During May–June of 2014 and 2015, we observed mass mortality of thousands of fish in the families Apogonidae, Balistidae, Muraenidae, Pomacentridae, and Scorpaenidae in addition to cephalopods, gastropods, and crustaceans. The dead organisms were found in green turbid waters that were trapped inside the lagoon (Fig. 1). In the dead patch observed offshore, the decaying organisms were found only in a warm surface layer that was approximately 2 m thick, which we interpret as originating in the lagoon and then drifting off the reef.
The onset of mortality tended to coincide with neap tides and/or calm weather. Neap tides occurred on May 22, 2014, and June 10, 2015, each preceding a die-off event by 1–2 days (Fig. 1B). Although the mortality events observed on June 1, 2014, and June 16, 2015 did not occur during neap tides, current speeds on the adjacent reef flat at both of these times were relatively slow (<5 cm s-1) due to weak winds and small waves. Temperatures reached 30.2°C on May 23, 2014, 30.5°C on June 1, 2014, and 33.9°C on June 11, 2015 (data downloaded from http://www.bco-dmo.org/dataset/560618). Coral bleaching occurred on the eastern part of the reef flat during both 2014 and 2015 as a result of unusually calm weather and warm waters compared to the preceding several years. Bleaching on the reef flat in May–June 2014 was transient with little or no coral mortality, whereas bleaching in the later half of June through July 2015 caused 40% coral mortality. The coral bleaching events were separated from the die-offs that we report here by 1–2 km, and in 2015 the coral mortality occurred 1–2 months after our observations. Thus, while the coral bleaching and the die-offs that we observed seem to be distinct events, they both correspond to relatively calm weather and high water temperatures.
We suspect that the die-offs were caused by hypoxia because suffocating fish from Apogonidae and Pomacentridae families were swimming to the surface to gulp air. Oxygen saturation on the adjacent reef flat is typically <10% around dawn, even reaching anoxia for 30–60 min on some days, due to high rates of community metabolism. This depletion of oxygen might be exacerbated in the lagoon during calm conditions. Stagnant lagoon waters, combined with weak winds and high temperatures, could limit the input and solubility of oxygen in seawater. The reef fauna and flora- including any plankton blooms may rapidly consume the available oxygen, and its low concentration could be reinforced by the decomposition of decaying organisms. This situation may be viewed as a “fish-bowl” effect in which organisms are trapped in an isolated volume of water that becomes warm and possibly oxygen-starved.