August 17, 2022

A helping fin: dolphin poop is helping reefs in the Maldives grow strong and resilient

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Just like manure fertilizes croplands, dolphin poo can help feed marine ecosystems with the “reef-enhancing nutrients” they contain.

A Helping Fin: Dolphin Poop Is Helping Reefs In The Maldives Grow Strong And Resilient
Reef near Baulhagallaa Island, Gaafu Dhaalu Atoll, Maldives. Image credits Jim Wang via Flickr.

Spinner dolphins (Stenella longirostris) are famous for their playful nature, high intelligence, and spectacular acrobatics at marina displays. But, according to new research, they can also boast having some very special excrement.

The study reports that the dolphins are giving threatened coral reefs in the Maldives and Chagos Archipelago some much-needed help: by defecating in the shallow lagoons during their daily commute, the dolphins are injecting nitrogen, an essential nutrient, into the area. In turn, this helps improve the productivity and resilience of the reefs, helping them cope with the increasingly-unwelcoming conditions of the planet’s warming oceans.

All in all, the animals are playing “a significant role” in supporting the vulnerable ecosystems of this area.

One dolphin’s trash…

“Coral reefs are facing profound threats around the world, including climate change and biodiversity loss, but this research has identified a clear ally for them: spinner dolphins,” says lead author and marine research fellow at ZSL’s Institute of Zoology, Dr Tom B Letessier.

“Simply by going to the toilet in the shallow atoll lagoons, these dolphins are providing a vital nutrient supply for the corals – making the strongest possible case for protecting the dolphins in order to save these reefs.”

Spinner dolphins grow to about 2 meters (6.5 feet) in length, with a thin snout. They’re dark gray with white bellies and live in warm ocean waters around the world: near Thailand, along the Pacific coast of Central America, and around the Hawaiian islands.

The species is not classified as endangered on the IUCN red list, but it does face pressures from marine debris — including becoming entangled in fishing nets — from ships through noise disturbance or direct impacts, and from habitat degradation caused by human activity.

For the study, the team used a combination of visual observations of the dolphins’ activity together with underwater recordings. This data was sourced, partially, from amateur whale-watchers. All in all, the team estimates that a population of around 105 dolphins reside in this area.

The team reports that the dolphins enter atoll lagoons formed inside the ring-shaped coral reefs of the area in the morning. Here, they spend around half the day resting, before moving out in the afternoon to hunt in the deeper ocean during the night.

They estimate that the dolphins excrete around 288 kgs of nitrogen into the lagoons every year, providing a steady supply of an essential nutrient into the ecosystem. The increase in productivity this nitrogen makes possible gives the corals that make the reef much better access to nutrients, allowing them to grow faster and increasing their resilience to outside pressures.

“It’s exciting to have found a likely important mechanism by which the dolphin’s behaviour could be sustaining the health of surrounding reefs,” says Dr Letessier. “This study is just the first piece in a bigger puzzle which we will explore further as part of our new regional scientific cetacean project.”

Coral reefs are essential marine ecosystems. They feed and house an incredible amount of life, and directly sustain the livelihoods of 6 million people worldwide. They also form a physical barrier against storms, protecting the shore against erosion, while providing important economic resources in the forms of jobs and tourism destinations for local communities.

This is particularly true in the Maldives, where locals heavily rely on coral reefs to survive. The current climate crisis and the risk it poses to corals by causing massive bleaching events thus leaves them particularly vulnerable. The findings showcase how the daily lives of dolphins can, however, provide hope to these reefs and the communities that rely on them.

The paper “Spinner dolphin residency in tropical atoll lagoons: Diurnal presence, seasonal variability and implications for nutrient dynamics” has been published in the Journal of Zoology.