The Typical Length of the demersal fish community has been adversely impacted by fishing and UK targets are unlikely to have been met in either UK North Sea or UK Celtic Seas despite a recovery in some areas since 2010. In contrast, no change in the Typical Length of the pelagic fish community is evident.

Background

UK Target for size composition in fish communities

This indicator is used to assess progress against the following target, which is set in the UK Marine Strategy Part One (HM Government, 2012): “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 community.

Key pressures and impacts

Over long periods of time, fishing leads to fish communities that are dominated by relatively small and young fish. Not only do fisheries preferentially select larger and older fish that tend to be prime breeders, but the simple reduction in biomass of a fish stock can lead to a truncation of the size-range present in the population. Under high fishing levels, the Typical Length of fish gradually declines as fish communities become dominated by smaller individuals that mature earlier and grow faster. When overfished, the transfer of mass and energy from prey species to larger predators (trophic function) is reduced. Therefore, trends in the Typical Length indicator will reflect a corresponding decline or improvement in the trophic function of the foodweb.

Measures taken to address the impacts

The UK Marine Strategy Part Three (HM Government, 2015) states that all parts of the marine fish community have been impacted by human activities, but there have been recent improvements in the status of some of these fish communities primarily as a result of a reduction in fishing pressure. With the existing policy in place and the introduction of the reformed Common Fisheries Policy, it is expected that there will be a further reduction in the overall fishing pressure reducing fishing impacts on both target and non-target species and sensitive species.

Monitoring, assessment, and regional cooperation

Areas that have been assessed

The assessment was based on data collected by 13 groundfish surveys carried out across two UK Marine Strategy Framework Directive Sub-Regions: Greater North Sea and Celtic Seas. Results from the OSPAR assessment (OSPAR Commission, 2017) in adjacent waters are shown for comparison.

Monitoring and assessment methods

The Typical Length represents the average length of fish (bony fish and elasmobranchs) and provides information on the size composition within communities of fish that are composed of many species. The indicator is calculated using data on the length of fish caught by scientific surveys. Changes in typical length over time were estimated separately for demersal fish species that live on or near the sea floor and for pelagic species that live in the water column.

The lack of data prior to large-scale commercial fishing means it is difficult to determine what Typical Length values have been indicative of sustainable populations of demersal and pelagic fish. The time-series of data for most areas assessed started in the 1980s or 1990s when fishing pressure had been high for some years. For the UK target to be met, Typical Length would be expected to be increasing compared to earlier points in the time-series. If Typical Length is not increasing, further investigation is required to identify if reductions in the size structure of communities are due to human activities, food web interactions or prevailing climatic conditions.

Assessment thresholds

No thresholds were set, but changes in Typical Length were categorised as (1) ‘long-term decrease to a minimum state’, (2) ‘long-term decrease’ (typical length in the current period is lower than in an earlier period but higher than a more recent one), (3) ‘long-term increase’ or (4) ‘no change’. The periods were identified using statistical analyses (see details in the Assessment Method section, below). Additionally, in each case, the minimum value observed over the time-series prior to the last six-years, was considered as a lower limit that should be avoided in future

Regional cooperation

The UK has been the joint lead for this indicator in OSPAR, and the UK results have been used in the OSPAR Intermediate Assessment (OSPAR Commission, 2017).

Further information

The justification for the indicator

The distribution of biomass over body sizes (size spectra; Kerr and Dickie, 2001) is an emergent property of food webs, therefore, size-based metrics that are sensitive and specific to pressures can be used as indicators of food web structure. Jennings and others (2007) found that body size was related to trophic level in fish in the North Sea at the community level (see also Reum and others, 2015). Barnes and others, (2010) demonstrated the relationship between fish size and trophic transfer efficiency. Riede and others (2011) demonstrated that the logarithm of mean body size was significantly related to trophic level in marine invertebrates, ectotherm and endotherm vertebrates using data on multiple ecosystems. Model simulations (Rossberg and others, 2008) have demonstrated that in foodwebs, where trophic interactions dominate over other interactions, large species at high trophic levels are highly sensitive to loss of diversity at lower trophic levels (ICES, 2014a).

