C7: Healthy seas: fish and shellfish populations

Short Description

This indicator tracks the health of our seas using assessments of demersal fish populations (fish that live and feed on or near the bottom of seas). It consists of 2 metrics. The first metric looks at the size of the fish in a community (typical length) and the second looks at the composition of fish communities (mean maximum length). Together these metrics tell us about the health and status of fish communities. A healthy fish community will be made up of species in the expected ratio of numbers of individuals, and with individual species showing the age classes and sizes consistent with a healthy population. Typical length: a reduction in the proportion of larger, older, fish (as measured by typical length) of several species, suggests the top (predator) level of the food web is in poor condition. Mean maximum length: if the species that tend towards larger individuals are depleted and smaller-bodied species become more abundant (shown by a reduction in mean maximum length), the species composition of the community can change, suggesting prolonged periods of pressure. When the community is dominated by slow growing species (as expected at low maximum mean length), the size structure is limited in its ability to recover (reduced typical length).

Readiness and links to data

An interim indicator is presented here based on assessments of demersal species, published as part of the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) Quality Status Report 2023. Data on the typical length and mean maximum length of demersal fish are available.

Data on fish populations, published as part of the UK Marine Strategy Part One (2019) are also available, and UK Biodiversity Indicator: Fish Size Classes provides additional data on fish sizes. Further development of this indicator is required to incorporate UK Biodiversity Indicator: Fish Size Classes and assessment thresholds.

Notes on indicator

It has been agreed between the OSPAR member states to cease updating the typical length and mean maximum length indicators for pelagic species. Whilst earlier assessments are still available in previous versions of the Outcome Indicator Framework (which can be accessed via the National Archives), they will no longer be included here. This is due to size-structure of pelagic fish being less affected by fishing activity given that there is only a small number of pelagic species - they are all typically small sized, and therefore reach sexual maturity and maximum size much faster than demersal species. It is thought that the assessment of feeding guilds (presented in Indicator C8: Healthy seas: marine food webs functioning) is a more appropriate indicator for these species as a community and a more appropriate indicator for populations of these species individually is given within Indicator C11: Status of sensitive fish species.

Indicator components

Table C7a: Long-term changes in the typical length of demersal fish communities in UK waters and surrounding areas, 1983 to 2020

