Wednesday, May 19, 2010

Sea Turtles at Work for the Oceans

During the summer months sea turtles swim ashore to nest on beaches throughout the Caribbean, but sea turtles spend the rest of their life in the ocean and can be seen year-round in our waters. Most sea farers know that greens are often found feeding on sea grass patches, while hawksbills can be seen tucked into their favourite corners of the reef. Just as many sea turtles are loyal to their nesting beach, returning to the same beach each nesting season, many turtles are also loyal to their feeding grounds. Our “resident feeding” turtles actually play an important part in maintaining the balance of our coastal waters.
Green Gardeners
Green sea turtles are known to be particularly loyal to their feeding grounds, often staying close to their chosen plot of sea grass. Beaks with finely serrated edges allow the green turtle to tear off grasses or algae from hard surfaces. Turtle grass or Thalassia, is the grass of choice of most greens, they can often be seen crawling on the sea bottom, “grazing” (almost like cows). But some might also call this “pruning”, for the sea turtles actually increase the productivity and nutrient content of the grass plot by removing the older upper portions of the grass and allowing the nutrient rich shoots to flourish. Older grasses are often overgrown with algae and fungus, and their accumulation on the sea bottom drains important nutrients from the food chain, resulting in less productive fisheries. Healthy grasses on the other hand, benefit the food chain and fish stocks. In Florida Bay, die off of sea grasses was directly linked to the local extinction of greens in the 1980s. The above photo is of a green sea turtle on a bed of turtle grass (courtesy WIDECAST).

A Diet of Glass?
Hawksbill turtles are known to take up residence in coral reefs that provide a rugged landscape for hiding out and a ready buffet of their favourite foods, especially sponges. Researcher Anne Meylan examined the stomach contents of 61 hawksbills caught by fishermen in the Caribbean and found that sponges accounted for almost 95% of the total digested food. The turtles also showed a strong preference for the most well defended sponges (d
emosponges), which are inedible and toxic to other species. The hardy skeletons of demosponges are made of spicules – tiny silica rich spines similar to glass needles! The above photo (courtesy CENSEAM) shows the glass like spicules. The strong beak-like mouth of the hawksbill allows them to rip through these sponges, and other animals then feed on the softer material that is left exposed once the turtle has moved on. Without hawksbills to keep them in check, sponges could easily take over from corals in the reef community. This would dramatically change the physical structure of the reef and diversity of species that it supports. Who would have thought that the small, shy hawksbills were such fearless guardians of the reef?

Jelly Hunters
Leatherbacks are the largest of all sea turtle species and even those that nest in the Caribbean will often circumnavigate the Atlantic in between nesting seasons. Leatherbacks have a varied habitat, but are usually in the open ocean following the cooler currents that carry their food – jellyfish. Whatever the jellyfish lacks in substance and appeal the leatherback makes up for by consuming great amounts of it - up to 440 pounds in one day! That’s the same weight as an adult lion! As one of the largest jellyfish consumers in the world, leatherbacks evidently play a key role in keeping jellyfish populations in check. Without leatherbacks and other top predators it is possible that jellyfish could become more plentiful than commercially important fish species, which are currently on the decline. This could have a devastating effect on already stressed fish stocks. It is also worth considering what our beaches would be like without the great appetite of the leatherbacks, with just one single turtle removing hundreds of pounds of jellyfish from our waters each day.

Though out of sight much of the time, our sea turtles are working hard each day to keep the oceans in check. If we can conserve the local nesting beaches and feeding areas of these endangered species, it seems that they will also do a lot in return. It is even possible that we might be saved from a painful jellyfish sting or two! The Antigua Sea Turtle Project collects information on sea turtle nesting on beaches throughout Antigua. To report sightings of nesting turtles, fresh turtle tracks, or nest hatchings on beaches around the island telephone 720 6955.

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