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D1a. Fish size classes

Type: State / Benefit Indicator

Introduction

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.

Key results

Since the previous publication additional data have been provided for the North Sea. No updates have been provided for the Scottish Continental Shelf, Irish Sea and the Celtic Sea.

In 2019, large fish in the North Sea survey made up 6% of the weight of the fish community. This is below the value of 15% recorded in 1983, but above the low of 2% in 2001. There was a clear decline in the indicator from 1983 to 2001, followed by a rapid recovery to 13% in 2015 and a dramatic fall between 2018 and 2019.

Figure D1ai. Percentage of large fish (equal to or larger than 50 cm), by weight, in the North Sea, 1983 to 2019.

A line graph showing how the percentage of large fish (equal to or larger than 50 cm), by weight, in the North Sea has changed from 1983 to 2019. The indicator declined from 15% in 1983 to a low of 2% in 2001. This clear decline in the indicator from 1983 to 2001, is followed by a rapid recovery to 13% in 2015 and a dramatic fall between 2018 and 2019.

Notes:

  1. The line graph shows the unsmoothed trend (dashed line) and a LOESS smoothed trend (solid line) with the shaded area showing the 95% confidence intervals around the smoothed trend.
  2. The black horizontal dashed line shows the assessment target from OSPAR (2017).
  3. 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

Deteriorating
1983–2019

Deteriorating
2014–2019

Decreased (2019)

Note: The long-term and short-term assessments have been made by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) by assessing change in the fitted LOESS smoothed trend.

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Indicator description

The indicator shows changes in the proportion, by weight, of large individuals equal to or over 50 cm in length in demersal fish populations in the North Sea. The indicator is based on standardised trawls from international scientific surveys conducted annually. During the 1980s, large fish in the North Sea fish community included cod (Gadus morhua), ling (Molva molva), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), anglerfish (Lophius piscatorius) and rays (Batoidea). Recently, lesser spotted dogfish (Scyliorhinus canicula), hake (Merluccius merluccius) and smoothhounds (Mustelus spp.) have also formed a substantial part of the large-fish catches in the northern North Sea. In 2019, a substantial reduction in large cod and saithe was evident in the survey along with an increase in small haddock, whiting (Merlangius merlangus), and dab (Limanda limanda).

The measure for the North Sea is used as the main indicator because it is based on the largest dataset that includes an assessment target period (early 1980s), and it is supported by publications, and therefore provides the most reliable indicator of change.

Figure D1ai presents the LOESS smoothed trend that is used to assess both long-term and short-term trends in the proportion of large individuals in North Sea fish populations.

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Relevance

Change in the relative abundance of large fishes is likely to affect marine ecosystems in several ways. Fewer large fishes will reduce the amount of predation on smaller prey species and allow increases in their abundance and biomass. In turn, this will affect the structure and stability of the ecosystem. The indicator responds to fishing impacts on the fish community because larger fish are more likely to be caught by trawls, and because larger species of fish are more likely to decline in number for a given rate of fishing (Engelhard et al. 2015; Greenstreet et al. 2012; 2011). When fish communities are more heavily fished the proportion of large fish is expected to fall, and when fishing is reduced the proportion of large fish is expected to rise, albeit with a multi-year delay. Some variation in the proportion of large fish will be driven by environmental variation and, in the long term, increases in temperature may lead to decreased body size of demersal fish in the North Sea (Queirós et al. 2018). However, in the short term (over periods of a few years) environmental effects on this indicator are expected to be relatively small in relation to fishing effects.

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Background

The indicator is compiled using methods based on those developed by the International Council for the Exploration of the Sea (ICES) for the analysis of International Bottom Trawl Survey data (ICES, 2007). These methods were further developed for the OSPAR Intermediate Assessment 2017, and have since been updated (Greenstreet and Moriarty, 2017; Moriarty et al 2017; Moriarty and Greenstreet, 2020).  Data on fish length are taken from surveys carried out in the North Sea during the first quarter of each year. All fish are measured as part of the survey, but only demersal fish species are used in the indicator.  

To compile the indicator, the proportions of fish in the North Sea greater than or equal to 50 cm are estimated by weight. For other seas, a different target for ‘large’ is selected based on signal-to-noise analyses. Technical documentation for the indicator is available from https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017/biodiversity-status/fish-and-food-webs/proportion-large-fish-large-fish-index/. The method involves additional averaging of catch densities across hauls within ICES rectangles, prior to summation of the large and total fish components for the large fish indicator (LFI) ratio calculation. 