Fishing is a size-selective process, therefore, the body size of fish decreases during overexploitation (Boudreau and Dickie, 1992). A gradual, steady decline in Typical Length is expected in response to high fishing pressure because the size structure of the community integrates the impacts of fishing pressure over long periods of time (Rossberg, 2012; Fung and others, 2013). Processes related to increases in sea temperature also serve to reduce body size of fish (Daufresne and others, 2009; Gibert and DeLong, 2014). The indicator can respond to pressures on the marine environment that impact individual fish directly (entrapment activities) or indirectly (through a change in their seabed or pelagic habitat, primary production, and food web interactions). Although species are combined within the habitat-based feeding assemblages, it is possible to compute the indicator for each species individually.

Assessment method

Indicator Metric

The Typical Length metric is the weighted geometric mean length of fish in units of centimetres (cm), with weights given by the standardised catch rate of individuals in an area and defined as shown in Figure 1.

Figure 1. The formula for calculating the Typical Length (TyL) metric. Where Mi is the body mass (standardised to kilogram per unit area fished) of the i-th fish with length Li (cm) in a sample of N fish.

Data for this indicator come from scientific fisheries surveys, which ideally sample the entire fish community but in practice do not. The indicator requires that each survey is conducted at regular intervals (for example, annually) in the same area with standard fishing gear. Sufficiency of sample sizes can be judged using re-sampling techniques (Shephard and others, 2012). The absolute biomass of individuals in length classes present in the environment is not recorded directly by surveys, rather observations are made from samples with detection error (including many false negatives). The detection error is further complicated by differing catchabilities over length classes and species such that the relative abundance between species and length classes observed is different for each of the surveys. Where available, catchability estimates can be used to attempt to correct for this component of the systematic measurement error (for example, Fraser and others, 2007). However, such estimates are sparse in the scientific literature and prone to high uncertainty. In future, the recent catchability corrections estimated by Walker and others (2017) could be considered. Alternatively, model-based estimates of absolute species abundance can be used to rescale observed abundances, but here model uncertainty is also large (ICES, 2014b). For simplicity, Typical Length is measured without correction for detection error and with a varying limitation by species to the size range sampled with each fishing gear. For each survey, this indicator is calculated for subdivisions of the Marine Strategy Framework Directive Sub-Regions that represent different habitats and communities, where possible.

The data are collected under the national programmes and the Data Collection Framework (European Commission, 2008). Currently, the most important data source for Typical Length are those groundfish surveys that are coordinated by the International Council for the Exploration of the Sea (ICES). The International Bottom Trawl Survey programme in the North Sea and the Celtic Seas is particularly important since the trawl is a general-purpose design aimed to catch both demersal and pelagic species. However, beam trawl surveys are more efficient at catching benthivorous species (such as sole) and acoustic surveys, supplemented with pelagic trawling, are more suitable for pelagic species (such as mackerel), and time-series of Typical Length from such surveys may be preferable should sufficient length sampling of fish be made.

Data used and quality assurance

This assessment draws on raw data from the ICES database of trawl surveys (DATRAS). These data have been quality controlled within OSPAR to generate a data product for assessment purposes (Greenstreet and Moriarty, 2017). Time-series of Typical Length for fish and elasmobranchs were derived from each available groundfish survey, where the community was separated into demersal and pelagic habitat-based feeding assemblages.

Time-series of Typical Length by assemblage were determined for 13 surveys (ten using otter trawl and three using beam trawl) that sample within UK waters and are carried out in the Greater North Sea and Celtic Seas (Table 1). Ecological sub-divisions were determined for the North Sea using a simplification of those strata proposed by the EU financed project Towards a Joint Monitoring Programme for the North Sea and Celtic Sea (JMP NS/CS)” that took place 2013 and building upon work of the EU project “Vectors of change in European Marine Ecosystems and their Environmental and Socio-Economic Impacts” that examined the significant changes taking place in European seas, their causes, and the impacts they will have on society. In the Celtic Seas, the strata used in the design of each survey were considered appropriate to represent the ecological sub-divisions.

Table 1. List of groundfish surveys, the region in which they operate, and the period over which they have been undertaken. The survey acronym naming convention consists of (1) first 2–3 capitalised letters indicate the region (CS: Celtic Seas; GNS: Greater North Sea), (2) subsequent capitalised and lowercase letters indicate the country involved (Fra: France; Eng: England; Ire: Republic of Ireland; NIr: Northern Ireland; Sco: Scotland; Ger: Germany; Int: International ICES North Sea bottom trawl survey; Net: Netherlands), (3) two capitalised letters indicate the type of survey (OT: otter trawl; BT: beam trawl), (4) final number indicates the season in which the survey is primarily undertaken (1: January to March; 3: July to September; 4: October to December).

