Deep refuges: the distribution of marine fish in warming subtropics

In light of global climate change, identifying critical marine habitats and conserving them is essential. Marine conservation planning recommends designating cooler habitats as marine protected areas. The ‘deep-reef refugia' hypothesis suggests that deeper, suitable habitats may allow species to undergo the evolutionary changes necessary to adapt to the growing environmental threats they face. This hypothesis has rarely been tested outside tropical ecosystems. This study, using a systematic approach, is the first to evaluate this hypothesis regarding fish communities in the east Mediterranean Sea (EMS), which is warming at an unprecedented rate. Fish were surveyed twice a year from 2015 to 2022 across three rocky habitats: shallow (10 m depth, 23% ± 11 of 1 m photosynthetically active radiation, PAR), upper mesophotic (25 m depth; 8% ± 4 of 1 m PAR), and lower mesophotic (45 m depth; 3% ± 2 of 1 m PAR), where summer maximal temperatures reach a mean of 29.69°C, 28.66°C, and 27.9°C, respectively. Data collected from 357 belt transects indicate that: 1) species composition and functional diversity of the shallow habitat are encompassed within those of the deeper habitats; 2) species diversity is greater in the upper mesophotic community compared to the shallow community; and 3) abundance is reduced at mesophotic depths. Unlike most findings on tropical coral ecosystems, our results suggest that a mixed fish community of indigenous and immigrant species is currently thriving at upper mesophotic depths. This habitat appears to act as a slightly cooler climate change refuge for a less diverse, shallower community, or a step along the way to tropicalization. The unique position of the EMS as a transitional marine environment emphasizes its potential role as an early indicator of changes in fish depth distributions that could globally impact subtropical ecosystems.