The background to this indicator previously included information from sub-divisional assessments within the North Sea, but current assessments (last updated in 2016) are only available at the regional sea level (Figures D1aii and D1aiii). From the previous extensive OSPAR assessment, 3 otter trawl surveys were selected to provide the key data for each regional sea area, considering the length of the time-series, spatial coverage and quality of data. These surveys are: the Scottish otter trawl survey in Quarter 1 for the Scottish Continental Shelf, the Northern Irish otter trawl survey in Quarter 1 for the Irish Sea, and the northern part of the French otter trawl survey in quarter 4 for the Celtic Sea. Key findings for these areas are as follows:

Scottish Continental Shelf. In 2016, large fish (≥ 35 cm) made up approximately 33% of the weight of the surveyed demersal fish community. This was lower than the 42% observed in 1985 and lower than the 50% target, but there are signs of recovery in the data. While there was a clear decline in the indicator during the 1990s to 12% in 2002, the smoothed trend has shown increases with fluctuations since that time.  

Irish Sea. In 2016, large fish (≥ 45 cm) made up approximately 10% of the weight of the surveyed demersal fish community. This was higher than the 7% recorded in 1992, but lower than the peak of 17% recorded in 2014 of 17%. Although recovery was prominent until 2014, a subsequent fall was observed in 2015 and 2016. This fall is due to a rapid increase in the biomass of small fish, predominately whiting (Merlangius merlangus) that do not contribute to the large fish component. The surveyed biomass of large fish has, in fact, continued to increase since 2014 but at a slower rate than the biomass of small fish.

Celtic Sea. In 2016, large fish (≥ 40 cm) made up approximately 19% of the weight of the surveyed demersal fish community. This was slightly higher than the 16% recorded in 1997 but below the target of 46%. The smoothed trend has been largely stable throughout the survey period.  

Figure D1aii. Proportion of large fish (survey specific threshold for ‘large’ size), by weight, in the Scottish Continental Shelf, Irish Sea and Celtic Sea.

Scottish Continental Shelf, 1985 to 2016.

This figure shows the percentage of catch weight of large fish for the Scottish Continental Shelf (1985 to 2016). Trends are described in the text.

Irish Sea, 1992 to 2016.

This figure shows the percentage of catch weight of large fish for the Irish Sea (1992 to 2016). Trends are described in the text.

Celtic Sea, 1997 to 2016.

This figure shows the percentage of catch weight of large fish for the Celtic Sea (1997 to 2016). Trends are described in the text.

Notes:

1. Each line graph shows the unsmoothed trend (dashed line) and the LOESS smoothed trend (solid line) with the shaded area showing the 95% confidence intervals around the smoothed trend.
2. The black horizontal dashed line show the assessment target from OSPAR (2017).
3. 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.

Figure D1aiii. Areas surveyed to generate the fish size class indicator for the seas around the UK: North Sea – dark grey, Scottish Continental Shelf – green, Irish Sea – yellow and Celtic Sea – dark blue.

This figure shows a map covering western continental Europe, the British Isles and the surrounding seas. The areas surveyed to produce the fish size class indicators for the Scottish Continental Shelf, the Irish Sea and the Celtic Sea are highlighted.

Source: Centre for Environment, Fisheries and Aquaculture Science.

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Goals and Targets

Aichi Targets for which this is a primary indicator

Strategic Goal B. Reduce the direct pressures on biodiversity and promote sustainable use.

Aichi Target 6 icon

Target 6: By 2020, all fish and invertebrate stocks and aquatic plants are managed and harvested sustainably, legally and applying ecosystem based approaches, so that overfishing is avoided, recovery plans and measures are in place for all depleted species, fisheries have no significant adverse impacts on threatened species and vulnerable ecosystems and the impacts of fisheries on stocks, species and ecosystems are within safe ecological limits.

Strategic Goal D. Enhance the benefits to all from biodiversity and ecosystems.

Aichi Target 14 icon

Target 14: By 2020, ecosystems that provide essential services, including services related to water, and contribute to health, livelihoods and well-being, are restored and safeguarded, taking into account the needs of women, indigenous and local communities, and the poor and vulnerable.

Aichi Targets for which this is a relevant indicator

Strategic Goal A. Address the underlying causes of biodiversity loss by mainstreaming biodiversity across government and society.

