Food webs
Assessment of progress towards the achievement of Good Environmental Status of food webs.
Extent to which Good Environmental Status has been achieved
The extent to which Good Environmental Status has been achieved is uncertain: plankton communities are changing; some fish communities are recovering, but others are not; breeding seabird populations are in decline; grey seal numbers are increasing and trends in cetacean populations are unclear. It is known that components of the marine food web are changing, but it is not clear how they are affecting each other.
In both the Greater North Sea and Celtic Seas, the extent to which Good Environmental Status has been achieved is uncertain.
We are not able to assess whether the UK’s aim of no significant adverse change in the function of different trophic levels in marine food webs as a result of human activities, will be achieved by 2020. Whilst there are some indications that fish communities, which are a key component of the food web, are recovering due to fisheries management measures, it is likely that these changes have contributed to and will continue to contribute to changes in prey availability for seabirds and marine mammals. It is unknown what the full extent of these changes in predator-prey interactions will be, or how climatically-driven changes in the plankton will affect the rest of the food web. There is still a substantial task to develop suitable indicators with other countries that provide a robust assessment of food web health.
How progress has been assessed
In the UK Marine Strategy Part One (HM Government, 2012),, the UK set out the following ‘Characteristics of Good Environmental Status’ for D4 food webs:
“At the level of the MSFD sub-regions, populations of key species groups within the food web have an age and size structure indicative of sustainable populations and occur at levels that ensure the long-term sustainability of the marine ecosystem of which they are part, in line with prevailing conditions, as defined by specific targets for species and pelagic habitats. There should be no significant adverse change in the function of different trophic levels in marine food webs as a result of human activities, including as a result of by-catch and discards.”
The progress towards achievement of the Good Environmental Status Characteristic under Food webs was assessed by using the assessments of fish, birds, seals, cetaceans and pelagic habitats that were conducted under Biological Diversity. Together the indicators selected for this assessment cover the Good Environmental Status Characteristics set out by the UK and provide an overview of ecosystem structure (species and size composition of fish communities and the abundance of seabirds and marine mammals) and functioning (changes in plankton communities to the production of seabirds and seals).
The extent that Good Environmental Status, as articulated in the criteria for Biological Diversity in the European Commission Decision (2010/447/EU), had been achieved, was assessed using targets set out for food webs in the UK Marine Strategy Part One (Initial Assessment) (HM Government, 2012), and their associated indicators (Table 1). All but one of the indicators, kittiwake breeding success, were agreed, in cooperation with OSPAR and they have been developed since the Initial Assessment 2012 (HM Government, 2012). Kittiwake breeding success was proposed by the UK as an OSPAR Common Indicator for the Greater North Sea but, was not adopted by other Contracting Parties because they lacked the data time-series necessary to construct the indicator.
Table 1. Summary of the sub-regional and UK environmental targets and associated indicators for fish, marine mammals, birds and pelagic habitats under Biological Diversity.
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Productivity of key species |
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Target |
Indicators |
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Marine mammal target: At the scale of the Marine Strategy Framework Directive sub-regions, marine mammal productivity is not significantly affected by human activities: there should be no statistically significant decline in seal pup production caused by human activities. |
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Bird target: At the scale of the Marine Strategy Framework Directive sub-regions, marine bird productivity is not significantly affected by human activities: annual breeding success of black-legged kittiwakes should not be significantly different, statistically, from levels expected under prevailing climatic conditions (sea surface temperature). |
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Proportion of selected species at the top of the top of food webs |
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Fish target: The size composition of fish communities should not be impacted by human activity such as to indicate any adverse change in trophic function within the community¹: a specified proportion (by weight) of fish in any defined marine region should exceed a stipulated length threshold. |
Proportion of large fish (Large Fish Index - LFI) (Demersal species only) |
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Species composition in fish Communities (Mean Maximum Length - MML) (Demersal & pelagic species) |
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Size composition in fish communities (Typical Length - TyL) (Demersal & pelagic species) |
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Abundance/distribution of key species/trophic groups |
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Pelagic habitat target: At the scale of the Marine Strategy Framework Directive sub-regions, abundance/distribution of plankton community is not significantly adversely influenced by anthropogenic drivers, as assessed by indicators of changes in plankton functional types (life form) indices. |
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Marine mammal target: At the scale of the Marine Strategy Framework Directive sub-regions abundance of cetaceans is not decreasing as a result of human activity: in all the indicators monitored, there should be no statistically significant decrease in abundance of marine mammals caused by human activities.
