Marine management

The Indo-Pacific lionfish Pterois volitans is an invasive species that was released along the U.S. Atlantic coast and spread to the Caribbean. Lionfish can inhabit a wide range of depths and they lack predation in their invasive range, leading to a successful invasion into their introduced range. Their large appetite combined with having no predators may pose an ecological threat to the Caribbean coral-reef ecosystem. Pterois volitans invaded Bonaire, Dutch Caribbean in 2009, and the extent of their presence and the impact they have upon the reefs surrounding Bonaire has yet to be investigated. Basic components to managing an invasive species are monitoring their occurrence and controlling their abundance. Around Bonaire, managers and diver volunteers catch and kill P. volitans to control their numbers in an attempt to reduce their negative impacts on the ecosystem. This study investigated the effectiveness of the current management strategy, P. volitans removal, by comparing their densities at frequented hunting sites with no-hunting sites. To achieve this, the densities of P. volitans at eight hunted and less frequented sites were calculated by searching for their presence in 50 x 4 m belt transects at five different depths (6 m, 12 m, 18 m, 24 m, 30 m) at selected sites along the west coast of the island. All P. volitans recorded were categorized into three size classes (small: 0-10 cm, medium: 11-30 cm, large: ≥ 30 cm). The results of this study show no significant differences between the densities of lionfish found at hunted sites (10/ha) compared to less frequented sites (16/ha). This study is the only known effort that attempts to determine the effectiveness of P. volitans removal from invaded areas in Bonaire.

This student research was retrieved from Physis: Journal of Marine Science IX (Spring 2011)19: 64-69 from CIEE Bonaire.

Abstract:

Species invasions have a range of negative effects on recipient ecosystems, and many occur at a scale and magnitude that preclude complete eradication. When complete extirpation is unlikely with available management resources, an effective strategy may be to suppress invasive populations below levels predicted to cause undesirable ecological change. We illustrate this approach by developing and testing targets for the control of invasive Indo-Pacific lionfish (Pterois volitans and P. miles) on Western Atlantic coral reefs. We first developed a size-structured simulation model of predation by lionfish on native fish communities, which we used to predict threshold densities of lionfish beyond which native fish biomass should decline. We then tested our predictions by experimentally manipulating lionfish densities above or below reef-specific thresholds, and monitoring the consequences for native fish populations on 24 Bahamian patch reefs over 18 months. We found that reducing lionfish below predicted threshold densities effectively protected native fish community biomass from predation-induced declines. Reductions in density of 75- 95%, depending on the reef, were required to suppress lionfish below levels predicted to over-consume prey. On reefs where lionfish were kept below threshold densities, native prey fish biomass increased by 50-70%. Gains in small (<6cm) size classes of native fishes translated into lagged increases in larger size classes over time. The biomass of larger individuals (>15cm total length), including ecologically important grazers and economically important fisheries species, had increased by 10-65% by the end of the experiment. 

Crucially, similar gains in prey fish biomass were realized on reefs subjected to partial and full removal of lionfish, but partial removals took 30% less time to implement. By contrast, the biomass of small native fishes declined by more than 50% on all reefs with lionfish densities exceeding reef-specific thresholds. Large inter-reef variation in the biomass of prey fishes at the outset of the study, which influences the threshold density of lionfish, means that we could not identify a single rule-of-thumb for guiding control efforts. However, our model provides a method for setting reef-specific targets for population control using local monitoring data. Our work is the first to demonstrate that for ongoing invasions, suppressing invaders below densities that cause environmental harm can have a similar effect, in terms of protecting the native ecosystem on a local scale, to achieving complete eradication.