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Across long-running surveys of the Northeast Atlantic and Mediterranean, fish populations shifted latitude in almost equal numbers poleward and equatorward. Many also moved deeper. The result is not a census showing that half of all Mediterranean commercial species moved: it describes populations tracked in a broad regional survey dataset, and local Mediterranean studies find still different patterns, including southward and shallower shifts.
What “nearly half moved” means
A 2026 Nature Communications study analyzed 92,961 bottom-trawl samples collected by 21 long-term survey programs between 1983 and 2019. The surveys covered 556 fish species across the Northeast Atlantic and Mediterranean. Among the analyzed populations, 49% shifted poleward and 51% shifted equatorward (Nature Communications, 2026).
The unit matters: these percentages describe populations in survey time series, not individual fish. Nor do they mean that exactly half of every commercially fished Mediterranean species moved. The study spans a wider region than the Mediterranean alone, and standardized bottom-trawl surveys observe sampled areas and taxa rather than every fish, fishery, or species in the sea.
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The near-even split in latitude challenges the simple expectation that marine fish distributions shift northward as waters warm. Depth movement, however, was commonly downward. The paper reports two related but distinct results:
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| Measure | Reported result | What it counts |
|---|---|---|
| Average depth | Increased for 60% | Analyzed populations |
| Median depth | Shifted deeper for 66% | Species-level summary |
These figures use different measures and denominators, so they should not be combined or treated as interchangeable. A fish population can shift deeper without shifting poleward; latitude, depth, and movement toward or away from shore are separate dimensions of distribution change (Nature Communications, 2026).
Western Mediterranean shifts include southward and shallower movement
A separate study of demersal communities in the western Mediterranean, using monitoring from 1994 to 2019, found numerous southward and southwestward shifts. It also reported that shelf species moved toward shallower, more inshore waters under high climate velocity (Ecological Indicators, 2024).
This does not contradict the broader survey result. The studies cover different places and periods, examine different sets of species, and use different indicators of movement. Together, they show why a single “fish are moving north and deeper” rule can miss local variation. In the western Mediterranean analysis, responses differed by species and subregion; southward movement and shallower inshore movement are part of that regional picture, not a universal pattern for the whole sea.
Why fish move in different directions
The broader study examines how environmental conditions, species traits, ecological niche tracking, and fishing pressure relate to observed shifts. Its results should not be read as proof that warming alone caused every population to move in a particular direction (Nature Communications, 2026).
The western Mediterranean study calculated climate velocity from spatial and temporal variation in sea-surface temperature and assessed its relationship to species’ temperature and depth preferences. That analysis identifies climate velocity as one factor associated with distribution patterns; it does not establish a single cause for every shift. Species characteristics, location, and other interacting influences matter when interpreting why observed movements differ (Ecological Indicators, 2024).
Quick Recap
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How to read these findings
- Check the geography: the 49%/51% latitude split covers survey populations across the Northeast Atlantic and Mediterranean; the southward and shallower examples come from western Mediterranean subregions.
- Check what moved: the main study reports population shifts, while its separate depth summary also reports a species-level statistic.
- Separate dimensions: latitude, depth, and inshore or offshore movement are not equivalent measures.
- Distinguish observation from cause: trawl surveys track distribution patterns, and modeled associations help investigate drivers; neither makes every shift a direct, isolated effect of climate warming.
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