Eel Census

The Burrishoole programme tracks European eel migration to support research into population change, life history, and environmental drivers.

European eel monitoring at Burrishoole

The Newport programme provides one of Europe’s longest continuous datasets on European eel (Anguilla anguilla) migration.

Circular European eel life-cycle diagram showing eggs, pre-leptocephalus larva and leptocephalus in the ocean, followed by glass eel, elver or yellow eel, silver eel and mature eel stages associated with the continent.

Silver eel migration

Silver eels represent the final freshwater life stage before their spawning migration. During this stage, they:

undergo physiological changes known as silvering to prepare for marine migration
typically migrate downstream in autumn
move in strong association with high-discharge events and low-light conditions
migrate to the Sargasso Sea, where spawning occurs

Long-term monitoring and data collection

Silver eel monitoring began in 1959. Comprehensive trapping has operated since the Salmon Leap Trap was installed in 1970.

Catchment census

The trapping system enables a near-complete census of eels migrating downstream within the Burrishoole catchment.

Individual-level measurements

The programme records daily migration counts, body length and weight, sex, and the timing and duration of migration events.

Changes in silver eel abundance and body weight
Measure	Period	Finding
Mean annual catch	1971–1982	~4,400 individuals
Mean annual catch	1983–1989	Declined to ~2,200 individuals
Annual catch	1990s	Partially recovered to >3,000 individuals annually
Mean body weight	Early 1970s	~95g
Mean body weight	1990s–2000s	Peaked at ~215g
Mean body size	Past 15 years	A declining trend was observed

Grayscale microscope view of a translucent European eel otolith with curved concentric growth rings.

Investigating size and growth patterns

Changes in eel size may reflect density-dependent processes, altered growth conditions, or recruitment variability. Otolith growth rings support age and growth analysis.

Sex ratio shifts

The proportion of males fell from approximately 94% in early records, including 1962, to approximately 37.5% by the late 1980s. This indicates a marked increase in females within the migrating population. Potential drivers include:

reduced recruitment of glass eels, also known as elvers
changes in environmental conditions within the catchment
density-dependent sex-differentiation mechanisms

Research significance

The Burrishoole dataset contributes to wider European efforts to conserve and recover this critically impacted species. Researchers use it to:

understand long-term population trajectories
investigate drivers of recruitment failure
analyse life-history plasticity in response to environmental change
support national and EU-level eel management frameworks

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