F-S-What?: Fish Spawning Aggregations
& How Do We Study Them?
& How Do We Study Them?
Written by Dr. Krista Sherman, Senior Scientist, Perry Institute of Marine Science
Birds do it, bees do it, and fish do it too! Sex. Breeding. Reproduction. Fish spawning aggregations (FSAs) are a type of reproductive strategy used by various fish species to breed or produce offspring. Fish that undertake long migrations from their home ranges to FSAs to produce annually at specific times of the year are known as transient spawners. The Nassau grouper (Epinephelus striatus) is an example of a transient spawner. In The Bahamas, these long distance spawning migrations occur between November-March around the week of the full moon. During this period, Nassau grouper migrate in groups from their home reefs to selected sites where they aggregate in large schools of hundreds to thousands of fish.
Photo2: Nassau grouper FSA. Photo © Perry Institute for Marine Science
Courtship or mating behaviour increases in the afternoons, where fish transition between three colour phases – bicolour, white belly and dark. Around sunset, smaller groups break away from the main aggregation and move higher up in the water column to spawn or release their gametes (eggs and sperm). This is incredible to see! Once gametes have been released, fish re-join the aggregation and typically leave the site within 1-3 days to begin their homeward migrations.
We have used multiple survey methods to study FSAs over the past nine years. Manta tows allow us to cover expansive areas to locate spawning aggregations without the use of SCUBA diving equipment (Photo 1). Acoustic telemetry, which is used to track fish migrations, has also been used to identify FSAs, determine migratory routes, peak spawning months, and study movement patterns within a spawning site (Photo 2).
Dive teams conduct underwater visual surveys to record data on Nassau grouper abundance, size, colour phase, spawning behaviour and activity once FSA locations have been verified (Photo 3). Photos and video surveys are also used to validate diver estimates and document spawning events.
Photo 1. Lindy Knowles completing a manta tow survey to locate aggregating Nassau grouper in San Salvador. Photo © Perry Institute for Marine Science
Photo 2. Dr. Sherman surgically implanting an acoustic transmitter into Nassau grouper. Photo © Giselle Dean
Photo 4. Lindy Knowles setting-up equipment for ship-based bathymetry surveys. Photo © Perry Institute for Marine Science
GPS points and depth profile data are also collected to produce bathymetry maps of FSAs (Photo 4). “Mapping the spawning grounds is vital to learning more about the spawning habitats and behaviour of the species”, stated Lindy Knowles (Senior Science Officer, Bahamas National Trust). “We’re familiar with when mating seasons are, but we still need to learn more about where and why. As more FSAs are mapped, we can then use this information to predict possible locations that may not be known as yet and help fill in gaps”.
Almost all Nassau grouper reproduction or breeding occurs at FSAs. Therefore, healthy FSAs are critical to the survival of Nassau grouper and are essential to replenish the fishery. The Bahamas has more reported Nassau grouper FSAs than any other country, but we have limited to no data for many of these sites. In partnership with scientists, fishers, government, non-governmental organizations and other stakeholders, the Perry Institute for Marine Science (PIMS) is working to verify and evaluate the status of all reported FSAs in The Bahamas (Photo 5).
We will continue using established survey techniques to monitor fish abundance and map FSAs, but are excited to test other survey methods such as passive acoustic monitoring, environmental DNA (eDNA), and baited remote underwater video surveys (BRUVs; Photo 6) that do not require divers. Collectively, these data provide valuable information about Nassau grouper that are needed to sustainably manage this critically endangered species.