Abstract
Shipwrecks can have significant localized effects when grounded on shallow coral reefs. These effects are not limited to the immediate physical damage, but can have wide-spread and lasting impacts due to alteration of the chemical makeup of the surrounding water column. This can subsequently impact the growth of benthic organisms, often leading to phase shifts and high levels of mortality of corals in the vicinity of the wreck. At Palmyra atoll, the grounding of a longline fishing vessel on the shallow reef terrace is associated with a phase shift to the corallimorph, Rhodactis howesii. In 2013, a wreck removal effort initiated by the US Department of Fish and Wildlife resulted in the successful extraction and disposal of the wreckage, after which the density and percent cover of R. howesii in the immediate vicinity of the wreck site dropped precipitously. Here, we document the response of the fish community to the wreck removal and localized decline in R. howesii. We show that the biomass of scarid parrotfishes and acanthurid surgeonfishes and unicornfishes (primarily herbivores) increased after the removal of the wreck, while biomass of chaetodontid butterflyfishes (primarily invertivores, many species are known to feed on coelenterate polyps) declined over the study period. The density of small scarids and acanthurids also increased, but only after a few years post removal. Overall these results indicate that Palmyra’s unfished herbivore population has rapidly responded to the removal of the wreck and associated decrease in corallimorph cover, can maintain high levels of grazing where space is made available for colonization of early successional algae species, and may have the potential to facilitate reef recovery.
Highlights
Ship groundings can have many significant deleterious effects on coral reefs
Some hypothesize that benthic phase shifts on coral reefs may be extremely difficult or even in some cases impossible to reverse, due to hysteresis in these systems favoring processes that prevent the successful reestablishment of corals once another benthic group becomes established (Knowlton, 1992; Norström et al, 2009; Hughes et al, 2010)
After the Hui Feng shipwreck was removed from the shallow reef of Palmyra atoll in 2013, there was a decline in the density and percent cover of the corallimorpharian, Rhodactis howesii, from the reef surrounding the wreck site (Work et al, 2018)
Summary
Ship groundings can have many significant deleterious effects on coral reefs. In addition to a number of consequences from the physical impact itself (Precht et al, 2001; Riegl, 2001), ship material has been shown to have wider-reaching impacts due to processes that alter species composition and competitive dynamics within benthic assemblages. There is convincing evidence that wrecks can cause changes in water chemistry resulting from metals and contaminants that leach over time from the wreck debris. This alteration of water chemistry can facilitate certain benthic species, species whose growth is limited by specific metals such as iron. Often, these opportunistic species begin to outcompete corals and other benthic organisms for space, resulting in wreck-associated benthic phase shifts where the enriched species (e.g., fleshy algae, cyanobacteria, or sessile invertebrates) become dominant (Kelly et al, 2012). Recovery potential is likely to be context-dependent, and will vary based on the initial benthic community state, restoration effort and timing, the prevalence and intensity of additional ecological stressors, and the abundance and diversity of consumers in the system that can prevent or reverse a phase shift
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