Data from: 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, using closed-circuit
rebreather systems, 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; 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 as 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.