Sub-Region

Survey Accronym1

Survey Period

Celtic Seas

(including OSPAR Wider Atlantic)

CSFraOT42

1997 – 2015

CSEngBT3

1993 – 2015

CSIreOT4

2003 – 2015

CSNIrOT1

1992 – 2015

CSNIrOT4

1992 – 2015

CSScoOT1

1985 – 2016

CSScoOT4

1995 – 2015

WAScoOT3

1999 – 2015

Greater North Sea

GNSEngBT3

1990 – 2015

GNSFraOT4

1988 – 2015

GNSIntOT1

1983 – 2016

GNSIntOT3

1998 – 2016

GNSNetBT3

1999 – 2015

Standard data collected on these surveys consists of numbers of each species of fish sampled in each sample, measured to defined length categories (1 cm below, so a fish with a recorded length of 14 cm would be between 14.00 cm and 14.99 cm in length). By dividing these species catch numbers-at-length by the area swept by the trawl on each sampling occasion, catch data are converted to standardised estimates of fish density-at-length, by species, at each sampling location. However, the indicator is based on biomass rather than abundance, so these abundance densities were converted to biomass density data through the application of species weight at length relationships (of the form weight = aLb, where a and b are species-specific parameters). Density estimates per length category per species based on biomass (kilogram per square kilometre) are referred to as catch per unit area.

These trawl-sample density-at-length estimates were averaged (retaining year, species and length category information) across all trawl samples within each sampling stratum (i.e. survey specific strata following the survey design, which is a rectangular grid in the North Sea and generally depth-based strata elsewhere).

Where multiple surveys were available for assessment, key surveys were prioritised given the length of time series available and spatial coverage of the regional sea in question. If these measures were equal between surveys, then whichever surveyed the greatest biomass by assemblage was selected as key for indicator assessment. Where disagreement between surveys exists in the survey area, these are described in the text. The following surveys were considered key:

Greater North Sea

GNSIntOT1 for both demersal and pelagic assemblages was selected as the key survey (preferred) for the Greater North Sea, given that it is the longest survey with the best spatial coverage. For the eastern English Channel, GNSEngBT3 was preferred for demersal assemblage given more consistent sampling here than GNSIntOT1 and GNSFraOT4. GNSFraOT4 was preferred for the pelagic assemblage in the eastern English Channel given the length of time series available.

Celtic Seas

CSScoOT1 for both demersal and pelagic assemblages was preferred over CSScoOT4 and CSIreOT4 due to the length of the time series. CSIreOT4 for both demersal and pelagic assemblages was preferred for sub-divisions to the west of Ireland and in the northern Celtic Sea, but not in the north where there was overlap with CSScoOT1. CSFraOT4 for both demersal and pelagic assemblages was preferred in the southerly sub-divisions of the Celtic Sea, except where overlap occurred with CSIreOT4.

CSEngBT3 for the demersal assemblage was preferred for the Irish Sea over CSNIrOT1 and CSNIrOT4 given its greater spatial coverage (inclusion of the St Georges Channel stratum to the south of the Irish Sea throughout the time-series). However, as a beam trawl survey CSEngBT3 catches flatfish species such as sole and plaice in preference to roundfish such as cod and haddock. CSNIrOT1 was chosen for the pelagic assemblage was preferred for the Irish Sea over CSNIrOT4 given relatively high biomass of the assemblage caught and identical coverage spatially and temporally.

Trend analysis

The indicator is aggregated at the survey level within each region assessed and complemented by sub-divisional analyses at a scale appropriate to pressures and habitats that can be highly localised. Sub-divisional metrics are aggregated through a weighted average where those weights are given by the total surveyed biomass of relevant assemblage in each sub-division. The long-term trend in each time-series (sub-division and survey level) was modelled through the application of a locally weighted scatterplot smoother with a “fixed span” of one decade and breakpoint analyses were used to define stable underlying periods.

At both sub-divisional level and survey level, breakpoint analyses were used to define stable underlying periods (see Probst and Stelzenmüller, 2015). The method allows us to say whether there is a significant change in the time series state over time such as whether the recent period is significantly different from the historically observed period. The method avoids the arbitrary choice of reference periods for assessment (how many years are used to calculate an average), which can lead to subjective assessments. The shorter the period chosen, the more likely that 'noise' or natural fluctuations in the data are being compared to one another. However, if too long a period is chosen actual changes in state could be averaged out. The minimum detectable period is defined in this analysis as three years. The analysis uses two statistical approaches. First a ‘supremum F test’ is applied to identify if a non-stationary time series or if a constant period for the entire time series is more suitable. If the time series is considered non-stationary, then a second statistical approach, breakpoints analysis, finds periods of at least three years duration.

