{"id":"river-fish-density","slug":"river-fish-density","name":"River Fish Density Dataset","description":"Density of Atlantic salmon, brown and sea trout, and European eel at 1,804 English river sites, from Environment Agency electrofishing depletion surveys since the 1970s. 5,412 series and 49,428 observations, of which the majority are zeros: a survey that looked and found none is a result, and for Atlantic salmon it is 84% of them.","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.","domains":["ecosystem"],"metricIds":["fish-density-atlantic-salmon","fish-density-brown-trout","fish-density-european-eel"],"subjectScope":"Environment Agency electrofishing survey sites in England with at least five years of survey","cadence":"irregular","version":"1.0.0","released":"2026-08-28","changelog":[{"version":"1.0.0","date":"2026-08-28","summary":"First release.","additions":["Salmon, trout and eel density at 1,804 English river sites, with reconstructed zeros."]}],"limitations":["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."],"context":{"hydrionisClass":"water-dependent-system","biologicalBasis":"population-estimate","whyHeld":"Fish are the part of a river’s condition the public recognises, and salmon and eel are the two whose lives are governed by whether the river works as a corridor — both migrate between river and sea and neither can complete a life cycle if flow, temperature or a weir stops them. Holding density beside flow lets a reader see whether the two moved together. None of it measures water: a fish density is not a flow, a temperature or a water quality value.","waterRelevance":["Whether rivers support migratory fish, which depends on flow and passage","Juvenile salmonid production, which responds to flow and temperature in summer","The state of a critically endangered catadromous species in English rivers","Biological response alongside drought and low-flow records"],"suitableUses":["Tracking density of one species at one river site across decades","Comparing sites surveyed under the same electrofishing depletion protocol","Showing density beside a river flow, drought or temperature record on a shared time axis","Seeing where a species is consistently absent, which the reconstructed zeros make visible"],"unsuitableUses":["Reading a salmon density as the size of a salmon run; these are juveniles in fresh water","Treating a zero as proof of absence rather than as a survey that caught none","Comparing these with single-catch, seine net or trap figures from the same publisher","Attributing a change in density to flow, temperature or a barrier because the two moved together; year-class strength, stocking, disease and habitat work all move fish numbers","Separating sea trout from resident brown trout","Treating the surveyed sites as representative of English rivers","Reading a single point as precise, given the standard error is not carried"],"joinDimensions":[{"kind":"measured-proximity","on":"Survey site coordinates, converted from the Agency’s own British National Grid references","limitation":"Proximity is not catchment membership. A site has a position and nothing more: Hydrionis holds no river network geometry, so a survey reach near a gauge may sit outside its catchment entirely, and nearness establishes neither that the gauge measures the water the fish were in nor which of the two is upstream."},{"kind":"time-only","on":"Survey dates, against any other Hydrionis series for the same period","limitation":"A survey is one day and the fish present reflect the preceding year — spawning success, winter flows, the previous summer’s low flow. Aligning a survey to same-day flow compares a season’s outcome with an instant."}],"relationshipConfidence":"contextual","eligibility":{"api":true,"download":true,"overlay":true,"publicPage":false}},"observationCount":49428,"seriesCount":5412,"subjectCount":1804,"from":1975,"to":2026,"sourceIds":["uk-ea-ecology"],"units":["fish/100m²"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), River Fish Density Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/river-fish-density/","downloads":[{"file":"/downloads/observations-ecosystem-uk-ea-ecology.csv","format":"CSV","label":"ecosystem observations from EA ecology (637,594 rows)"}],"apiPath":"/api/v1/datasets/river-fish-density.json","lastFetched":"2026-08-27","fields":[{"id":"fish-density-atlantic-salmon","name":"Atlantic salmon density","definition":"Estimated number of Atlantic salmon (Salmo salar) per 100 square metres of river bed, from an electrofishing depletion survey.","unit":"fish/100m²","nativeGeography":["monitoring-station"],"comparabilityNotes":"Juveniles, not returning adults: a rise is a good spawning year or good survival of young fish, and says nothing directly about the size of the run. A population estimate inferred from the fall in catch between successive electrofishing runs, not a count of fish seen. The inference assumes a closed