eat lionfish, at least occasionally. Humans have also been chowing down, which is a particularly good strategy; we’ve proven repeatedly that we can overfish just about any species to the brink of extinction, so why not use that power for good?

Unfortunately, as a recent paper in PLoS ONE shows, even human predation may not do the job. The finding is based on mathematical modeling, so it comes with the usual caveat that simulations are not reality, but it provides some testable predictions that field scientists can now check.

Model results suggested that a high level of sustained removal would be required to reduce lionfish population sizes below the SPR threshold of recruitment overfishing. Scaling the annual exploitation rate to a lionfish per hectare removal figure based upon published data on lionfish density [7], [15], suggests a yearly removal of 157–293 lionfish per hectare would be required to cause recruitment overfishing for a population based on M and CR values of 0.5 and 15. Thus, the control of lionfish populations through targeted removal efforts will be costly, and eradication through removal efforts is highly unlikely.

A hectare is 10,000 square meters, or about 2.4 acres. One large dive boat could probably drop enough divers into the water to spear 200 lionfish over the course of a two-dive trip, but they’d all have to be serious underwater hunters to pull it off. And that would only take care of one hectare’s worth of fishing for one year. Even if that’s multiplied by hundreds of lionfishing boat trips per season in a popular diving destination, the ocean is way too big for us to take care of the whole job ourselves. The fishermen can’t pick up the slack, either:

Furthermore, such a lionfish fishery would be limited to shallow water (<30 m) spearfishing and handnetting as lionfish have a low vulnerability to capture by hook and line [7]. This gear and depth limitation provides potential refugia from fishing, potentially making removal efforts less effective. Lionfish are being captured regularly as bycatch in reef fish trap fisheries [7], but feasibility of a lionfish specific trap capable of removing high densities of lionfish without high bycatch of native species is questionable.

There is one thing that could help: groupers. These diverse fish (several species in the subfamily Epinephelinae) are large predators that have traditionally been common throughout the Caribbean. They can grow to the size of small sharks, and they aren’t fussy eaters: if it swims and it’s smaller than the grouper, it’s potential grouper chow. Unsurprisingly, researchers have found lionfish in grouper stomachs. But how much lionfish does a grouper eat?

In another recent PLoS ONE paper, researchers took a crack at that question using a natural experiment: the Exuma Cays Land and Sea Park (ECLSP). Two decades ago, Bahamian officials declared this area of small islands and reefs a no-fishing zone. Since then, groupers, normally some of the most heavily fished species in the world, have become abundant inside the park. Comparing the populations of multiple fish species in the ECLSP and in nearby fishable waters, the scientists saw a striking trend:

The biomass of lionfish was significantly negatively correlated with the biomass of grouper, with predator biomass explaining 56% of the variance of prey biomass (linear regression p = 0.005, Fig. 2, Table 1). Unlike large-bodied groupers (mean total length 55 cm, range 30–110 cm), other smaller predatory fishes such as Cephalopholis spp., lutjanids, carangids and aulostomids had no significant bearing on lionfish biomass (p = 0.17, Table 1), which might imply that large-bodied fish are the primary predators of lionfish. The relationship of grouper on lionfish was strongly non-linear such that an 18-fold variation in predator biomass among sites (~170–3000 g 100 m−2) was related to a tenfold difference in lionfish density (~0.3–0.03 fish 100 m−2) and 7-fold difference in lionfish biomass (Fig. 2). A 50% reduction in lionfish biomass was achieved with a grouper biomass of 800 g 100 m−2. Reducing lionfish density to 30% its highest value required a further doubling of grouper biomass to approximately 1516 g 100 m−2 (Fig. 2). The mean body length of lionfish was 24.5 cm (SD 4.1, range 15–34 cm).

There are limitations to the study, of course. In particular, it doesn’t directly measure grouper predation on lionfish. All it really shows is that having lots of big groupers around correlates with having fewer lionfish, and that the relationship is nonlinear, i.e. you need a whole lot of groupers before you see a serious dent in the lionfish population. In any case, it strongly suggests that we should try to boost grouper populations elsewhere if we’re serious about getting rid of the lionfish.

That’s going to be tough, though. As I mentioned, grouper is heavily fished, for the good and simple reason that it’s delicious. Indeed, the data clearly show – and my own experience confirms – that big groupers are now uncommon outside protected marine reserves. The appropriate policy might be to protect more reefs from fishing, but the authors conclude with a blunt assessment of that strategy:

However, if the historical trend of poor management continues [25] then direct capture and eradication may be the only practicable form of lionfish control for much of the Caribbean.

And that brings us back to spearing them.

1. Mumby, P., Harborne, A., & Brumbaugh, D. (2011). Grouper as a Natural Biocontrol of Invasive Lionfish PLoS ONE, 6 (6) DOI: 10.1371/journal.pone.0021510

2. Barbour, A., Allen, M., Frazer, T., & Sherman, K. (2011). Evaluating the Potential Efficacy of Invasive Lionfish (Pterois volitans) Removals PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0019666

Attention Horror Movie Writers

Here’s your next script idea:

The remains of a prehistoric child were removed from an underwater cave in Mexico four years after divers stumbled upon the well-preserved corpse … The skeletal remains of the boy, dubbed the Young Hol Chan, are more than 10,000 years old and are among the oldest human bones found in the Americas.

In other news, members of a team of cave divers have begun to die, one by one, under mysterious circumstances.

Unanticipated Consequences of Beach Replenishment

I was amused to see the story in the New York Times about the surprising souvenirs now available on some New Jersey beaches:

The explosives problem arose on March 5 when a resident using a metal detector came upon a rusted military fuze, an ignition device incorporating mechanical or electric elements, buried in the sand. Believed to have been dumped off the sides of ships sometime during World War I, the discarded military munitions lay on the ocean floor for 90 years or more, according to Mr. Follett. Last fall, the Army Corps dredged up 500,000 cubic yards of sand from the bottom of the Atlantic as part of a $9 million beach replenishment program for Surf City and part of Ship Bottom.

US Navy antisubmarine weapons were dropped in huge numbers of American shores.

Why do I find this amusing? Well, a few years ago some of my scuba diving friends and I had a somewhat closer call with a piece of ammunition in New Jersey waters (I wrote about it on the old version of this blog). This stuff is definitely down there, and it doesn’t get safer over time. In fact, according to a Navy source one of my diving buddies talked to, it actually gets more dangerous, as the explosives become less stable. Yet another reason to question the wisdom of beach-replenishment projects.