C7: Healthy seas: fish and shellfish populations
Short Description
This indicator tracks the health of our seas using assessments of fish populations (here separated into demersal communities – fish that live and feed on or near the bottom of seas, and pelagic communities – fish that usually feed in the surface layers of the 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
This indicator is not available for reporting in 2024 in a finalised form. Further development of this indicator is required to incorporate the Large Fish Indicator (UK Biodiversity Indicator D1a) and assessment thresholds. An interim indicator is presented here with communities classified as demersal or pelagic. The assessments used for this interim indicator have been reported under the UK Marine Strategy Part One (2019). Data on fish populations, analytical methods and assessment are available and UK Biodiversity Indicator D1a provides additional data on fish sizes.
The demersal components of this indicator have recently been 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.
Notes on indicator
It has been agreed between the OSPAR member states to cease updating the typical length and mean maximum length fish size indicators for pelagic species. 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 and they are all typically small sized. It is generally thought that the assessment of feeding guilds 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 C11: Productive seas: status of sensitive fish and shellfish stocks. The assessments for pelagic species will remain here until further notice, but may be removed in a future update to the Outcome Indicator Framework.
Indicator components
Image C7ai: Long-term changes in the typical length of demersal fish communities in UK waters and surrounding areas, 1983 to 2020
Trend description for C7ai
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.
Image C7aii: Long-term changes in the typical length of pelagic fish communities in UK waters and surrounding areas, 1983 to 2016
Trend description for C7aii
In the northern North Sea, no change in the typical length in the pelagic fish community is evident. The typical length of pelagic fish generally shows no long-term change at the sub-regional level in the Celtic Seas.
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.
Image C7bi: Long-term changes in the mean maximum length of demersal fish communities in UK waters and surrounding areas, 1983 to 2020
Trend description for C7bi
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.
Image C7bii: Long-term changes in the mean maximum length of pelagic fish communities in UK waters and surrounding areas, 1983 to 2016
Trend description for C7bii
Within the southern and central North Sea, the mean maximum length of pelagic fish communities is declining suggesting the proportion of larger species is declining. There is no long-term change in the mean maximum length in the northern North Sea.
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.