Fish populations in long-running surveys across the Northeast Atlantic and Mediterranean shifted latitude in almost equal numbers toward the poles and toward the equator. Many also shifted deeper. But the headline result is not a census of every commercially fished Mediterranean species: it describes populations tracked in specific bottom-trawl surveys, and a western Mediterranean study found local movements that included southward, southwestward and shallower shifts.
What the “nearly half” figure actually measures
A 2026 Nature Communications study analyzed 92,961 bottom-trawl samples collected by 21 long-term survey programs from 1983 to 2019. Across 556 fish species, it assessed changes in the distributions of populations represented in those survey time series. The result was nearly even: 49% of analyzed populations shifted poleward and 51% equatorward. The study
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Those percentages refer to populations, not individual fish. Nor do they show that exactly half of all Mediterranean commercial species moved: the surveys covered the Northeast Atlantic and Mediterranean together, and bottom trawls provide standardized observations only for sampled areas and taxa. The finding is a broad pattern in monitored populations, not an exhaustive inventory of every species or fishery in the Mediterranean.
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The same study found a frequent shift toward deeper water. It reports increased average depth for 60% of populations. In a separate species-level summary, 66% of species had a median shift toward deeper water. These figures use different units and depth measures, so they should not be treated as competing estimates or combined into one percentage. Nature Communications
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Latitude and depth are distinct dimensions of distribution change. A population can move deeper without moving poleward, and a poleward or equatorward shift does not by itself say whether it moved inshore or offshore. The findings therefore do not reduce to a single route that every fish is taking.
Why western Mediterranean movements can look different
A 2024 study of demersal communities in the western Mediterranean, using monitoring from 1994 to 2019, found numerous southward and southwestward shifts. It also found that some shelf species moved toward shallower, more inshore waters in areas with high climate velocity. The authors describe varied responses across species and subregions, rather than one universal direction. Ecological Indicators
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This does not contradict the broader survey result. The studies differ in geography, time span, species sets and the indicators used to describe movement: one estimates population shifts across Northeast Atlantic and Mediterranean survey programs, while the other examines demersal distribution patterns within western Mediterranean subregions. A regional southward or inshore pattern can coexist with an almost even split in latitude direction across a much wider set of populations.
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The 2026 paper examines movement in relation to environmental conditions, species traits, ecological niche tracking and fishing pressure. Its reported directional percentages do not isolate climate warming as the cause of each shift. They describe observed distribution patterns and investigate interacting factors; they do not prove that one driver explains every population’s response. Nature Communications
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The western Mediterranean study calculated climate velocity from spatial and temporal variation in sea-surface temperature and assessed how temperature and depth preferences related to observed patterns. It found differences among species and subregions and noted evidence consistent with other interacting drivers. Climate velocity is one analytical factor in that work, not a complete causal explanation for all the movements. Ecological Indicators
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read claims about fish moving
When a report says fish are “moving,” check what was measured before applying the claim to a particular species or fishery:
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- Where: A result for combined Northeast Atlantic and Mediterranean surveys is not automatically a result for the Mediterranean alone; a western Mediterranean pattern is more geographically specific.
- Which unit: A percentage of surveyed populations is not a percentage of individual fish, species, or commercial catches.
- Which direction: Latitude, depth, and inshore–offshore movement describe different aspects of a range shift.
- How it was observed: Trawl-survey records and modeled associations show patterns in monitored areas; they do not amount to continuous observation of the entire sea or prove a single cause.
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