River Fish Density Dataset

What this looks like

One real series from the dataset, and real rows from the head of its published extract. Neither is a mock-up: the rows below are the same rows, unchanged, that are in the file you would download.

Atlantic salmon density at Fish survey site 711, East Anglia

The longest single series this dataset holds — 31 daily values, one of 5,412. The dataset is not this chart.

Drawn to show the shape and resolution of what the dataset contains, not to make a point about Fish survey site 711, East Anglia. Gaps are breaks, never interpolated.

View the data behind this chart

Underlying data
Period	Fish survey site 711, East Anglia (fish/100m²)
1981	0
1985	0
1988	0
1991	0
1994	0
1997	0
2000	0
2001	0
2002	0
2003	0
2004	0
2005	0
2006	0
2007	0
2008	0
2009	0
2010	0
2011	0
2012	0
2013	0
2014	0
2015	0
2016	0
2017	0
2018	0
2019	0
2021	0
2022	0
2023	0
2024	0
2025	0

Sample rows

12 of 637,594 rows Taken from the start of observations-ecosystem-uk-ea-ecology.csv. These are sample rows, not the dataset.

Sample rows from the River Fish Density Dataset extract
`metric_id`	`subject_id`	`subject_kind`	`value`	`censored`	`censor_operator`	`censor_threshold`	`censor_limit`	`censor_reported`	`unit`	`period_start`	`period_end`	`period_kind`	`basis`	`confidence`	`source_id`	`source_ref`	`qualifier`	`superseded`	`retrieved`
ept-family-richness	eco-ea-1000	geography	8	—	—	—	—	—	count	1991-07-29	1991-07-29	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	9	—	—	—	—	—	count	1995-03-22	1995-03-22	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	6	—	—	—	—	—	count	1995-10-17	1995-10-17	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	7	—	—	—	—	—	count	1996-04-03	1996-04-03	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	10	—	—	—	—	—	count	1996-06-27	1996-06-27	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	8	—	—	—	—	—	count	1996-10-10	1996-10-10	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	3	—	—	—	—	—	count	1997-03-12	1997-03-12	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	8	—	—	—	—	—	count	1997-07-14	1997-07-14	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	11	—	—	—	—	—	count	1997-09-30	1997-09-30	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	7	—	—	—	—	—	count	1998-04-21	1998-04-21	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	8	—	—	—	—	—	count	1998-07-28	1998-07-28	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28
ept-family-richness	eco-ea-1000	geography	11	—	—	—	—	—	count	1998-09-30	1998-09-30	day	calculated	high	uk-ea-ecology	INV_OPEN_DATA_TAXA.csv	—	—	2026-08-28

Download these rows as CSV — the same twelve, with the column header, for testing a loader before committing to the full file.

Methodology

Collected from the Agency’s Ecology and Fish Data Explorer. Only non-selective electrofishing depletion surveys of at least 50 m² are used — about 7% of published rows — because a single-catch count, a seine net haul and a depletion estimate are three different numbers and a series mixing them would step when the method changed rather than when the river did. The Agency publishes an estimated population inferred from the fall in catch across successive runs; Hydrionis divides it by the fished area and expresses it per 100 m², under method depletion-density-per-area 1.0.0. Zeros are reconstructed: a row exists only where a species was caught, so every qualifying survey is enumerated and a species with no row at that survey is recorded as zero. Sites are converted from British National Grid to WGS84 by projection inverse and Helmert datum shift.

What this dataset cannot tell you

Read before relying on these figures

A zero is a zero catch, not proof of absence. Electrofishing never catches every fish, and eels in particular hide in the substrate and are among the hardest to catch, so a zero eel density is weaker evidence of absence than a zero salmon density.
The values are estimates, not counts. A depletion estimate assumes the population is closed for the survey and that every fish has the same chance of capture; both weaken as rivers get deeper, wider and more turbid, so the estimate is least reliable where the river is largest.
The published standard error is not carried. A Hydrionis series point holds a date and a value with nowhere to put an uncertainty, so a density of 14 and one of 14 give or take 9 are stored identically.
Salmon and trout densities are juveniles. Adult Atlantic salmon are at sea, and what an electrofishing survey finds is fry and parr, so a rise means a good spawning year or good juvenile survival and not more fish returning to the river.
Brown trout and sea trout are one species and are not separated. Their juveniles cannot reliably be told apart in the field, so nothing here can be attributed to the migratory form.
Eel figures exclude elvers and glass eels, which the Agency records separately, so this is the growing stage only and not a measure of recruitment arriving from the sea.
About 93% of published fish rows are excluded as not comparable — single-catch surveys, seine netting, fixed traps and species-selective surveys — along with surveys fishing less than 50 m², where dividing by the area extrapolates a river from a few square metres. That is real data answering a different question, not missing data.
Sites are those surveyed in at least five separate years. They are not a random sample of English rivers: the Agency surveys where it has reason to.

