Modelling the effect of fibropapilloma disease on the somatic growth dynamics of Hawaiian green sea turtles |
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Authors: | Email author" target="_blank">Milani?ChaloupkaEmail author George?Balazs |
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Institution: | (1) Ecological Modelling Services Pty Ltd, University of Queensland, PO Box 6150, St Lucia, Queensland, 4067, Australia;(2) NOAA National Marine Fisheries Service, Pacific Islands Fisheries Science Center, 2570 Dole Street, Honolulu, HI 96822-2396, USA |
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Abstract: | The effect of the tumour-forming disease, fibropapillomatosis, on the somatic growth dynamics of green turtles resident in
the Pala’au foraging grounds (Moloka’i, Hawai’i) was evaluated using a Bayesian generalised additive mixed modelling approach.
This regression model enabled us to account for fixed effects (fibropapilloma tumour severity), nonlinear covariate functional
form (carapace size, sampling year) as well as random effects due to individual heterogeneity and correlation between repeated
growth measurements on some turtles. Somatic growth rates were found to be nonlinear functions of carapace size and sampling
year but were not a function of low-to-moderate tumour severity. On the other hand, growth rates were significantly lower
for turtles with advanced fibropapillomatosis, which suggests a limited or threshold-specific disease effect. However, tumour
severity was an increasing function of carapace size—larger turtles tended to have higher tumour severity scores, presumably
due to longer exposure of larger (older) turtles to the factors that cause the disease. Hence turtles with advanced fibropapillomatosis
tended to be the larger turtles, which confounds size and tumour severity in this study. But somatic growth rates for the
Pala’au population have also declined since the mid-1980s (sampling year effect) while disease prevalence and severity increased
from the mid-1980s before levelling off by the mid-1990s. It is unlikely that this decline was related to the increasing tumour
severity because growth rates have also declined over the last 10–20 years for other green turtle populations resident in
Hawaiian waters that have low or no disease prevalence. The declining somatic growth rate trends evident in the Hawaiian stock
are more likely a density-dependent effect caused by a dramatic increase in abundance by this once-seriously-depleted stock
since the mid-1980s. So despite increasing fibropapillomatosis risk over the last 20 years, only a limited effect on somatic
growth dynamics was apparent and the Hawaiian green turtle stock continues to increase in abundance. |
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