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.
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.
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
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).
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.
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.
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.
(including OSPAR Wider Atlantic)
1997 – 2015
1993 – 2015
2003 – 2015
1992 – 2015
1992 – 2015
1985 – 2016
1995 – 2015
1999 – 2015
Greater North Sea
1990 – 2015
1988 – 2015
1983 – 2016
1998 – 2016
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.
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.
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.
Findings from the 2012 UK Initial Assessment
This indicator was not considered as part of the UK Initial Assessment (HM Government, 2012).
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.
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).
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
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.
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.
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.
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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Barnes C, Maxwell D, Reuman DC, Jennings S (2010) ‘Global patterns in predator–prey size relationships reveal the size dependency of trophic transfer efficiency’ Ecology, 91(1):222-232 (viewed on 22 November 2018)
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Daufresne M, Lengfellner K, Sommer U (2009) ‘Global warming benefits the small in aquatic ecosystems’ Proceedings of the National Academy of Sciences of the United States of America, 106 (31):12788-12793 (viewed on 22 November 2018)
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|Assessment Type||UK MSFD Indicator Assessment|
D4 Food webs
Size composition in fish communities
|Point of contact firstname.lastname@example.org|
|Metadata date||Thursday, August 1, 2019|
|Title||Fisheries survey data from Research Vessels|
The Groundfish Survey Monitoring and Assessment Data Product data derived by Marine Scotland from data collected during Research Vessel surveys, co-ordinated by International Council for the Exploration of the Seas (ICES), from January 1983 to June 2017 for surveys of the northeast Atlantic shelf and marginal seas.
Acquisition during Research Vessel cruises
Sampling device: otter and beam trawls
Manual for the data product used in the assessment
Greenstreet, S.P.R and Moriarty, M. (2017) Manual for Version 3 of the Groundfish Survey Monitoring and Assessment Data Product. Scottish Marine and Freshwater Science Vol 8 No 18, 77pp. DOI: 10.7489/1986-1
Moriarty, M., Greenstreet, S.P.R. and Rasmussen, J. (2017) Derivation of Groundfish Survey Monitoring and Assessment Data Product for the Northeast Atlantic Area. Scottish Marine and Freshwater Science Vol 8 no 16, 240pp. DOI: 10.7489/1984-1
|Conditions applying to access and use|
© Crown copyright, licenced under the Open Government Licence (OGL).
The Groundfish Survey Monitoring and Assessment (GSMA) data product is a single set of fully standardised and quality assured data products for all the surveys operating in the Northeast Atlantic
Moriarty, M., Greenstreet, S. 2017. Greater North Sea Dutch Beam Trawl Quarter 3 Groundfish Survey Monitoring and Assessment Data Products. DOI: 10.7489/1967-1
Moriarty, M., Greenstreet, S. 2017. Greater North Sea International Otter Trawl Quarter 1 Groundfish Survey Monitoring and Assessment Data Products. DOI: 10.7489/1922-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea Irish Quarter 4 Otter Trawl Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1925-1
Moriarty, M., Greenstreet, S. 2017. Greater North Sea International Otter Trawl Quarter 3 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1923-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea Scottish Otter Trawl Quarter 4 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1924-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea Scottish Quarter 1 Otter Trawl Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1957-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea /Bay of Biscay French Quarter 4 Otter Trawl Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1958-1
Moriarty, M., Greenstreet, S. 2017. Greater North Sea French Otter Trawl Quarter 4 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1959-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea Northern Ireland Otter Trawl Quarter 1 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1961-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea Northern Ireland Otter Trawl Quarter 4 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/10.7489/1962-1
Moriarty, M., Greenstreet, S. 2017. Celtic Sea English Quarter 3 Beam Trawl Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1964-1
Moriarty, M., Greenstreet, S. 2017. Greater North Sea German Beam Trawl Quarter 3 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1965-1
Moriarty, M., Greenstreet, S. 2017. Greater North Sea English Beam Trawl Quarter 3 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1966-1
Moriarty, M., Greenstreet, S. 2017. Bay of Biscay Iberian Coast Portugal Otter Trawl Quarter 4 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1963-1
Moriarty, M., Greenstreet, S. 2017. Wider Atlantic Scottish Otter Trawl Quarter 3 Groundfish Survey Monitoring and Assessment Data Products. doi: 10.7489/1960-1
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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/
1Centre for Environment, Fisheries and Aquaculture Science