9/28/2023 0 Comments Size of giant manta rayCould this be in-line with their global population decline expressed in local or regional terms? This year 15 individuals have been identified so far, around 40% less than back in 20. The historic average seasonal abundance is 26 individuals. Then in June of this year it’s almost as if someone flicked a switch and they are back. Last year two giant mantas were photo-identified at La Reina but this wasn’t enough to declare their return. Over 20, no mantas had been sighted at La Reina and despite the site being an excellent dive destination, even in the absence of mantas, operators stopped running trips there due to the distance and they favoured the Espiritu Santo Archipelago instead. In the year 2003, the mantas did not return. 52 individual mantas were identified over the years 20 with an exact 50/50 split of males and females in the final analysis. In 2001, Pablo extended the study to include photo-identification of individuals so that the population size could be estimated. What is very interesting is that the mantas typical behaviour is to circle the islet very slowly throughout the daytime as if they are there to rest, perhaps using La Reina as an easy location to navigate around. Often mantas would be observed being cleaned by various cleaner fish species but this didn’t seem to be the primary reason for their occurrence.” and the behavioural study showed no evidence that they were there to feed. “All of the individuals were and still are juveniles instantly ruling out mating”, Pablo explains, “…. ![]() Felipe Galván of the research centre CICIMAR here in La Paz, as well as with several service providers, Pablo began to examine the behaviour of the animals in an attempt to determine why La Reina was so important to the species. In 1999, marine biologist Pablo Ahuja founded Manta Mexico, a community project whose purpose was to learn more about the mantas. This method of photo-identification is used to answer questions about population size and dynamics, seasonal abundance, time of site residency, site fidelity and even foraging or migratory movements of individuals. Not too unlike our own fingerprints, oceanic giant mantas have a pattern on their undersides that is unique to each individual, allowing scientists and citizen scientists to identify them through the examination of suitable photographs. The oceanic giant manta is listed on The International Union for Conservation of Nature’s (IUCN) Redlist of Threatened Species as “vulnerable” to extinction due to a drastic decrease in its global population over the last two decades resulting largely from overfishing. The remaining 70% of their diet is captured in the deep ocean and includes small fish. These animals are partial filter-feeders, straining plankton-rich surface water for around 30% of their food. They have a circumglobal distribution, typically associated with tropical and subtropical waters but are also periodically encountered in temperate waters. The oceanic giant manta ray is the largest species of ray on the planet, reaching sizes of up to 7 metres / 23 feet across. The oceanic giant manta - Mobula birostris ![]() It could just be that for those 15 years of absence the grass was greener on the other side of the fence, so to speak. Animals instinctively seek the best conditions in which to survive and thrive. Some would blame their sudden disappearance on the fisheries whilst others put it down to displacement caused by either predation pressure or the naturally changing dynamics of the surrounding sea. I’d previously learnt that these oceanic giants had once graced divers and snorkelers visiting the area but no one could reliably explain why they had all but vanished. But 15 years later, here they are once again and the big question on our minds of course is “why?” Although there had been several infrequent sightings of these mantas over the years, there had not been a report of an actual aggregation, which, in biological terms, is defined as when two or more individuals of the same species come together for a common cause such as feeding or breeding. Scrolling through my Facebook newsfeed, I stumble across the post of an acquaintance who stated that several oceanic giant manta rays (Mobula birostris) had been encountered at a rocky islet called “La Reina” which lies just off the northern-tip of Isla Cerralvo, which, itself, lies off the coast of La Paz. I remember the excitement of the moment very well.
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