population and equal catchability, and both assumptions weaken as rivers get deeper, wider or more turbid — so the estimate is least reliable where the river is largest. Held only for electrofishing depletion surveys of at least 50 m², which is about 7% of the Agency’s published fish rows. Single-catch surveys, seine netting and trap catches are real data and are not comparable with a depletion estimate, so they are excluded rather than combined. The Agency publishes a standard error for every estimate and Hydrionis cannot carry it: a series point holds a date and a value, so a density of 14 and a density of 14 give or take 9 are stored identically. Treat single points as indicative and trends as the readable signal. Describes a surveyed reach on one day, not the river or the catchment. Fish move, and a reach fished in August says little about the same reach in March. Density is an extrapolation to 100 m². The largest values in the record come from small reaches of dense juvenile habitat, where the arithmetic is correct and the extrapolation to a whole river would not be. Strongly seasonal and strongly age-structured — fry counts collapse and recover annually by nature, so a fall between two years is not a decline."},{"id":"fish-density-brown-trout","name":"Brown and sea trout density","definition":"Estimated number of Salmo trutta per 100 square metres of river bed, from an electrofishing depletion survey.","unit":"fish/100m²","nativeGeography":["monitoring-station"],"comparabilityNotes":"Resident brown trout and migratory sea trout are one species and are not separated here: the Agency records them together because juveniles of the two are not reliably distinguishable in the field. A change therefore cannot be attributed to the migratory form. A population estimate inferred from the fall in catch between successive electrofishing runs, not a count of fish seen. The inference assumes a closed population and equal catchability, and both assumptions weaken as rivers get deeper, wider or more turbid — so the estimate is least reliable where the river is largest. Held only for electrofishing depletion surveys of at least 50 m², which is about 7% of the Agency’s published fish rows. Single-catch surveys, seine netting and trap catches are real data and are not comparable with a depletion estimate, so they are excluded rather than combined. The Agency publishes a standard error for every estimate and Hydrionis cannot carry it: a series point holds a date and a value, so a density of 14 and a density of 14 give or take 9 are stored identically. Treat single points as indicative and trends as the readable signal. Describes a surveyed reach on one day, not the river or the catchment. Fish move, and a reach fished in August says little about the same reach in March. Density is an extrapolation to 100 m². The largest values in the record come from small reaches of dense juvenile habitat, where the arithmetic is correct and the extrapolation to a whole river would not be."},{"id":"fish-density-european-eel","name":"European eel density","definition":"Estimated number of European eels (Anguilla anguilla) beyond the elver stage per 100 square metres of river bed, from an electrofishing depletion survey.","unit":"fish/100m²","nativeGeography":["monitoring-station"],"comparabilityNotes":"Excludes elvers and glass eels, which the Agency records separately: this is the growing yellow and silver stage only, so it is not a measure of recruitment arriving from the sea. Eels hide in the substrate and are among the hardest fish to catch by electrofishing, so catchability is low and variable and the depletion estimate is correspondingly weaker than for salmonids. A population estimate inferred from the fall in catch between successive electrofishing runs, not a count of fish seen. The inference assumes a closed population and equal catchability, and both assumptions weaken as rivers get deeper, wider or more turbid — so the estimate is least reliable where the river is largest. Held only for electrofishing depletion surveys of at least 50 m², which is about 7% of the Agency’s published fish rows. Single-catch surveys, seine netting and trap catches are real data and are not comparable with a depletion estimate, so they are excluded rather than combined. The Agency publishes a standard error for every estimate and Hydrionis cannot carry it: a series point holds a date and a value, so a density of 14 and a density of 14 give or take 9 are stored identically. Treat single points as indicative and trends as the readable signal. Describes a surveyed reach on one day, not the river or the catchment. Fish move, and a reach fished in August says little about the same reach in March. Density is an extrapolation to 100 m². The largest values in the record come from small reaches of dense juvenile habitat, where the arithmetic is correct and the extrapolation to a whole river would not be."}]}