Results

Findings from the 2012 UK Initial Assessment

This indicator was not considered as part of the UK Initial Assessment (HM Government, 2012).

Trend assessment

Greater North Sea

Overall, the Typical Length of the assessed demersal fish community is low compared to the early 1980s (Figures 2 and 3). Areas of concern, with long-term decreases to lowest observed levels remain in the western and southern North Sea (Figure 2). Nevertheless, the demersal fish community at the Greater North Sea scale is recovering due to recent increases in Typical Length in some subdivisions with a high biomass of fish (Figure 2, left) including the Orkney/Shetland area in the northern North Sea and UK coast in the Channel. The pelagic fish community generally shows fluctuations without trend in the UK part of the North Sea.

Figure 2. Summary of long-term changes in the typical Length of demersal fish (left) and pelagic fish (right) communities in UK waters and surrounding areas (Exclusive Economic Zones shown by a solid black line) in key surveys selected for spatial coverage (Table 1). Assessment period starts in the 1980s or 1990s and ends in 2015 or 2016 depending on the survey (see Figure 3).

Figure 2. Summary of long-term changes in the typical Length of demersal fish (left) and pelagic fish (right) communities in UK waters and surrounding areas (Exclusive Economic Zones shown by a solid black line) in key surveys selected for spatial coverage (Table 1). Assessment period starts in the 1980s or 1990s and ends in 2015 or 2016 depending on the survey (see Figure 3).

Figure 3. Time series of the Typical Length of demersal fish (top plots) and pelagic fish (bottom plots) communities by surveys that sample strata that are located (fully or partially) within UK waters.Figure 3. Time series of the Typical Length of demersal fish (top plots) and pelagic fish (bottom plots) communities by surveys that sample strata that are located (fully or partially) within UK waters.

Figure 3. Time series of the Typical Length of demersal fish (top plots) and pelagic fish (bottom plots) communities by surveys that sample strata that are located (fully or partially) within UK waters.

Celtic Seas

Although the multiple surveys showed mixed signals within the Celtic Seas region for the Typical Length of the demersal fish assemblage (Figure 3), surveys in the north of the area suggest some recovery from previous low states with increases to the west of Scotland (Figure 2, left). However, decreases are also apparent for shelf edge waters to the west. Elsewhere the picture is similarly mixed with decreases near the Irish coast of the Irish Sea (although not evident in the otter trawl surveys and in the Clyde area, but increases to the south of Ireland, Isle of Man, Sea of the Hebrides, and The Minch.

The typical length of pelagic fish generally shows no long-term change at the Sub-Regional level (Figure 3). However, sub-divisional increases are seen to the south of Ireland and decrease in some northerly areas including the Sea of the Hebrides and coastal areas in the Irish Sea (Figure 2, right).

Further information

The summary results for each fish community (Figure 2) draw on multiple analyses for separate surveys. The individual results for surveys that sample UK waters well are shown below (Figures 4-30). A map of sub-divisions is given to identify which are measured by each survey and then these are followed by time-series analyses. Time series are shown for each survey sub-division and aggregated at survey level (title ‘sea’) for demersal and pelagic communities. Each subtitle shows the ‘p’ value for supremum ‘F’ test, which demonstrates whether a significant long- term change is evident (the changes are shown by the red dashed lines when significant or else a mean level is shown for the whole time series using a grey dashed line). Annual estimates are shown by blue circles with a fitted locally weighted scatterplot smoother (black line) with an estimate of spread shown (± one standard deviation). A solid horizontal blue line with minimum observed data point prior to the most recent six data points and two horizontal thin black lines showing the average indicator value for the first and last six years are also shown.

Greater North Sea

Figure 4. North Sea sub-divisions used as a basis for GNSIntOT1, GNSIntOT3, GNSNetBT3 analyses.

Figure 4. North Sea sub-divisions used as a basis for GNSIntOT1, GNSIntOT3, GNSNetBT3 analyses.

Figure 5. GNSIntOT1 demersal fish community.

Figure 5. GNSIntOT1 demersal fish community.

Figure 6. GNSIntOT1 pelagic fish community.

Figure 6. GNSIntOT1 pelagic fish community.