At the scale of the Marine Strategy Framework Directive sub-regions abundance of seals is not decreasing as a result of human activity: in any of the indicators monitored, there should be no statistically significant decrease in abundance of marine mammals caused by human activities. |
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Cetacean abundance and distribution (except inshore bottle nose dolphins |
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Seal abundance and distribution (grey seals and harbour seals) |
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Bird target: At the scale of the Marine Strategy Framework Directive sub-regions abundance of marine birds is not significantly affected by human activities: Changes in abundance of marine birds should be within individual target levels in 75% of species monitored. |
Marine bird abundance (breeding seabirds and non-breeding waterbirds) |
Progress since 2012
Since 2012, we have developed bespoke food web indicators describing fish community size structure and plankton communities in pelagic habitats. The remaining indicators for fish and pelagic habitats and for birds and marine mammals were also assessed under Biological diversity.
Further information
Fish
The Initial Assessment 2012 (HM Government, 2012), of fish biodiversity was based mainly on the Large Fish Index (LFI) of demersal fish. Since 2012, the LFI has been recovering in the Greater North Sea and northern part of the Celtic Seas and assessment thresholds are close to being achieved.
Since 2012, two new OSPAR indicators for demersal fish communities (relating to species composition and size composition) have been developed by the UK and these have been applied to pelagic fish for the first time. These used a newly created data product from surveys in the NE Atlantic. Pelagic shelf fish could be partially assessed for trends but not status.
Birds
The proportion of waterbird species meeting thresholds for non-breeding abundance has declined in both sub-regions and is still on target only in the Greater North Sea. The proportion of seabird species meeting thresholds for breeding abundance has remained stable but below target in both sub-regions. More seabird species in both sub-regions are now experiencing frequent widespread breeding failure. New measures include designation of marine Special Protection Areas offshore and black guillemot Marine Protected Areas (MPAs) in Scotland.
Seals
Grey seals have continued to increase in numbers throughout UK waters since monitoring began.
In the Greater North Sea, harbour seals have increased in English colonies, but have continued to decline in Scottish colonies. Research is underway to try to establish the potential causes of the decline. While the causal mechanisms for the decline have not yet been identified, several factors have been ruled out – fisheries bycatch, pollution, loss of habitat, entanglement in marine debris and legal shooting. In response to the observed declines, monitoring effort has been increased, with some North Sea populations surveyed annually.
In the Celtic Seas, harbour seal numbers have increased in West Scotland, but there are inconclusive indications of declines in two other assessment units and insufficient data in another. Since declines were observed in some harbour seal populations in the North Sea, survey effort has been increased around the UK coast which will provide more data to inform future trends.
Cetaceans
Since 2012, population estimates have been updated by an international survey. Sufficient data are now available to determine that numbers of minke whale have remained stable in the Greater North Sea over the last 20 years. For most other species the most recent estimates of abundance were similar to or larger than previous estimates made in comparable areas.
Pelagic Habitats
Since the Initial Assessment 2012 (HM Government, 2012), new indicators of zooplankton community structure and biomass have been developed. Phytoplankton communities continue to vary considerably over space and time and phytoplankton biomass has continued to increase in most areas.
Outstanding issues
It is likely that changes in the biomass and/or spatial distribution of fish populations, has contributed to the change in prey availability for seabirds and grey seals. However, the full extent of changes in the predator-prey interactions and the importance of climatically driven changes impacting the plankton requires further investigation.
Further information
Interpretation of trends in the indicators of pelagic fish communities was problematic because the underlying relationships between pressure, environment and state are not clear.
Most indicators used in this assessment are new and require further development to integrate results quantitatively across indicators to assess Good Environmental Status.
Fish
Assessments were limited to the scale of the entire sub-region or survey area. Assessments at a finer scale, as evidenced by the differing patterns seen between sub-divisions through the size-composition and species-composition indicators (Typical Length (TyL) and Mean Maximum Length (MML) respectively), would help to underpin the best advice for management of spatially-resolved pressures from activities. It would also provide evidence that could improve our understanding of the changes occurring in populations of higher predators (i.e. seabirds, fish and marine mammals).
