A higher sea temperature at coral bleaching after 2006 did not show a general gain in heat tolerance in four survey programmes, 1998-2017

Sully et al. (2019) found that bleached Reef Check coral surveys had a 0.5-0.6 C higher mean sea surface temperature (SST) in 2007-2017 than in 1998-2006, and said that corals can have adapted. We tested this claim with the public Global Coral Bleaching Database (BCO-DMO 773466 v2), four survey programmes and two heat-stress products (CoRTAD, as supplied, and NOAA OISST weekly 1 degree). In 10,032 Reef Check samples, the mean SST of bleached samples increased by 0.30 C (95% CI 0.07-0.52), but the mean SST of unbleached samples increased by 0.44 C. Thus the bleached surveys did not move to a warmer SST relative to the other surveys. Of the bleached Reef Check samples, 67-78% had zero degree heating weeks (DHW) at the survey date. Within the same sites, at equal SST anomaly and DHW, Reef Check had less severe bleaching in 2007-2017 (odds ratio 0.50, 0.32-0.79). This effect was only in the Atlantic (Pacific 0.90, 0.46-1.78). It went away when we removed 2006 (1.11, 0.63-1.95), when most severe bleaching came after the late-2005 Caribbean heat wave at zero DHW. With a 26-week OISST DHW it was 0.78 (0.49-1.25). For any bleaching, the later deficit was at zero heat stress and the heat response was steeper later. This agrees with a change in the recording of background bleaching. In Florida (FRRP, OR 1.72, 1.35-2.19), Japan (Kumagai, OR 4.9, 2.2-10.6) and the Caribbean (AGRRA, separation toward more bleaching), severe bleaching at equal heat stress was more frequent in the later period. These data do not support a general gain in coral heat tolerance. Limitations: the surveys are not random, observers are unknown, survey-date DHW does not measure past heat events completely, and FRRP and Kumagai have few spatial groups.

coral bleaching · thermal tolerance · degree heating weeks · marine heatwave · Reef Check · observation bias · conditional logistic regression · replication · sea surface temperature · acclimatization