Aliens in the twilight zone: using robots to study and manage invasive lionfish on Caribbean mesophotic coral ecosystems

Research context
Invasive lionfish (Pterois spp.) have spread widely across the western Atlantic and are a substantial threat to native marine biodiversity. Lionfish predate unchecked on native fish populations which, unlike in the lionfish's native Indo-Pacific range, have no evolved defence. Current management relies largely on opportunistic culling by recreational SCUBA divers yet this method only provides local short-term respite. This is because recreational divers tend to stay above 30 m, yet lionfish are at high abundance below this depth and are consequently missed. Also, recent evidence shows these deeper lionfish - in mesophotic coral ecosystems (MCEs: 30-150 m) - are larger and more fecund than those found in the shallows.

MCEs are poorly studied because they are too deep for conventional SCUBA diving but not deep enough to justify expensive research submersibles. Operation Wallacea has a new collaboration with US-based charity Robots in Service of the Environment (RSE), which has developed a small, cost-effective robot capable of accessing MCEs and capturing lionfish. The robot has successfully operated at 85m depth during 2019 pilot tests and is designed to reach beyond 200m.

Research questions
These deep populations of lionfish may contribute substantially to recruitment in shallow reefs, undermining management efforts by allowing rapid recolonisation after culls. How far this potential is realised, and thus how much it compromises management efforts, requires greater ecological understanding of MCE lionfish.
We aim to exploit these robots exciting new capability to (1) determine the role of MCE lionfish in the species' population dynamics, (2) quantify the impacts of invasive lionfish on the wider MCE fish community, and (3) use spatially-structured demographic models to develop best management practice for culling MCE lionfish populations.

Methods
Fieldwork will primarily be at Operation Wallacea's long-term marine research sites in Honduras (active since 2005; 23 peer-reviewed scientific papers since 2016). Using externally funded, bespoke prototype robots mounted with cameras, standardised data will be collected on lionfish and the ecosystems they inhabit, particularly focusing on MCEs. Specifically, video surveys will be used to survey the distribution of lionfish at their full depth range and determine prey availability. The robots will also catch lionfish and return them to the surface to enable gut content analysis and demographic changes across depth.
Using these data, a spatially-structured demographic model will be developed to understand lionfish population dynamics. With this model, we will explore different culling scenarios and develop hypotheses for optimal strategies. These strategies will be tested via field experiments involving robot-enabled deep culling. Based on the results, the population model will be refined for application to other locations, and to inform lionfish management globally.

Timeline
Year 1: intensive training/orientation. Literature review. Video observations (Honduras). Analysis of abundance distribution, population structure, predatory behaviour and wider MCE fish community.
Year 2: Gut content, fecundity and body condition analyses. Develop spatially-structured population model to inform culling experiment.
Year 3: Field experiments to determine optimal culling strategy and validate/refine model. Develop and implement best-practice guidelines for invasive lionfish management.