Birds
Lower availability of small fish, on which the seabirds feed, has been largely responsible for declines in seabird breeding abundance and frequent widespread breeding failures in some species. There is a lack of understanding about how climate change is driving changes in the food chain that have led to these reductions in food availability.
The decline in wintering waterbird abundance in the Celtic Seas is thought to be part of a north-east shift in their distribution in response to milder winters. Climate change is thought to be driving these changes but the role of additional human impact is unclear.
Seals
The reason(s) for the harbour seal declines in the Greater North Sea are unclear but could include several naturally occurring pressures such as predation (by grey seals and killer whales), competition with grey seals, and exposure to toxins from harmful algae.
Cetaceans
There are still insufficient data for most species to assess trends in the Greater North Sea or more widely at the NE Atlantic scale. For most species, abundance has been estimated only once prior to this assessment.
Pelagic Habitats
It is still unclear to what extent natural variability, climate change, ocean acidification, eutrophication and the impacts of fishing on the food web, may be contributing to changes in abundance, biomass and composition of plankton communities.
Achievement of targets and indicators used to assess progress
Marine food webs are covered by a number of the objectives, targets and indicators for the ecosystem components included in this updated Marine Strategy Part One (HM Government, 2012) (for example, species composition, size and abundance). However, they are complex and are not restricted to the UK parts of the Celtic Seas and the Greater North Sea which makes elaborating the relationships between all parts of the marine food web and assessing its health extremely challenging.
For the current assessments, the progress on food webs has been made using the results described in this section and in the Biodiversity section for fish, birds, seals, cetaceans and pelagic habitats. In both the Greater North Sea and Celtic Seas, plankton communities are experiencing changes in biomass, abundance, and community structure. Deterioration in fish populations has been halted and, in some areas, the size and species structure of fish communities are recovering. Trends in the proportion of large fish in the demersal fish community suggest recovery may continue in most of the areas if current fishing pressures do not increase and if current environmental conditions are maintained. However, breeding seabird populations are declining in abundance and experiencing widespread breeding failures, which for seabird species that feed on small fish (such as sandeels, sprat and herring), is a result of low food availability within their foraging areas. Grey seal numbers are increasing, while harbour seals are declining in some places, but the trends in cetacean populations are less clear.
The components of the food web are clearly changing, but it is unclear how these changes are affecting each other. Signs of recovery in fish communities should ultimately lead to improvements in populations of predatory species at higher trophic levels. Prevailing oceanographic and climatic conditions are likely to be driving these changes in productivity, particularly at the base of the food web. In addition, the cumulative effects of pressure from human activities on the food web are unclear.
Further information
In the Greater North Sea, the UK targets for Productivity of key species, were met in seals but not in seabirds (Table 2). The UK target for Proportion of selected species at the top of food webs showed evidence of recovery in the proportion of large demersal fish (Large Fish Index). This trend is driven strongly by recovery trends in both size-structure (TyL indicator) and species-composition (MML indicator) in the Northern Isles (and also in the Channel) but hampered by long term decreases in the central-west and south-western North Sea (Table 3). The UK targets for Abundance/distribution of key species/trophic groups, were not met for seals and seabirds and it remains uncertain if they were met for pelagic habitats, pelagic fish and cetaceans (Table 4).
In the Celtic Seas, the UK Targets for Productivity of key species, were met in seals and seabirds (Table 2). The UK target for Proportion of selected species at the top of food webs has not yet achieved for demersal fish, but a recovery in the proportion of large demersal fish (Large Fish Index) is underway across large parts of the subregion (Table 3). The recovery trends are driven by increases in both size-structure and species-composition in the Sea of the Hebrides, the Minch, the Celtic Sea and in the Irish Sea, but hampered by declines on the shelf edge and in the Clyde area (Table 3). The UK targets for Abundance/distribution of key species/trophic groups, were not met for seals, seabirds and waterbirds; it remains uncertain if they were met for pelagic habitats, pelagic fish and cetaceans (Table 4).
