D1. Biodiversity and ecosystem services

D1a. 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 50cm 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.




Since the previous publication additional data are provided for the North Sea and metrics for other regional seas have been restated.  The size structure has also been changed from 40cm to 50cm.


In 2016, large fish in the North Sea survey made up 16% of the weight of the fish community. This is close to the value of 20% recorded in 1983 and a noticeable increase from a low of 4% in 2001. While there was a clear decline in the indicator from 1983 to 2001, there has been rapid recovery since and this pace of recovery accelerated after 2010.  



Figure D1ai.  Proportion of large fish (equal to or larger than 50cm), by weight, in the North Sea, 1983 to 2016.

Figure D1ai. Proportion of large fish (equal to or larger than 50cm), by weight, in the North Sea, 1983 to 2016.


Notes: 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 threshold.       

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

2010 indicator stable

indicator improving

Decreased (2016)

Notes: 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.  LOESS is a non-parametric regression method; it may be understood as standing for "LOcal regrESSion”.


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Last updated: July 2018

Latest data:

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

Proportion of large fish by weight in the Scottish Continental Shelf – 2014

 Proportion of large fish by weight in the Irish Sea – 2015

Proportion of large fish by weight in the Celtic Sea – 2015