Figure 7. GNSIntOT3 demersal fish community.

Figure 7. GNSIntOT3 demersal fish community.

Figure 8. GNSIntOT3 pelagic fish community.

Figure 8. GNSIntOT3 pelagic fish community.

Figure 9. GNSNetBT3 demersal fish community.

Figure 9. GNSNetBT3 demersal fish community.

Figure 10. English Channel sub-divisions used as a basis for GNSEngBT3 analyses.

Figure 10. English Channel sub-divisions used as a basis for GNSEngBT3 analyses.

Figure 11. GNSEngBT3 demersal fish community in the Channel.

Figure 11. GNSEngBT3 demersal fish community in the Channel.

Figure 12. GNSFraOT4 pelagic fish community in the Channel.

Figure 12. GNSFraOT4 pelagic fish community in the Channel.

Celtic Seas

Figure 13. Irish Sea sub-divisions used as a basis for CSEngBT3, CSNIrOT1 and CSNIrOT4 analyses (Figures k-o). Note that the deep area of Beaufort’s Dyke is shown within the Irish Coast stratum, but this assessment does not represent Beaufort’s Dyke.

Figure 13. Irish Sea sub-divisions used as a basis for CSEngBT3, CSNIrOT1 and CSNIrOT4 analyses (Figures k-o). Note that the deep area of Beaufort’s Dyke is shown within the Irish Coast stratum, but this assessment does not represent Beaufort’s Dyke.

Figure 14. CSEngBT3 demersal fish community.

Figure 14. CSEngBT3 demersal fish community.

Figure 15. CSNIrOT1 demersal fish community.

Figure 15. CSNIrOT1 demersal fish community.

Figure 16. CSNIrOT1 pelagic fish community.

Figure 16. CSNIrOT1 pelagic fish community.

Figure 17. CSNIrOT4 demersal fish community.

Figure 17. CSNIrOT4 demersal fish community.

Figure 18. CSNIrOT4 pelagic fish community.

Figure 18. CSNIrOT4 pelagic fish community.

Figure 19. West of Scotland sub-divisions used as a basis for CSScoOT1 analyses (Figures q-r).

Figure 19. West of Scotland sub-divisions used as a basis for CSScoOT1 analyses (Figures q-r).

Figure 20. CSScoOT1 demersal fish community.

Figure 20. CSScoOT1 demersal fish community.

Figure 21. CSScoOT1 pelagic fish community.

Figure 21. CSScoOT1 pelagic fish community.

Figure 22. West of Scotland sub-divisions used as a basis for CSScoOT4 analyses.

Figure 22. West of Scotland sub-divisions used as a basis for CSScoOT4 analyses.

Figure 23. CSScoOT4 demersal fish community.

Figure 23. CSScoOT4 demersal fish community.

Figure 24. CSScoOT4 pelagic fish community.

Figure 24. CSScoOT4 pelagic fish community.

Figure 25. Celtic Sea sub-divisions used as a basis for CSFraOT4 analyses.

Figure 25. Celtic Sea sub-divisions used as a basis for CSFraOT4 analyses.

Figure 26. CSFraOT4 demersal fish community.

Figure 26. CSFraOT4 demersal fish community.

Figure 27. CSFraOT4 pelagic fish community.

Figure 27. CSFraOT4 pelagic fish community.

Figure 28. Rockall Bank sub-divisions used as a basis for WAScoOT3 analyses.

Figure 28. Rockall Bank sub-divisions used as a basis for WAScoOT3 analyses.

Figure 29. WAScoOT3 demersal fish community.

Figure 29. WAScoOT3 demersal fish community.

Figure 30. WAScoOT3 pelagic fish community.

Figure 30. WAScoOT3 pelagic fish community.

Conclusions

The UK target has probably not been met in the UK North Sea nor the UK Celtic Seas. In the North Sea, the Typical Length of demersal fish remains relatively low, but recovery since 2010 is underway, which is driven by increases in the northern North Sea. Long-term decreases, since the 1980s, were evident in the southern and central North Sea, implying that the demersal fish communities there are more dominated by small-bodied fish at present. In contrast, no change was evident in the North Sea pelagic fish community. Within the UK waters of the Celtic Seas either no change or long-term increases were observed throughout much of the area (with the exception of one survey). Some decreases were detected at smaller scales. Recovery appears underway in the demersal fish communities since 2000 in the West of Scotland and Rockall Bank area.

