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Collapse of seagrass beds threatens survival of marine life

RG - Jennifer Gray

JGray BTP.jpg

 

The Bermuda Turtle Project is anxious to get back out onto the water this month to get a clearer picture of sea turtle abundance. Due to the restrictions brought upon us all by the pandemic, we have been unable to do any in-water research for nearly two years and it would appear, from observations, there have been some drastic changes in our marine environment.

Once thriving seagrass beds have been in decline for some two decades, but over the last 48 months many acres of this critical habitat have completely collapsed with not a blade of grass left. Along with the disappearance of seagrass comes numerous consequences comprising disturbance and shifting of the marine sediments that were once held stable by the root systems of the seagrass and a vulnerability to the arrival of potentially invasive species, especially algae. Even more troubling is the seeming departure of once-visible species like fish and sea turtles.

BTP has been documenting a change in the size class of green turtles on the Bermuda platform that shows green turtles departing our shores at a smaller size than they once did. This could well be driven by diminishing food sources in Bermuda, leading to a search for ‘greener pastures’ across the ocean. The Bermuda Aquarium Museum and Zoo Wildlife Rehabilitation Centre has also observed a scarcity of large green turtles and an increase in the number of small green turtles stranding in recent years.

The ‘Bermudian’ population of nesting green turtles was driven to extinction by man more than 100 years ago. The green turtles we have observed over the last century are not of Bermudian origin – rather, they represent many nesting colonies far to our south. After spending the early phase of their lives adrift in the open ocean, the adolescents arrive at our shores as a mixed genetic stock of juveniles. After spending a period of years here, they leave Bermuda to complete their maturation somewhere else – and eventually return to their natal beach to reproduce and lay eggs as adults.

It may well be that our young turtles are making shorter visits to Bermuda compared to findings from the BTP’s early research, which documents stopovers for nearly 20 years. It makes sense that if the energy it takes to forage outweighs the benefits of intake it would be better to find a more bountiful habitat to sustain a large or growing body.

Sea turtles are highly migratory, crossing vast expanses of ocean and many geographic boundaries throughout their long lives, and, while making a migration as a juvenile has a higher risk than as an adult, they have evolved to know when and how to make these long journeys to find food or mates.

This is not to say that all the green turtles have left Bermuda. We need to get out there and conduct our research to obtain a clearer picture of what is happening. We know there are still turtles here, and that they appear to be utilising new habitats around the island. There have been observations of small green turtles feeding on algae and even mangrove leaves, perhaps making a diet change rather than risking a migration.

Further studies are essential and will help fill in the blanks on what, exactly, is happening with sea turtles in Bermuda.

I have spent considerable time on the ocean and remember, in my childhood years, it was a rare and exciting event to see a sea turtle.

These were the years when sea turtles were hunted for food in Bermuda and across the globe. Bermuda banned turtle hunting in 1972 and during the same period conservation efforts on nesting beaches in South and North America and the Caribbean were ramping up to save the species from extinction.

During the 80s and 90s we all observed a burst in sea turtle sightings in island waters, a direct result of successful conservation and reproductive output on nesting beaches to our south, particularly Florida, Costa Rica and Mexico.

Studies have shown that grazing by sea turtles actually stimulates seagrass growth, nutrient cycling, and overall biomass turnover. This makes for healthy and productive seagrass meadows. With sea turtle populations recovering, but still nowhere near historic numbers, we must question why such far-reaching changes have occurred in Bermuda and particularly why our seagrass meadows have declined.

Since sea turtles inhabited Bermuda’s productive waters, feeding on and coexisting with seagrass, for thousands of years prior to the arrival of humans on the island, it is most likely that the answers to what is causing our seagrasses to disappear will be found when looking at what is newest to the equation. In other areas around the world, climate change and nutrient run-off have been found to be the major factors involved in seagrass loss.

Bermuda is the most northern limit for both seagrass and sea turtle survival which means that even a subtle environmental change can lead to extraordinary effects on any species on the edge of where it is viable.

