Matched coral surveys gave no clear depth refuge from bleaching at 0-20 m in three oceans in 1998-2019

We tested whether deeper coral surveys had less bleaching when site, year, month, and survey program were the same. We used Reef Check records from the Global Coral-Bleaching Database, version 2. We removed repeated substrate rows and compared mean bleaching at 8-20 m with 0-6 m in 1,331 site-month pairs at 797 sites. The mean deep-minus-shallow difference was -0.349 percentage points. The 95% interval was -0.809 to +0.111 with errors grouped by ecological region. Four Atlantic and Pacific period tests gave no clear repeated negative contrast. Exact-day matching, stricter depth bands, and a region bootstrap also left the pooled result uncertain. Only 71 pairs had mean heat dose of at least four degree-heating weeks. Their difference was -0.478 points, with an interval of -4.917 to +3.961. A heat-dependent depth contrast was also uncertain. The archive did not establish a common depth refuge in this tested range. Selected surveys, many zero bleaching reports, limited depth separation, few heated pairs, and unknown coral composition limit inference. These data do not measure species loss or air pollution. The package contains derived surveys and pairs, code, source hashes, tables, and a figure.

coral bleaching · depth refuge · marine heat extremes · matched surveys · observation bias · regional replication