Given that the methodology is new there is moderate/low confidence in the method for this assessment and high confidence in the data availability.

Further information

The Typical Length of fish and elasmobranch community responds to changes in the dynamics in the size distribution across the full assemblage including both large and small fish, yet the indicator is still robust to outliers in the data. Typical Length can be directly compared across geographical regions, and the indicator can be computed for pelagic or demersal species. The sub-divisional strata are a useful means to capture local patterns in indicators for specific benthic and water column habitats and the often-local impacts of pressures.

Within the North Sea there are clear sub-divisional differences where the demersal and pelagic assemblages in the northerly areas are recovering, while the southerly areas continue to decline. For the Celtic Seas, decreases in the demersal fish assemblage appear greatest at the shelf edge, while decreases in pelagic fish occur in coastal areas. While fishing may have contributed to this depletion, it is unclear whether warming temperatures have led to increases in small-bodied fish (immature fish) or small species.

Key surveys were chosen for the assessment of each sub-region. Assessment of additional surveys generally confirmed the overall conclusions, with the exception of the Irish Sea. Further details of these conclusions, by survey are given:

  • The increase in the Typical Length of demersal fish since 2010 in the Greater North Sea, evident in the International Bottom Trawl Survey from quarter 1 survey, was also shown in the quarter 3 survey (GNSIntOT1 and GNSIntOT3), while the more spatially restricted groundfish surveys showed no significant change (GNSNetBT3, GNSEngBT3).
  • For pelagic fish, no change was evident in the two the International Bottom Trawl Surveys in the North Sea.
  • Within the Celtic Seas, increases in typical length of demersal fish were evident to the west of Scotland in two surveys (CSScoOT1 since a low value in 2010 and CSScoOT4 since 2005),
  • Increases were evident for the Irish Sea overall in two surveys (CSNIrOT1 and CSNIrOT4 since 2010) with no change in a third (CSEngBT3) and disagreement between surveys was greatest in the Irish coast subdivision.
  • The pelagic fish in the Celtic Seas generally showed no significant changes in typical length, except for a decrease in the Irish Sea in one survey (CSNIrOT4 in 1998).
  • An overall increase since 2002 in the typical length of demersal fish in the Rockall Bank was significant while no change in the typical length of pelagic fish was evident.

Knowledge gaps

Further work is required to evaluate appropriate baselines and thresholds for this indicator. This work is needed because any historical baseline for the fish and elasmobranch community is likely to represent an impacted state. Thresholds should preferably be identified through multi-species modelling.

The use of beam trawl data for demersal fish community assessment and otter trawl data for pelagic fish community assessment is new and requires further investigation to improve the assessment.

Further information

The setting of assessment values for this indicator should consider their relation to the Common Fisheries Policy, targets aiming at Maximum Sustainable Yield and in relation to other fish community indicators. Until more comprehensive investigations are complete, the minimum observed Typical Length in the available time-series can be considered as a precautionary limit for the indicator. If indicator scores are at a minimum observed state, a positive (increasing) trend should be evident to avoid falling below the limit.

While reductions in fishing pressure in recent years appear to be stimulating improvements in the size structure of demersal fish in some areas, it should not be forgotten that the OSPAR maritime area has also warmed significantly recently (IPCC, 2014). These prevailing conditions may mean that the species composition is changing. Climatic influences in the North Eastern Atlantic have been reported to lead to the northward expansion of small pelagic fish (Alheit and others, 2014) and are likely to have significant effects on recruitment of fish stocks and thus potentially recovery times for communities (Beggs and others, 2014). Since Lusitanian (warm-water southern) species tend to be smaller bodied than boreal (cold-water northern) species, the size-structure may require longer than expected to recover to its historical values, if possible.

The importance of potential density-dependent growth effects for species (for example, Marshall and Frank, 1999) at the community level are unknown.

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Acknowledgements

Assessment Metadata

Please contact marinestrategy@defra.gov.uk for metadata information

Recommended reference for this indicator assessment

Lynam, C.P.1, Moriarty M.2, & Greenstreet, S.P.R.2 2018. Size composition in fish communities (Typical Length) *.  UK Marine Online Assessment Tool, available at: https://moat.cefas.co.uk/biodiversity-food-webs-and-marine-protected-areas/fish/size-composition/

* Adapted from OSPAR Intermediate Assessment 2017 on Size Composition in Fish Communities

1Centre for Environment, Fisheries and Aquaculture Science

2Marine Scotland