Table 2: Assessment of targets for the productivity of key species
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Target |
Indicator assessment results |
Target assessment result |
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Greater North Sea |
Celtic Seas |
Greater North Sea |
Celtic Seas |
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Seal Target: There should be no statistically significant decline in seal pup production caused by human activities. |
Grey Seal pup production: the number of grey seals born each year has increased substantially since 1992 and has continued to rise in recent years (from 2009 to 2014). |
Target met |
Target met |
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Bird Target: Widespread seabird colony breeding failures should occur rarely |
Marine bird breeding success/failure: 35% of species experienced frequent widespread breeding failures between 2010 and 2015 |
Marine breeding success/failure: 75% of species did not experience frequent widespread breeding failures between 2010 and 2015 |
Target not met |
Target met |
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Bird Target: Annual breeding success of black-legged kittiwakes should be in line with prevailing climatic conditions. |
Kittiwake breeding success: Target was met at kittiwake colonies on the UK mainland coast of the North Sea, but no colonies passed the assessment in Shetland and Orkney, where the population is in steep decline. |
Kittiwake breeding success: The target was not assessed in the Celtic Seas because we were unable to take prevailing climatic conditions into account and to thus distinguish impacts from human activities. |
Target not met |
Target not assessed |
Table 3: Assessment of targets for the proportion of selected species at the top of food webs
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Target |
Indicator assessment results |
Target assessment result |
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Greater North Sea |
Celtic Seas |
Greater North Sea |
Celtic Seas |
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Demersal shelf fish Target: The size composition of fish communities should not be impacted by human activity such as to indicate any adverse change in trophic function within the communities. |
Proportion of large fish (Large Fish Index - LFI): The proportion of large fish does not currently meet assessment thresholds, but strong recovery of trends is evident and extrapolation of these trends infers that targets could be met by the next cycle. |
Proportion of large fish (Large Fish Index - LFI): The proportion of large fish met thresholds in the north of the sub-region and are approaching recovery in the east, where recovery is strong. The proportion of large fish did not meet thresholds in the south and west, where evidence for recovery is weak. |
Target not met |
Target not met |
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Species composition in Fish Communities (Mean Max Length - MML): Recovery evident in northerly areas, an increase is evident in the Channel but a decrease is seen in south western and central-western North Sea. |
Species composition in Fish Communities (Mean Max Length - MML): Mean Max Length has increased in central and eastern Irish Sea, Sea of Hebrides, the Minch; but decreased along shelf edge and in the Clyde area.
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Size composition in fish communities (Typical Length - TyL): Overall typical length is low compared to 1980s but is recovering due to increases in the north and Channel. |
Size composition in fish communities (Typical Length - TyL): Similar pattern of change to Mean Max length with stronger increase in the northern Celtic Sea to the south-west of the UK. |
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Pelagic shelf fish target: The size composition of fish communities should not be impacted by human activity such as to indicate any adverse change in trophic function within the communities. |
Species composition in Fish Communities (Mean Max Length - MML): No long-term change in northerly areas observed, limited evidence of a decrease in south western and central-western North Sea. Insufficient knowledge of the pelagic fish community to set quantitative targets. |
Species composition in Fish Communities (Mean Max Length - MML): Increase in central Irish Sea extending into Celtic Sea but decrease in many northerly areas west of Scotland. Insufficient knowledge of the pelagic fish community to set quantitative targets. |
Target not assessed |
Target not assessed
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Size composition in fish communities (Typical Length - TyL): Fluctuations observed, but without trend. |
Size composition in fish communities (Typical Length - TyL): No long-term change in most of the sub-region, but decrease in eastern Irish Sea, The Clyde, Sea of Hebrides. |
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Table 4: Assessment of targets for the abundance/distribution of key species/trophic groups
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Target |
Indicator assessment results |
Target assessment result |
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Greater North Sea |
Celtic Seas |
Greater North Sea |
Celtic Seas |
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Birds Target: Changes in abundance of marine birds should be within individual target levels in 75% of species monitored |
Marine bird abundance – Non-breeding waterbirds: Target met in 78% of non-breeding waterbird species. |
Marine bird abundance – Non-breeding waterbirds: Target met in in only 53% of non-breeding waterbird species. |
Target met for non-breeding waterbirds
Target not met for breeding seabirds |
Target not met |
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Marine bird abundance – breeding seabirds: Target met in only 59% of breeding seabird species |
Marine bird abundance – breeding seabirds: Target met in only 63% of breeding seabird species |
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Seals Target: in all of the indicators monitored, there should be no statistically significant decrease in abundance of marine mammals caused by human activities |
Harbour seal abundance: Harbour seal abundance is stable or increasing along the English North Sea coast but has declined along the Scottish North Sea coast. The causes of these declines are currently unknown, although a number of potential factors (for example, fisheries bycatch) have been discounted as contributing to the decline. |
Harbour seal abundance: There was a significant increase in abundance in west Scotland where the majority of seals are, but there was inconclusive evidence of declines elsewhere. |
Target met for grey seals
Uncertain if target met for harbour seals |
Target met for grey seals
Uncertain if target met for harbour seals |
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Grey Seal abundance: Grey seal abundance has been increasing across its range in the UK. |
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Cetaceans Target: in all the indicators monitored, there should be no statistically significant decrease in abundance of marine mammals caused by human activities. |
Inshore bottlenose dolphin: The UK target of ‘no statistically significant decrease in abundance’ was met.
