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.
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 , , 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 . 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 , 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  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.
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
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