Alarm bells ring as turtle growth rates decline

An unprecedented regional study of turtle growth rates by Bjorndal et al, compiled scientific data on hawksbill turtle growth rates collected over a 33 year period from 24 sites around the Caribbean. The study concluded that since 1997 turtle growth rates have declined dramatically and that this appears to be closely tied to climate change effects, El Niño Southern Oscillation fluctuations and increasing sea surface temperatures.

Hawksbill turtle by Brenda Kirkby

As a result of hundreds of years of overexploitation for their shell, meat and eggs, hawksbills occur on the IUCN Red List (2015) and are considered “critically endangered” They are particularly at risk because they are highly migratory as well as their strong dependence on coral reefs, which themselves are suffering regional degradation as a result of climate change (Jackson 2014).

Hawksbill turtles, (Eretmochelys imbricata), make excellent subjects for studies of this kind. Turtles are cold-blooded animals (ectotherms) and environmental factors therefore have a considerable impact on their productivity. The rate at which they grow (somatic growth) depends strongly on temperature, salinity, habitat quality as well as food quality and quantity. Turtles are long lived and have large range states, migrating over thousands of kilometers, which decreases the potential for local genetic variation to have a significant impact on their productivity. As juveniles and adults they are highly dependent of coral reef ecosystems. Any changes in turtle productivity are therefore likely to carry a strong environmental signal.

Hawksbills spend the first few years of life in open water and only come inshore once they are about 20cm in length. Young turtles will then spend their time at foraging sites, migrating every few years from one area to another. Once they mature they can become much more territorial, spending years on the same reefs where their diet consists of sponges, zoathids, corallimorpharians and other benthic invertebrates associated with coral reefs.

Study sites ranged from Bermuda to Brazil and included data from the Dutch Caribbean, collected by Sea Turtle Conservation on Bonaire. Data showed that turtle growth rates throughout the Caribbean were very similar indicating that these tend to be driven by non-local factors. The study also determined that between 1997 and 2013 mean hawksbill growth rates in the Caribbean have decline steadily and significantly by 18%.

Decreases in turtle growth rates correlated most closely to climate change indices, specifically Multivariate El Nino Southern Oscillation Index (MEI), which combines six variables associated with the oscillation namely, sea surface temperature, surface air temperature, sea level pressure, surface winds and cloudiness. There was also a strong negative correlation between turtle growth rate and sea surface temperature for water temperatures above 26.4 ºC.

The authors believe that hawksbill growth and coral reef decline are linked through climate change, but that the relationship is not a simple one. What the final effect will be on hawksbill populations has yet to be determined and will depend on how good they are at finding healthy productive reefs, on their foraging behavior, prey selection and food abundance as well as their reproductive success in the face of ocean acidification and increasing sea surface temperatures.


References

Bjorndal, K. A., et al. 2016. Somatic growth dynamics of West Atlantic hawksbill sea turtles: a spatio-temporal perspective. Ecosphere 7(5): e01279

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