Aichi Target 4 icon

Target 4: By 2020, at the latest, Governments, business and stakeholders at all levels have taken steps to achieve or have implemented plans for sustainable production and consumption and have kept the impacts of use of natural resources well within safe ecological limits.

Strategic Goal D. Enhance the benefits to all from biodiversity and ecosystems.

Aichi Target 15 icon

Target 15: By 2020, ecosystem resilience and the contribution of biodiversity to carbon stocks has been enhanced, through conservation and restoration, including restoration of at least 15 per cent of degraded ecosystems, thereby contributing to climate change mitigation and adaptation and to combating desertification.

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Reference Title
OSPAR Intermediate Assessment 2017 Proportion of Large Fish (Large Fish Index)
UK Marine Strategy Assessment 2019 Proportion of Large Fish (Large Fish Index) UK Marine Online Assessment Tool 2019.
Defra Science Cotter et al. 2008. Development of a Marine Trophic Index for UK waters and recommendations for further indicator development (PDF, 1.4 Mb)
International Council for the Exploration of the Sea ICES home page

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References

Cotter, J., Rogers, S., Ellis, J., Mackinson, S., Dulvy, N., Pinnegar, J., Jennings, S. & Greenstreet, S. (2008) Marine Ecosystem Integrity: Development of a Marine Trophic Index for UK waters and recommendations for further indicator development. Final report for Defra, Centre for Environment, Fisheries and Aquaculture Science (Cefas).

Engelhard, G.H., Lynam, C.P., Garcia-Carreras, B., Dolder, P.J. & Mackinson, S. (2015) Effort reduction and the large fish indicator: spatial trends reveal positive impacts of recent European fleet reduction schemes. Environmental Conservation 42: 227-236 https://doi.org/10.1017/S0376892915000077

Greenstreet, S.P.R., Rogers, S.I., Rice, J.C., Piet, G.J., Guirey, E.J., Fraser, H.M., Fryer, R.J. (2011). Development of the EcoQO for fish communities in the North Sea. ICES Journal of Marine Science 68: 1-11.

Greenstreet, S.P.R., Rogers, S.I., Rice, J.C., Piet, G.J., Guirey, E.J., Fraser, H.M., Fryer, R.J. (2012). A reassessment of trends in the North Sea Large Fish Indicator and a re-evaluation of earlier conclusions. ICES Journal of Marine Science 69: 343-345.

Greenstreet, S.P.R and Moriarty, M. (2017) Manual for Version 3 of the Groundfish Survey Monitoring and Assessment Data Product. Scottish Marine and Freshwater Science Vol 8 No 18, 77pp. https://doi.org/10.7489/1986-1

International Council for the Exploration of the Seas (ICES). (2007) Development of EcoQO on changes in the proportion of large fish and evaluation of size-based indicators. International Council for the Exploration of the Sea, Copenhagen. ICES ACE Report 2007.

Moriarty, M., Greenstreet, S.P.R. and Rasmussen, J. (2017) Derivation of Groundfish Survey Monitoring and Assessment Data Product for the Northeast Atlantic Area. Scottish Marine and Freshwater Science Vol 8 no 16, 240pp. https://doi.org/10.7489/1984-1

Moriarty, M., Greenstreet, S. 2020. Greater North Sea International Otter Trawl Quarter 1 Groundfish Survey Monitoring and Assessment Data Products - 2017-2019 Update. https://doi.org/10.7489/12310-1

OSPAR (2017). Intermediate Assessment 2017. Available at: https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017

Queirós A.M., Fernandes J., Genevier L. & Lynam C.P. (2018) Climate change alters fish community size‐structure, requiring adaptive policy targets. Fish and Fisheries. 2018; 00:1–9. https://doi.org/10.1111/faf.12278

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Downloads

Download the Datasheet from JNCC's Resource Hub.

and

Full details of this indicator, including a datasheet and technical documentation are available at: https://moat.cefas.co.uk/biodiversity-food-webs-and-marine-protected-areas/fish/large-fish-index/

https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017/biodiversity-status/fish-and-food-webs/proportion-large-fish-large-fish-index/ 

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Last updated: October 2020

Latest data:   

Proportion of large fish by weight in the North Sea – 2019

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

 

This content is available on request as a pdf in non-accessible format. If you wish for a copy please go to the enquiries page.

 

Categories:

UK Biodiversity Indicators 2020

Published: .

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