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Inshore bottlenose dolphin: The UK target of ‘no statistically significant decrease in abundance’ was met in the largest group in the Celtic Seas. No assessment has been possible for the other two smaller Celtic Seas groups. |
Uncertain if target met |
Uncertain if target met |
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Harbour porpoise, minke whale, white-beaked dolphin and other species: The UK abundance target of no significant decline was met for, minke whale. The abundance of harbour porpoise and white-beaked dolphin appeared to be stable but uncertainty in the data meant a decline could not be ruled out.
The extent to which UK target has been achieved for other species is unknown. |
Harbour porpoise, minke whale, white-beaked dolphin and other species: There are insufficient data over time to assess if populations of these species have changed in the Celtic Seas (abundance in Celtic Seas has been estimated only once prior to this assessment). |
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Pelagic habitats Target: abundance/distribution of plankton community is not significantly adversely influenced by anthropogenic drivers, as assessed by indicators of changes in plankton functional types (life form) indices. |
Changes in plankton communities: The lifeforms (functional groups of species) that make up the plankton communities changed between the Initial Assessment (2004-2008) and the current assessment period (2009-2014). These changes could have implications for ecosystem functioning. It is unclear whether these changes are a result of pressures from human activities. Therefore, it is not yet possible to say with confidence if the UK target has or has not been met. |
Uncertain if target met |
Uncertain if target met |
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Moving forward
To get a more robust assessment of whether marine food webs are adversely affected by human activities, it will be necessary to address several knowledge gaps and to develop suitable indicators that can provide a more robust assessment. As the food web extends well beyond UK seas, our intention is to do this through OSPAR and build on the recent research outcomes from the Defra funded Marine Environmental Research Programme, which will improve our understanding of the ecosystem processes that underpin the marine food web, how they are responding to environmental change and management scenarios for improving their status.
As our monitoring improves for ecosystem components including birds and mammals, the relationships between trophic levels will become clearer. By using refined ecosystem models, we will be able to evaluate food web status under different environmental and management scenarios.
Further information
Further development of food web indicators will examine change in the biomass of feeding guilds for fish predators to identify changes between fish communities. This will complement the current community indicators that indicate change within fish communities in terms of the size-composition and species-composition indicators. Biomass based feeding guild indicators should help identify predator-prey relationships between fish and both higher levels (seabirds and marine mammals) and lower levels (planktonic production) of the food web. Pelagic fish community indicators shall be further examined to develop understanding of pressure-state relationships. Targets for size-composition and species-composition indicators shall be developed where possible.
The feasibility of developing production indicators shall also be investigated in relation to UK waters, including the OSPAR primary production of plankton pilot-assessment for the North Sea. For fish, datasets on abundance of larvae will be reanalysed with the aim to produce productivity indicators for “keystone species” such as sandeel or key forage fish families such as Clupeidae (includes herring and sprat), that support important prey stocks for seabirds and marine mammals.
Further study of predator-prey interactions between plankton, fish, seabirds and marine mammals shall be undertaken to identify foraging areas supporting top-predators
References
Acknowledgements
Commission Decision 2010/447/EU, REPORT FROM THE COMMISSION TO THE EUROPEAN PARLIAMENT AND THE COUNCIL assessing Member States' monitoring programmes under the Marine Strategy Framework Directive COM/2017/03 final (viewed on 20 March 2019)
HM Government (2012) ‘Marine Strategy Part One: UK Initial Assessment and Good Environmental Status’ (viewed on 5 July 2018)