Newly published research on the movement and diet of Ascension’s grouper and spotted moray
1 May 2024
Conservation
New publication highlights different management approaches recommended for these two diverse species
Written By Dr Danielle Orrell
Grouper and spotted moray are two common fish in Ascension's nearshore waters that are found in the same habitats and often in very high numbers. Given their abundance, how do these two animals co-exist? Furthermore, can we use the same management measures to protect their populations, e.g. in the wake of increasing fishing pressure or climate change? From animal movement data collected by acoustic telemetry and diet information from stomach content analysis and chemical analysis of tissues, we found these two species have similar diets. However, subtle differences in their movement behaviour suggest that there is no "one rule fits all" in their effective management.
Researchers from the University of Windsor, Canada, partnered with AIG Conservation to track the fine-scale movements of 46 grouper and 15 spotted morays in English Bay from 2017 to 2021. This project used acoustic telemetry, which involves inserting an acoustic pinger about the size of an AA battery into an animal that emits a unique coded signal. When a nearby moored acoustic receiver picks up this signal, it is decoded and stored alongside the time of detection.
Acoustic tracking data found that grouper had incredibly small home ranges of 0.0001 to 0.3114 km2. A home range describes a core area where an animal spends most of its time, e.g., sleeping, eating, and transiting. An area of 0.0001 km2 is just bigger than Long Beach football pitch, which means some grouper don't go very far (at least in the several years we studied them!). For example, a grouper tagged in 2017 was detected 3,549 times from 2018-2020, with most of these detections only 50 metres from its original tagging location! We found that the depth occupied by each grouper was different, and they were a tiny bit deeper at night than during the day. High site fidelity and residency (staying in the same area) suggest that grouper have the potential for local depletion as they don't move very far. This finding suggests that spatial management, e.g., via closed areas, could effectively support the recovery of fish populations if they decline. Future work will need to assess whether larval grouper are transported around the island on Ascension's currents and if they have the potential for replenishment. Understanding whether long-ranging movements occur around the island, e.g., by tagging grouper towards South East Bay, Mars Bay and Pan Am and tracking their movement using an island-wide acoustic array could be another area of research.
Spotted moray were much more challenging to track, as they spent a lot of time amongst the rocks, which prevented the acoustic signal being recorded on the nearby listening receivers. Tracking data found that morays occupied areas for several months before moving to a new space. This finding is similar to observations collected from snorkel surveys and other tracking studies in the Caribbean, which found spotted moray move between burrows to find new resources (food, shelter) or to reduce competition with other fish around them. The depth of spotted morays differed between individuals, with the lunar cycle and the day of the year. For example, spotted morays were in deeper waters in March and June, shallow in April and from September to October. Based on their movement behaviour, fisheries-based management would be more effective if there was a significant decline, e.g. bag limits.
Using stomach content analysis (identifying what was in the stomachs of grouper and spotted moray) and chemical analysis (using different tissues, you can study diet from a few weeks to a few months), it was found that the diet of these two animals was very similar. Both species ate mainly fish and crustaceans (crabs and shrimp). Differences in chemical analysis between years suggest grouper and spotted moray eat different prey over time. Seasonal prey pulses could also explain the depth changes in these two species. For example, grouper may feed on baby turtle hatchlings near the water's surface from January to June. Feeding on more abundant spawning prey like long jaw squirrelfish and mole crab means they would spend less energy trying to find food.
This work was made possible thanks to the support and expertise of the Ascension Island community and the fishing community. Dr Danielle Orrell would like to share a heartfelt thank you to people including, but not limited to, Jaco Ferriera, Shaun Scipio, Alex Knipe, Clay Brogdon, Dwayne Joshua, Adrienne Levknecht, Rick Lutman, James Richards, Mike Greentree, Richard Cook, Eugene Bennett, Siobhan Stewart, Ken Williams, Ollie Jones, Gary Picky, Alex Brittan, Hugh Carville, and Geston Wayne Burckhalter. Additionally, Ascension Island Government staff, including but not limited to Victoria Knight, Andrew Richardson, Alys Perry, Marcia Blyth, Diane Baum, and Jolene Sim. Thank you also to the Schuyler Line Navigation Company for supporting the equipment deployment and data collection that made this work possible.
This work has now been published in the Journal of Fish Biology and is available open access (free to read) using this link: http://doi.org/10.1111/jfb.15744.
