All species of sea turtle are either listed as threatened or endangered, but despite the continual growth of their population they are still plagued by the disease fibropapillomatosis, also referred to as FP. Members of the National Oceanic and Atmospheric Administration (NOAA) in Honolulu, Hawaii and the Nicholas School of Environment and Earth Sciences at Duke University have made new progress in discovering the effects and origin of the disease by studying the Hawaiian green turtles.
FP is a severe problem for sea turtles because it is an extremely incapacitating disease, which causes the development of tumours. It is not known for sure how FP is contracted; however a herpesvirus has been connected to the disease. Studies have observed that sea turtles with FP tumours also have the herpesvirus and that turtles free of one are also free of the other. The recent study takes a closer look at how the disease varies according to the size of turtles, whether the disease is more prevalent is specific geographic areas, and if so what are the reasons for the different geographic areas to be so afflicted. The answers to these questions give further insight to the possibility of the herpesvirus being the cause of FP.
To determine disease rates the researchers used records of 3,939 stranded green sea turtles that had been collected over 28 years from the National Marine Fisheries Service, Pacific Islands Fisheries Science Center.The study found that after the disease rates were calculated, there was indeed a relationship between turtle size and the risk of contracting FP. The results indicated that the age group of turtles which were most affected were those that had nearly reached adulthood.
Sea turtles breed every 3-4 years, which means that where they spend their time in-between breeding seasons has a significant effect on their health. These locations are referred to as their nearshore habitats. On the islands of Oahu, Maui, and Hawaii these habitats were split into twelve different regions for the purpose of the study, it was then determined which regions had the highest disease rates. Due to the variance among the regions, land use was then examined to determine if there was an explanation for why different regions had higher disease rates.
To examine the land use of the regions the researchers calculated a Nitrogen-footprint for each area based on ten factors they chose that are known major nitrogen contributors in the Hawaiian area. They chose to examine nitrogen because it is linked to the green turtles food source the macroalgae and because it is a well-known contributor to the balance of ecosystems. The chosen contributors were sewage injection wells, urbanization, the cultivation of pineapple and sugar, aquaculture and fishponds, the water flow of streams and rivers, estuaries and wetlands, ports, coastal lagoons, rigorous poultry and hog farming, and cattle grazing/dairy production. When the disease rates of FP were compared to the calculated N-footprints a strong correlation was found between having a high N-footprint in the same areas that have high disease rates.
This means that human production of N is possibly contributing to FP rates in green sea turtles. To discover how the N is affecting the sea turtles, their diet that is composed of mostly macroalgae, was examined. It was found that 93% of the regions where there is macroalgea there is also high disease rate and high N-footprints. The consumption of the macroalgae may therefore be significant to rates of FP in green turtle populations.
Since the macroaglae is found in N rich regions, and plants are known to produce large amount of arginine (Arg, an amino acid) in those conditions it is possible that the macroalgae is producing Arg. Arginine is connected to the herpesvirus, which has been suspected of causing the FP tumours. Therefore, this study suggests that high N-footprints could be the cause of the herpesvirus that may be the cause of FP tumours. However, the authors of the study warn that there are still many factors that may not yet have been explored which could cause the contraction of the infectious disease.
Reference:
Van Houtan KS, Hargrove SK, Balazs GH (2010) Land Use, Macroalgae, and a Tumor-Forming Disease in Marine Turtles. PLoS ONE 5(9): e12900. doi:10.1371/journal.pone.0012900
By: Katherine Robinson 0719523
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