The recent media coverage of two tiger sharks caught and landed dead, should not be celebrated but rather serve as a bold reminder of how important these keystone predators are in maintaining a balanced ecosystem and why they should be protected.

Their feeding can affect prey population numbers, but also prey distribution as they select a habitat to avoid being eaten. For example, the presence of tiger sharks has been shown to prevent green turtles from overgrazing seagrass beds, making healthy populations of sharks and turtles critical to the structure and function of seagrass ecosystems, including their role as carbon sinks.

Could a lack of sharks be responsible for overgrazing by sea turtles in Bermuda? It is possible but more likely just a component of many factors culminating in an ecosystem imbalance. The time is long overdue, and the case undeniably strong, for the protection of shark species in Bermuda waters.

Ecosystems are complex and there is a need to expand research in Bermuda to inform conservation and management of our marine environment. The loss of Bermuda’s seagrass meadows is well documented, but the reasons why are not well understood. Efforts by the Department of Environment and Natural Resources to protect seagrass habitats are to be applauded and along with their current studies we have learnt a great deal. Their work should be supported and expanded.

Research is needed on seagrass pathogens and disease, some of which have wiped out similar habitats on the US coast and could have easily arrived in Bermuda. We need to understand the presence, or lack thereof, of organisms in the habitat sediments that are key to the transfer of nutrients and influence successful seagrass photosynthesis and reproduction.

Understanding the effects of sedimentation from cruise ships, fast ferries and the like and the effects of dredging, coastal development and shading by docks is essential. We need to prudently study the effects of our own input into the ocean – sewage, road run-off pollutants, herbicides and fertilisers, outputs from energy plants and incinerators, cleaners, paint products, fuels, and oils, all of which find their way from our boats and our landmass into our surrounding ocean. And of course, rising water temperatures caused by climate change is, of course, another major factor in seagrass loss around the world.

While science and knowledge can take decades to provide us with answers, there are immediate changes we should make, based on sound science and proven conservation management. Some of these should have been implemented years ago when Bermuda first noticed a decline in seagrass habitats.

The most obvious is protecting sharks and a return to a more natural balance in the food web.

Introducing marine protected areas, where harmful fishing practices, anchoring, engine outputs and propeller scarring would be eliminated, would afford a better chance for seagrass recovery and the return of juvenile fish which, when they multiply, would spill outside the protected area providing an increase in abundance in areas where fishing does take place. The scarring of the sea floor caused by swing moorings is especially devastating in a seagrass habitat. Studies have shown it could take turtle grass forty years or more to fill in a simple scar made by a propeller. Imagine the years it would take to recover from these mooring scars.

Bermuda should make environmentally friendly moorings mandatory. We also must ensure that septic tanks and other sources of nutrient runoff are being property maintained.

There are bigger issues that we cannot easily control in the form of sea level rise and increased storm activity which will have devastating consequences on all ecosystems if humanity can’t come to grips with our wasteful and damaging ways. But this is no excuse for ignoring the smaller things we can do that will make an immediate difference, provided we implement and enforce before it’s too late.

Our focus should be on anything we can do to protect and restore critical habitats. While the vast beds of seagrass have collapsed there are meadows, albeit smaller ones, still surviving and even signs of new growth in some areas. Sadly, we have waited and pondered the obvious for too long and there is no one fix but rather several efforts that should be implemented if we wish to establish the return of balance in the ecosystem.

Perhaps most important is a human mindset change that comprehends our place in ecosystems, appreciates how vital they are to our own existence and sets out a strategy that sees humans living in harmony with nature.

So “where have all the turtles gone, long time passing”? BTP will be out and about this summer to try to find some answers.

Jennifer Gray is the director of the Bermuda Turtle Project, a collaborative effort by the Bermuda Zoological Society and Sea Turtle Conservancy, whose mission is to promote the conservation of sea turtles through research and education. The hotline number for sea turtles in distress is 2932727 #99