Location Description	Survey	Change Direction
Bristol Channel (StratumC)	CSEngBT3_Bchannel	Increase
Eastern Channel - Deep Water	GNSEngBT3	No change
Eastern Channel - English Coast	GNSEngBT3	Increase
Eastern Channel - French Coast	GNSEngBT3	Decrease
Far West of Scilly Isles - Small Deep Area	CSFraOT4	Increase
Far West of Scilly Isles - Small Mid Depth Area	CSFraOT4	Increase
Irish Sea - Eastern Coast	CSEngBT3	Increase
Irish Sea - Isle of Man	CSEngBT3	Increase
Irish Sea - South	CSEngBT3	Increase
Irish Sea - Western Coast	CSEngBT3	Decrease
North Sea - Central West	GNSIntOT1	Decrease
North Sea - North East	GNSIntOT1	No change
North Sea - Orkney/Shetlands	GNSIntOT1	Increase
North Sea - South East	GNSIntOT1	Decrease
North Sea - South West	GNSIntOT1	Decrease
North West Coast of Scotland	CSScoOT1	Increase
North West of Scotland - 'Windsock'	CSScoOT1	Increase
North West of Scotland - Deep Water (red1_lam)	CSScoOT1	No change
North West of Scotland - Deep Water (red2_lam)	CSScoOT1	No change
North and West of Scotland - Mid Depth	CSScoOT1	No change
North of Ireland - Coast	CSScoOT1	No change
South East of Ireland - Coast	CSIreOT4	Increase
South East of Ireland - Mid Depth	CSIreOT4	Increase
South West of Ireland - Coast	CSIreOT4	No change
South West of Ireland - Deep	CSIreOT4	Increase
South West of Ireland - Mid Depth	CSIreOT4	No change
Southern Celtic Sea - Deep	CSFraOT4	Decrease
Southern Celtic Sea - Mid Depth	CSFraOT4	No change
Southern Celtic Sea - Shallow	CSFraOT4	No change
West of Ireland - Coast	CSIreOT4	Increase
West of Ireland - Deep	CSIreOT4	No change
West of Ireland - Mid Depth	CSIreOT4	Increase
West of Scilly Isles - Small Mid Depth Area	CSFraOT4	Increase
West of Scilly Isles - Small Deep Area	CSFraOT4	No change
West of Scotland - Clyde	CSScoOT1	Decrease
West of Scotland - Deep Water	CSScoOT1	Decrease
West of Scotland and North West of Ireland	CSScoOT1	Decrease
West of Scotland and North of Ireland	CSScoOT1	No change
West of Scotland and West of Clyde	CSScoOT1	No change
Wider Atlantic - Porcupine Bank - Deep Western Waters	WASpaOT3	Decrease
Wider Atlantic - Porcupine Bank - East	WASpaOT3	No change
Wider Atlantic - Porcupine Bank - Far South	WASpaOT3	No change
Wider Atlantic - Porcupine Bank - North	WASpaOT3	No change
Wider Atlantic - Porcupine Bank - South	WASpaOT3	Increase
Wider Atlantic - Porcupine Bank - Very Far South	WASpaOT3	No change
Wider Atlantic - Rockall Bank Deep	WAScoOT3	Increase
Wider Atlantic - Rockall Bank Mid Depth	WAScoOT3	No change
Wider Atlantic - Rockall Bank Shallowest	WAScoOT3	No change
Wider Atlantic - Rockall Bank Very Deep	WAScoOT3	No change

Source

The Convention for the Protection of the Marine Environment of the North-East Atlantic, Size Composition in Fish Communities

Footnotes

The assessment period differs depending on the survey, starting at the earliest in 1983 in five of six Greater North Sea subdivisions (i.e. excluding the Eastern English Channel) and ending in either 2019 or 2020.

The legend entries are defined as follows:

Long-term increase: an increase in the size of fish within the community over the period
No long-term change: no change in the size of fish
Long-term decrease: the size of fish in the community has decreased from an earlier period but has increased from a more recent period
Decrease to minimum observed state: the fish in the community are currently at the lowest size recorded

In the 2025 update of this indicator, table C7a has been added, summarising the assessment results which feed into the map. Whilst the table and map are linked, surveys often overlap. Where this happens, an aggregated result is shown in the map. More information about the surveys, their location and the technical areas within the surveys (strata) can be found in OSPARs Coordinated Environmental Monitoring Programme (CEMP) guidelines for common indicators FC1, FC2, FC3, and FW3.

Download CSV

Image C7a: Long-term changes in the typical length of demersal fish communities in UK waters and surrounding areas, 1983 to 2020

Source

The Convention for the Protection of the Marine Environment of the North-East Atlantic, Size Composition in Fish Communities

Footnotes

The assessment period differs depending on the survey, starting at the earliest in 1983 in five of six Greater North Sea subdivisions (i.e. excluding the Eastern English Channel) and ending in either 2019 or 2020.

The legend entries are defined as follows:

Long-term increase: an increase in the size of fish within the community over the period
No long-term change: no change in the size of fish
Long-term decrease: the size of fish in the community has decreased from an earlier period but has increased from a more recent period
Decrease to minimum observed state: the fish in the community are currently at the lowest size recorded

In the 2025 update of this indicator, table C7a has been added, summarising the assessment results which feed into the map. Whilst the table and map are linked, surveys often overlap. Where this happens, an aggregated result is shown in the map. More information about the surveys, their location and the technical areas within the surveys (strata) can be found in OSPARs Coordinated Environmental Monitoring Programme (CEMP) guidelines for common indicators FC1, FC2, FC3, and FW3.