Coverage

Subjects	Environment Agency electrofishing survey sites in England with at least five years of survey — 1,804 held
Period	1975–2026
Units	fish/100m²
Update cadence	Irregularly, when the publisher releases
Identifier	`river-fish-density` v1.0.0

Fields

Data dictionary →

Every metric this dataset carries, with the unit it is stored in and the geography level it is reported at. Full definitions are in the data dictionary.

Identifier	Field	Unit	Reported at
`fish-density-atlantic-salmon`	Atlantic salmon density Estimated number of Atlantic salmon (Salmo salar) per 100 square metres of river bed, from an electrofishing depletion survey.	fish/100m²	monitoring-station
`fish-density-brown-trout`	Brown and sea trout density Estimated number of Salmo trutta per 100 square metres of river bed, from an electrofishing depletion survey.	fish/100m²	monitoring-station
`fish-density-european-eel`	European eel density Estimated number of European eels (Anguilla anguilla) beyond the elver stage per 100 square metres of river bed, from an electrofishing depletion survey.	fish/100m²	monitoring-station

Rights and access

Licence position

Open with attribution. Derivative use true; redistribution true; commercial reuse true.

Permitted uses

Public display
Derived analysis
API delivery
Bulk download
Commercial use

Available to: Public, Developer, Research, Professional, Enterprise. These follow from the rights of the contributing publishers, not from a Hydrionis pricing decision — no access tier can widen what a publisher withheld.

Contributing sources

Environment Agency — Ecology and Fish Data Explorer — freshwater macroinvertebrates (Open Government Licence v3.0)

Attribution and citation

Cite and reuse →

Two different obligations, deliberately kept apart. One is a condition of somebody else's licence; the other is a request Hydrionis has no standing to enforce.

Attribution you are required to give

These are conditions of the licences the underlying data is published under. They are obligations to the publishers, not to Hydrionis, and they apply to you directly whether or not you mention Hydrionis at all.

Contains public sector information licensed under the Open Government Licence v3.0.

Environment Agency — Open Government Licence v3.0 (licence text)

Suggested citation for Hydrionis

Hydrionis (2026), River Fish Density Dataset, version 1.0.0, accessed 2026-09-02, hydrionis.com/data/datasets/river-fish-density/

Cite Hydrionis where you rely on the compilation, the normalisation, the eligibility judgements or a chart. Where you use an unmodified published value, the publisher’s attribution is the one that is required; crediting Hydrionis as well is welcome and is not something Hydrionis can require of you.

Recommended, not required. Hydrionis does not claim rights over unmodified public data it relayed, and doing arithmetic on somebody else’s measurements does not make them Hydrionis’s. Cite the version and the access date: River Fish Density Dataset changes as publishers release and as methods are revised, and a citation without both cannot be checked later. Full guidance is on cite and reuse.

Changelog

Version 1.0.0 — 2026-08-28

First release.

Additions

Salmon, trout and eel density at 1,804 English river sites, with reconstructed zeros.

Use this data

Download ecosystem observations from EA ecology (637,594 rows) (CSV)
Machine-readable JSON /api/v1/datasets/river-fish-density.json
How to cite this, and who you must credit
How these figures were produced
Request this at a different scale, format or geography

Every bulk extract ships with a companion metadata file carrying this dataset’s version, its sources, their licences, the attribution you are required to give and the suggested citation — so the terms travel with the file rather than staying on this page.

About this data

The fields a professional user needs to judge whether this is fit for a particular purpose. Deliberately not a score.

About this data

Everything below is a property of the data rather than a rating of it. There is deliberately no single confidence score: how much a gap matters depends on what you are about to do with the series, and only you know that.

Source	Environment Agency — Ecology and Fish Data Explorer — freshwater macroinvertebrates · Open Government Licence v3.0
How the values came to exist	Mixed across the series — the basis is recorded on every individual value.
Period held	1975–2026
Known gaps	A zero is a zero catch, not proof of absence. Electrofishing never catches every fish, and eels in particular hide in the substrate and are among the hardest to catch, so a zero eel density is weaker evidence of absence than a zero salmon density. The values are estimates, not counts. A depletion estimate assumes the population is closed for the survey and that every fish has the same chance of capture; both weaken as rivers get deeper, wider and more turbid, so the estimate is least reliable where the river is largest. The published standard error is not carried. A Hydrionis series point holds a date and a value with nowhere to put an uncertainty, so a density of 14 and one of 14 give or take 9 are stored identically.
Continuity	Version 1.0.0, released 2026-08-28. 1 recorded version; breaking changes and corrections are listed in the changelog above.
Last read from the publisher	2026-08-27 · publisher updates irregularly, when the publisher releases
Redistribution	Permitted by the upstream licences
Commercial reuse	Permitted by the upstream licences. No Hydrionis access tier can widen this.
Methodology	How this was produced