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D1a. Biodiversity and ecosystem services

a. Fish size classes in the North Sea

Type: State / Benefit Indicator

Indicator Description

The indicator shows changes in the proportion, by weight, of large individuals equal to or over 50 cm in length in demersal (bottom-dwelling) fish populations in the North Sea. Changes in the size structure of fish populations and communities reflect changes in the state of the fish community. Fluctuations in values between years are expected given inter-annual fluctuations in the distribution and abundance of North Sea fish populations and sampling variation.

Contents

  1. Summary
  2. Downloads

Summary

Since the previous publication additional data have been provided for the North Sea, Celtic sea, Irish Sea and Scottish Continental Shelf. The entire dataset has been revised, and updated to include all available years. The assessment value thresholds are specific to time periods and therefore change with any updates.

In 2017, large fish in the North Sea survey made up 12% of the weight of the fish community. This is approaching the value of 15% recorded in 1983 and shows a noticeable increase from a low of 2% in 2001. While there was a clear decline in the indicator from 1983 to 2001, there has been recovery since, and this pace of recovery accelerated after 2012.  

Figure D1ai. Percentage of large fish (equal to or larger than 50 cm), by weight, in the North Sea, 1983 to 2017

A line graph showing the proportion of large fish (equal to or larger than 50cm) by weight in the North Sea between 1983 and 2016. The horizontal dashed line shows the assessment threshold. The LOESS smoothed trend shows a considerable decline between 1983 and 2001 and from then on there has been a recovery and this pace of recovery accelerated after 2012.  The unsmoothed trend shows considerable year-on-year variation.

Note: The line graph shows the unsmoothed trend (dashed line) and a LOESS smoothed trend (solid line) with the shaded area showing the 95 per cent confidence intervals around the smoothed trend. The horizontal dashed line shows the assessment LOESS is a non-parametric regression method; it may be understood as standing for "LOcal regrESSion”.

Source: Centre for Environment, Fisheries and Aquaculture Science; Marine Scotland.

 

Assessment of change in the proportion of large fish, by weight

 

Long term

Short term

Latest year

North Sea


1983–2017


2012–2017

Increased (2017)

Note: The long-term and short-term assessments have been made by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) by fitting a LOESS smoothed trend to the index (see the note under Figure D1ai).

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Downloads

Download the Fiche and Datasheet from the JNCC Resource Hub

Technical documentation for the indicator is available from the OSPAR Assessment Portal

Last updated: September 2019

Latest data:   

Proportion of large fish by weight in the North Sea – 2017

Proportion of large fish by weight in the Scottish Continental Shelf – 2016

Proportion of large fish by weight in the Irish Sea – 2016

Proportion of large fish by weight in the Celtic Sea – 2016

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Categories:

UK Biodiversity Indicators 2019

Published: .

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