Download Figure Image

Trend description for C7a

In the central and southern North Sea and on the shelf edge to the west of Scotland, the proportion of individuals within demersal communities, relative to the early 1980s, has shifted towards smaller fish (low typical length) indicating this community is in poorer health. In the northern North Sea, eastern English Channel, Irish Sea, the Bristol Channel and much of the Celtic Seas south of Ireland, the health of the demersal fish community has improved since the 1990s, with an increasing contribution of large individuals (increasing typical length).

Assessment of change

No assessment of change was undertaken for this indicator as a suitable time series is not yet available in the Outcome Indicator Framework.

Table C7b: Long-term changes in the mean maximum length of demersal fish communities in UK waters and surrounding areas, 1983 to 2020

Location Description	Survey	Change Direction
Bristol Channel (StratumC)	CSEngBT3_Bchannel	Increase
Eastern Channel - Deep Water	GNSEngBT3	Increase
Eastern Channel - English Coast	GNSEngBT3	Increase
Eastern Channel - French Coast	GNSEngBT3	Increase
Far West of Scilly Isles - Small Deep Area	CSFraOT4	No change
Far West of Scilly Isles - Small Mid Depth Area	CSFraOT4	No change
Irish Sea - Eastern Coast	CSEngBT3	Increase
Irish Sea - Isle of Man	CSEngBT3	Increase
Irish Sea - South	CSEngBT3	Increase
Irish Sea - Western Coast	CSEngBT3	No change
North Sea - Central West	GNSIntOT1	Decrease
North Sea - North East	GNSIntOT1	No change
North Sea - Orkney/Shetlands	GNSIntOT1	No change
North Sea - South East	GNSIntOT1	Decrease
North Sea - South West	GNSIntOT1	Decrease
North West Coast of Scotland	CSScoOT1	No change
North West of Scotland - 'Windsock'	CSScoOT1	Increase
North West of Scotland - Deep Water (red1_lam)	CSScoOT1	No change
North West of Scotland - Deep Water (red2_lam)	CSScoOT1	No change
North and West of Scotland - Mid Depth	CSScoOT1	No change
North of Ireland - Coast	CSScoOT1	No change
South East of Ireland - Coast	CSIreOT4	Increase
South East of Ireland - Mid Depth	CSIreOT4	Increase
South West of Ireland - Coast	CSIreOT4	No change
South West of Ireland - Deep	CSIreOT4	No change
South West of Ireland - Mid Depth	CSIreOT4	No change
Southern Celtic Sea - Deep	CSFraOT4	Decrease
Southern Celtic Sea - Mid Depth	CSFraOT4	No change
Southern Celtic Sea - Shallow	CSFraOT4	No change
West of Ireland - Coast	CSIreOT4	Increase
West of Ireland - Deep	CSIreOT4	Decrease
West of Ireland - Mid Deep	CSIreOT4	No change
West of Scilly Isles - Small Mid Depth Area	CSFraOT4	No change
West of Scilly Isles - Small Deep Area	CSFraOT4	No change
West of Scotland - Clyde	CSScoOT1	Decrease
West of Scotland - Deep Water	CSScoOT1	Decrease
West of Scotland and North West of Ireland	CSScoOT1	No change
West of Scotland and North of Ireland	CSScoOT1	No change
West of Scotland and West of Clyde	CSScoOT1	No change
Wider Atlantic - Porcupine Bank - Deep Western Waters	WASpaOT3	No change
Wider Atlantic - Porcupine Bank - East	WASpaOT3	No change
Wider Atlantic - Porcupine Bank - Far South	WASpaOT3	Increase
Wider Atlantic - Porcupine Bank - North	WASpaOT3	No change
Wider Atlantic - Porcupine Bank - South	WASpaOT3	Increase
Wider Atlantic - Porcupine Bank - Very Far South	WASpaOT3	Increase
Wider Atlantic - Rockall Bank Deep	WAScoOT3	No change
Wider Atlantic - Rockall Bank Mid Depth	WAScoOT3	Decrease
Wider Atlantic - Rockall Bank Shallowest	WAScoOT3	Decrease
Wider Atlantic - Rockall Bank Very Deep	WAScoOT3	No change

Source

The Convention for the Protection of the Marine Environment of the North-East Atlantic, Size Composition in Fish Communities

Footnotes

The assessment period differs depending on the survey, starting at the earliest in 1983 in five of six Greater North Sea subdivisions (i.e. excluding the Eastern English Channel) and ending in either 2019 or 2020.

