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Ecological Intelligence

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Ecological Intelligence / Subject

fish

North America

Young fish are shifting in space and season on different clocks

Evidence
Peer-reviewed multi-program observational synthesis
Geography
Northeastern Pacific from Alaska through the California Current
System
Marine pelagic / early-life-stage fish habitat

WHAT THE EVIDENCE SAYS

Six sampling programs contributed 21,737 unique events from 1995–2018. Significant changes appeared in different metrics for different taxa: for example, chub and jack mackerel shifted seasonal centers of gravity while several sanddabs and other species shifted latitudinal metrics. Spatial and phenological variability were not tightly coupled, and most environmental covariates did not show simple linear trends across the full domain.

WHY IT MATTERS / OUR READ

Range shift and timing shift should not be treated as interchangeable evidence of climate response. Survey design and life stage matter, and management may need to track geography and phenology separately for recruitment-sensitive species.

CAVEATS / UNCERTAINTY

The synthesis combines programs with different regions and sampling structures, and significance varies by species and metric. The results describe early life stages rather than whole-stock distribution and do not assign every shift to a single environmental cause.

SOURCE

ICES Journal of Marine Science — primary study
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A 1995–2018 synthesis of 21,737 Northeastern Pacific sampling events found significant spatial or seasonal shifts in only subsets of species and metrics, rather than one coherent climate-response direction.

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Peer-reviewed Africa

One invasive fish can suppress another—and leave a native remnant in the middle

Evidence
Peer-reviewed
Geography
Breede River, Western Cape, South Africa
System
River fish community

WHAT THE EVIDENCE SAYS

A river survey recorded 2,546 fish representing 17 species, seven of them introduced. Sharptooth catfish frequency was negatively associated with fish diversity, while invasive black bass showed the opposite broad association; flow barriers, high catfish abundance and turbidity appeared to suppress visually hunting bass enough for endangered Berg-Breede whitefish to persist at very low frequency.

WHY IT MATTERS / OUR READ

In invaded systems, management can face interacting pressures rather than one clean native-versus-invasive axis. Removing one stressor can change the effect of another, which makes food-web and barrier context relevant to control decisions.

CAVEATS / UNCERTAINTY

The whitefish remnant is persistence, not recovery. The evidence is observational and site-specific, and antagonism among invasive fishes is not an argument to retain invasive species or barriers; intervention effects would need direct testing.

SOURCE

Biological Invasions
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In South Africa’s Breede River, flow barriers, turbidity and abundant sharptooth catfish appear to damp black-bass predation enough for an endangered whitefish to persist at extremely low frequency.

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Peer-reviewed North America

Dam removal changed Cuyahoga fish assemblages most where reconnection opened a real route to source populations

Evidence
Peer-reviewed
Geography
Cuyahoga River, Ohio, USA
System
Urban river fish assemblages and longitudinal connectivity

WHAT THE EVIDENCE SAYS

A Restoration Ecology analysis of Cuyahoga River fish data from 1984–2024 found that removal of Brecksville Dam in 2020 increased connectivity to Lake Erie by 39 river km and more than doubled free-flowing habitat below the remaining Gorge Plant Dam. Downstream reaches gained 15 newly detected taxa and shifted strongly through time; 12 of those taxa were native and three were nonnative. Upstream assemblages showed limited change consistent mostly with redistribution of resident species rather than colonization-driven turnover.

WHY IT MATTERS / OUR READ

Barrier removal is not a binary treatment. Its ecological value depends on what the removal reconnects to, what barriers remain and whether source populations can reach the restored reach. Reconnection can support native recolonization while also opening pathways for nonnative spread, so network position and invasion risk belong in restoration prioritization rather than connectivity being treated as an automatic benefit.

CAVEATS / UNCERTAINTY

The river also experienced decades of water-quality improvement, remediation and other restoration, so not all assemblage change can be attributed solely to one dam removal. Historic sampling intensity and timing also differed from recent standardized surveys. Three of the 15 newly detected downstream taxa were nonnative, and the authors explicitly flag invasion risk—including upstream expansion of Round goby—as a tradeoff of reconnection.

SOURCE

Restoration Ecology
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Four decades of data show strong downstream turnover after the Brecksville Dam removal, while reaches still isolated by another major barrier changed far less.

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