Two decades of body length measurements in size-structured larval and juvenile fish populations in English rivers.

Long term ecological datasets are valuable in providing context and understanding to complex ecological processes that occur over broad temporal scales, and provide a baseline for analysing change. Monitoring of fish populations in UK waterbodies and elsewhere is typically through measuring the length of individual fish caught in surveys. Through this method, the age structure of fish populations can be determined, as well as over winer survival rates and future recruitment success and cohort sizes can be predicted. The larval and juvenile period are when fish are considered most vulnerable to predation, competition, disease and environmental perturbations. 

This study presents the first long-term larval and juvenile fish lengths dataset for 67 survey sites over two decades (1999-2018) from the rivers Ancholme, Warwickshire Avon, Don, Trent, and Yorkshire Ouse (including the Swale, Ure, Nidd and Wharfe) in the United Kingdom. These rivers represent a range of topographical and biotopical characteristics. For the majority of this study, surveys were conducted on a monthly or fortnightly basis making both annual and seasonal analyses of size structure, growth and body length possible. Although there is some variation in the sampling frequency and some locations varied throughout the study according to requirements. In total, more than 380,000 larval or juvenile fish of 30 species were measured, likely representing one of the most comprehensive datasets of its type.

Surveys were conducted in river margins, where the velocity was slowest and larval and juvenile fish tend to aggregate. Fish were captured using a 25 x 3 m micromesh (3 mm mesh size) seine net that was set in a rectangle parallel to the bank. This net capture fish as small as 5 mm and is the most appropriate method of catching larvae and juvenile fish, although occasionally some larger adult fish may have also been captured and measured as part of this dataset for completeness. All fish were identified to species and measured to standard length (mm) and released at the point of capture. The exception was the smallest larvae, which were euthanised with an overdose of methanesulphonate (MS-222) and preserved in 4% formalin solution for microscopic examination.

The dataset contains 384,090 rows and 13 columns. Each row corresponds to a single fish that was measured at each site and date. Associated site information (site name, location, area fished (m2) and survey date) is reported for each row. When only a fraction of the catch was processed, the sub-sample size was reflected in the Count column (e.g. when half the sample was processed, the numbers of fish measured or only counted were multiplied by two). This enables accurate densities to be calculated as the total number of both measured and unmeasured fish is recorded.

Description of columns found in the dataset:

Column heading

Column description

Data type

Units

Fish _Catchment

The river catchment/basin location of each fish site

Text

n/a

Fish_River

The river/watercourse location of each fish site.

Text

n/a

Fish_SiteName

The name of each fish site

Text

n/a

Fish_Latitude

The latitude of each fish site (WGS 1984)

Integer

Decimal degrees

Fish_Longitude

The longitude of each fish site (WGS 1984)

Integer

Decimal degrees

Fish_Area

Area of fish site surveyed

Integer

m-2

Fish_SurveyDate

Date fish survey was carried out

Integer

dd/mm/yyyy

Fish_Year

Year fish survey was carried out

Integer

yyyy

Common_Name

The common/vernacular name of each fish taxon recorded in the dataset.

Text

n/a

Latin_Name

The scientific name of each fish taxon recorded in the dataset

Text

n/a

Net_Number

The net number the fish in a given survey were caught on

Integer

n/a

Length_mm

Length of individual fish caught

Integer

mm

Count

Count of fish caught accounting for sub- sampling

Integer

Number of fish