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

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

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

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

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