The legend entries are defined as follows:

Long-term increase: the fish are shifting back towards larger species
No long-term change: no change in the composition of fish communities
Long-term decrease: more smaller species than in an earlier period but less than in a more recent one
Decrease to minimum observed state: the community has the highest proportion of small species recorded

In the 2025 update of this indicator, table C7b was added, summarising the assessment results shown in the map. Whilst the table and map are linked, surveys often overlap. Where this happens, an aggregated result is shown in the map. More information about the surveys, their location and the technical areas within the surveys (strata) can be found in OSPAR’s Coordinated Environmental Monitoring Programme (CEMP) guidelines for common indicators FC1, FC2, FC3, and FW3.

Download CSV

Image C7b: Long-term changes in the mean maximum length of demersal fish communities in UK waters and surrounding areas, 1983 to 2020

Source

The Convention for the Protection of the Marine Environment of the North-East Atlantic, Size Composition in Fish Communities

Footnotes

The assessment period differs depending on the survey, starting at the earliest in 1983 in five of six Greater North Sea subdivisions (i.e. excluding the Eastern English Channel) and ending in either 2019 or 2020.

The legend entries are defined as follows:

Long-term increase: the fish are shifting back towards larger species
No long-term change: no change in the composition of fish communities
Long-term decrease: more smaller species than in an earlier period but less than in a more recent one
Decrease to minimum observed state: the community has the highest proportion of small species recorded

In the 2025 update of this indicator, table C7b was added, summarising the assessment results shown in the map. Whilst the table and map are linked, surveys often overlap. Where this happens, an aggregated result is shown in the map. More information about the surveys, their location and the technical areas within the surveys (strata) can be found in OSPAR’s Coordinated Environmental Monitoring Programme (CEMP) guidelines for common indicators FC1, FC2, FC3, and FW3.

Download Figure Image

Trend description for C7b

In the central and southern North Sea and on the shelf edge to the west of Scotland, the balance of species within demersal communities, relative to the early 1980s, has shifted towards smaller species (low mean maximum length), indicating this community is in poorer health. There has been no long-term change in mean maximum length of demersal fish communities in the northern North Sea. In the eastern English Channel, Irish Sea, the Bristol Channel and much of the Celtic Seas south of Ireland, the health of the demersal fish community has improved since the 1990s, with an increasing contribution of species that can grow large (increasing mean maximum length).

Assessment of change

No assessment of change was undertaken for this indicator as a suitable time series is not yet available in the Outcome Indicator Framework.

Indicator Metadata

Headline indicator status: Health of our seas
Primary goal in the 25 Year Environment Plan: Thriving plants and wildlife
Relevant goal(s) in the 25 Year Environment Plan: Thriving plants and wildlife
Relevant target(s) in the 25 Year Environment Plan: Making sure populations of key species are sustainable with appropriate age structures; Reversing the loss of marine biodiversity and, where practicable, restoring it; Ensuring seafloor habitats are productive and sufficiently extensive to support healthy, sustainable ecosystems
Position in the natural capital framework: Condition of assets - seas
Related reporting commitments: UK Marine Strategy (Marine Strategy Regulations 2010); Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR); Relevant to Sustainable Development Goal 14; International Council for Exploration of the Seas (ICES); Stock sustainability and reducing bycatch under the Joint Fisheries Statement
Geographical Scope: UK (Celtic Seas and Greater North Sea)
Status of indicator development: Interim
First Reported: 2020
Last Updated: 2025
Latest Data: 2020