The Marine Megafauna Foundation Finds a City-Dwelling Population of Young Mantas
The first study of manta rays off the coast of Florida, recently published in the online journal Endangered Species Research, has discovered a potential urban nursery ground for manta rays.
Conducted over a four year period between 2016 and 2019, the study found that almost all of the mantas sighted in the area were juvenile oceanic manta rays (Mobula birostris), some of which remained in the area for long periods of time, with the habitat repeatedly used over successive years – all of which are 'key definitions of a nursery ground,' according to the study.
As well as the discovery of a potential manta nursery – which would, if confirmed, be only the third such habitat in the world known to science – it is the first time a nursery habitat has been identified in such close proximity to a highly developed urban environment.
'Manta rays are rarely seen by divers on Florida’s reefs and most divers are surprised to learn that we have a potential manta nursery ground along our coast,' said lead author Jessica Pate of the Marine Megafauna Foundation (MMF). 'In many places, manta rays are seen at remote cleaning stations along coral reefs systems or feeding offshore. Here in Florida, we regularly see manta rays swimming in 5ft (1.5m) of water over the sand in front of multi-million dollar mansions or high-rise hotels and apartment blocks. They rest in man-made inlets and feed at the mouths of inlets which have heavy boat traffic.'
During the course of the study, MMF researchers conducted 175 boat-based surveys, reporting a total of 150 manta ray encounters from which 59 individual manta rays were identified (which were added to the online global manta ray database at www.mantamatcher.org). All of the males and 98 per cent of the females were identified as sexually immature based on the development of claspers (the reproductive organs of male elasmobranchs), and the absence of mating scars or visible pregnancies in the females. Twenty-five individuals were observed more than once during the study, and eight were sighted over multiple years.
'Very few nursery habitats for manta rays have been identified and this is the first to be found in such an urban environment,' said Dr Andrea Marshall, co-founder and principal scientist of the Marine Megafauna Foundation. 'We were immediately concerned for the safety of these small juvenile individuals who appear to use the highly populated coastline of southeastern Florida during the first years of their lives. They face so many threats in this area, intense boat traffic, recreational fishing, raw sewage and toxic algal blooms. The list goes on and on.'
The study also found that the mantas experience 'frequent anthropogenic impacts', particularly fishing line entanglements and vessel strikes. Of the 59 mantas observed in the studies, sixteen had been caught by hooks or entangled in fishing line – and six had suffered on multiple occasions. Some of the mantas had been injured by impacts with boats, with 30 per cent of the injuries caused by boat propellors – including one individual with a wound which had been cut completely through its wing. Injuries were observed to heal rapidly, however, with deep propeller injuries closing completely within just a few weeks, indicating that the overall number of injuries in the population would be greater.
Although the Florida manta population consists of animals identified as oceanic mantas, it may well be comprised of a third species which has yet to be formally described, a discovery made by MMF co-founder and principal scientist Dr Andrea Marshall while researching manta rays off the coast of Mexico in 2010.
'Manta rays are highly vulnerable and we know very little about this presumed new species,' said Dr Marshall, 'It is tragic to think that the Floridian population, one of their largest we currently know about, is subject to so many anthropogenic pressures. This project was designed to rapidly assess more about Florida’s mantas so that better management plans can be developed.'
As the survey area for the study only covered a small portion of the Florida coastline, the full extent of the nursery habitat is still unknown. The Marine Megafauna is planning future studies to better understand the potential nursery habitat, which will include the use of satellite and acoustic telemetry, aerial surveys and plankton tows (a method of collecting plankton for identification). MMF’s researchers are also currently developing educational materials aimed at helping fishers and boaters better understand the presence of manta rays in Florida's waters, in order to reduce the harmful impacts of humans on 'urban' manta rays.
This article was originally published by Jessica Pate, Lead Scientist of the Florida Manta Project and Marine Megafauna Foundation on the Marine Megafauna website at www.marinemegafaunafoundation.org/blog. Follow the team on Twitter, Facebook, Instagram and LinkedIn.