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R Soc Open Sci
2017 Aug 02;48:170396. doi: 10.1098/rsos.170396.
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Bias associated with the detectability of the coral-eating pest crown-of-thorns seastar and implications for reef management.
Kayal M
,
Bosserelle P
,
Adjeroud M
.
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Outbreaks of the predator crown-of-thorns seastar (COTS) Acanthaster planci cause widespread coral mortality across the Indo-Pacific. Like many marine invertebrates, COTS is a nocturnal species whose cryptic behaviour during the day can affect its detectability, particularly in structurally complex reef habitats that provide many refuges for benthic creatures. We performed extensive day and night surveys of COTS populations in coral reef habitats showing differing levels of structural complexity and COTS abundance. We tested whether estimations of COTS density varied between day and night observations, and if the differences were related to changes in COTS abundance, reef structural complexity and the spatial scale of observation. Estimations of COTS density were on average 27% higher at night than during the day. Differences in COTS detection varied with changing seastar abundance but not reef structural complexity or scale of observation. Underestimation of COTS abundance in daytime was significant for a broad seastar density range, thus potentially affecting most outbreak events. Our study suggests that portions of COTS populations can be undetected during conventional surveys and control campaigns, which are exclusively conducted by day, and significantly affect the trajectory of coral reefs. Accounting for bias in COTS detection can strengthen coral reef management broadly.
Figure 1. Variability in reefscape across sites and depths as observed around Moorea, French Polynesia. The nine reef locations consisted of three water depths (vertically; 6 m, 12 m, 18 m) at each of three sites (horizontally; H, Haapiti; T, Tiahura; V, Vaipahu). Values in italic indicate mean (s.e.) substrate rugosity as estimated in 2008 by the chain-and-tape method. Note that reef structural complexity varies in time and space with changing coral community abundance and structure. Pictures were taken in November 2007.
Figure 2. Density trajectories of the coral-killing seastar COTS as counted by day and night time. Surveys were performed in permanent-transects established at nine reef locations that consisted of three water depths (vertically; 6 m, 12 m, 18 m) at each of three sites (horizontally; H, Haapiti; T, Tiahura; V, Vaipahu). Dots represent actual observations and lines illustrate mean trajectories.
Figure 3. Contrast curve identifying the domain of significant difference in estimations of seastar density between day and night surveys. Densities of the coral-killing seastar COTS were estimated every six months over a period of 2 years by day and night counts performed in permanent-transects established at nine reef locations (figure 2). The semi-parametric contrast curve [28,29] represents variation in the difference between day and night estimations (difference = day density − night density, y-axis) along the seastar density range (x-axis). The domain of significant difference is identified as the portion of the covariate (x-axis) for which the 95% confidence interval of the contrast curve (shaded area) does not cross the no-difference threshold (horizontal dashed line): COTS abundance was significantly lower in day counts compared to night in the log-density range 0.13–1.31, corresponding to the COTS density interval 0.4–19.3 seastar 200 m−2.
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