The UK target has only been met in the northern part of the Celtic Seas. In central parts of the Celtic Seas where assessment thresholds have yet to be reached, strong recovery trends are also evident. However, in the southern Celtic Seas region, the Large Fish Index shows little or slow recovery. In the Greater North Sea, assessment thresholds have yet to be achieved, but strong recent recovery trends in the Large Fish Index suggest that assessment thresholds could be met by 2022.

Background

UK Target on the proportion of large fish

This indicator is used to assess progress against two targets, the first a target for biodiversity and the second for food webs, which were set in the UK Marine Strategy Part One (HM Government, 2012):

  1. “The size-composition of fish communities should reflect a healthy status and not be significantly impacted by human activity.”
  2. “The size composition of fish communities should not be impacted by human activity such as to indicate an adverse change in trophic function within the community: A specified proportion (by weight) of fish in any defined marine region should exceed a stipulated length threshold”

Key pressures and impacts

Fishing mortality constrains the age structure of fish communities, reducing the proportion of older individuals. Fishing is also size-selective, preferentially removing larger and older fish, and therefore fundamentally affects fish community size composition.

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 OSPAR Regions: the Greater North Sea and the Celtic Seas.

Monitoring and assessment methods

The Large Fish Index has been developed to respond to fishing pressure on the proportion of large fish in demersal fish communities (species living on or near the sea floor). The Large Fish Index uses estimates of fish abundance-at-length provided by international bottom trawl surveys. These are standardised monitoring programmes that occur each year at the same time of the year taking representative samples according to specific guidelines. The assessment of each survey involves determining the suite of species constituting the demersal fish community and the length criteria defining large fish. The proportion of the demersal fish biomass that has a length exceeding the length criteria can then be calculated.

Assessment thresholds

Specific assessment thresholds are derived from the time series used for each regional assessment.

Regional cooperation

The UK has been the indicator lead for the Large Fish Index in the recent OSPAR Intermediate assessment process (OSPAR Commission, 2017), the UK results of which are presented here.

Further information

The Large Fish Index was developed to support the Ecological Quality Objective for the Greater North Sea (Greenstreet and others, 2011; 2012). The index uses estimates of fish biomass density-at-length provided by the first quarter international bottom trawl survey. Firstly, the suite of species constituting the ‘demersal fish community’ is established. Secondly, the length value (LLF) defining ‘large fish’ is determined. The mathematical derivation of the Large Fish Index based on these values is shown in Figure 1.

Equation for calculating the Large Fish Index (LFI) from the biomass density (B, in kilogram per square kilometre: kg kg-2) of demersal fish at a given length (L, in centimetres), and the length value defining “large fish” (LLF).

Figure 1. Equation for calculating the Large Fish Index (LFI) from the biomass density (B, in kilogram per square kilometre: kg kg-2) of demersal fish at a given length (L, in centimetres), and the length value defining “large fish” (LLF).

The methodology to derive the Large Fish Index for the Greater North Sea has subsequently been adapted to derive Large Fish Indices for the Celtic Seas (Shephard and others, 2011) and the Northern Iberian coastal shelf (Modica and others, 2014). For example, in the initial Greater North Sea study, the length defining “large fish” was set at 40 cm, whereas in the Celtic Seas and Northern Iberian coast this length is 50 cm and 35 cm, respectively. To establish targets, these studies have either used actual Large Fish Index values observed at a time when the Large Fish Index I had yet to be adversely impacted by fishing, or they have used the observed relationship between the Large Fish Index and fishing mortality to estimate Large Fish Index values consistent with sustainable mortality across a suite of species. In the North Sea and Celtic Sea studies the former approach suggested Large Fish Index target values of 0.3 and 0.4 respectively, whereas in the Northern Iberian Coast the latter approach inferred a target value of 0.35. Thus, rather than applying a prescribed Large Fish Index definition to other areas and data-sets, a prescribed methodology has instead been used to derive area- or data-specific Large Fish Indices for each new situation.

In all three studies, the relationship between variation in fishing mortality and the Large Fish Index response was time-lagged, with the Large Fish Index response following as much as 16 years after changes in fishing mortality. Modelling studies have since confirmed that recovery of demersal fish community size composition was likely to occur only over decadal time-scales (Fung and others, 2013; Shephard and others, 2013; Spiers and others, 2016). In the three studies where the Large Fish Index protocol has been established, these time-lagged responses suggest that the necessary fisheries management measures required for the Large Fish Index to reach the assessment thresholds stipulated for each community have already been implemented and that these assessment thresholds should be achieved within ten-years with no further management action necessary. However, a subsequent study has suggested that the recovery of the Large Fish Index in the Greater North Sea may have faltered (Fung and others, 2012).

The assessment of the Greater North Sea Ecological Quality Objective on the proportion of large fish (OSPAR Commission, 2009) showed that from the early 1980s, the percentage of fish greater than 40 cm declined from around 30% to a low point of less than 5% in 2001. By 2008 it had recovered to around 22%, but the need to make further progress to reach the level of the Ecological Quality Objective (30%) was recognised.

Assessment method

Indicator metric and data collection

To assess the state of demersal fish size composition across the North East Atlantic Region, Large Fish Index I time-series were determined for 13 groundfish surveys carried out across two separate regions: The Greater North Sea and Celtic Seas. Between six and seven Large Fish Index time-series were derived for each region (Table 1).

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). *Survey CSFraOT4 is a single survey that operates in the Celtic Seas and the Bay of Biscay and Iberian Coast regions, from the southern coast of the Republic of Ireland and down the western Atlantic Coast of France and was split into its two regional components for the purposes of the Large Fish Index assessment.

Region

Survey Acronym

Survey Period

Celtic Seas

CSFraOT4*

1997-2015

CSEngBT3

1993–2015

CSIreOT4

2003–2015

CSNIrOT1

1992–2015

CSNIrOT4

1992–2015

CSScoOT1

1985–2016

CSScoOT4

1995–2015

Greater North Sea

GNSEngBT3

1990–2015

GNSFraOT4

1988–2015

GNSGerBT3

2002–2015

GNSIntOT1

1983–2016

GNSIntOT3

1998–2016

GNSNetBT3

1999–2015

Standard data collected on these surveys comprises numbers of each species of fish sampled in each trawl 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, the catch data are converted to estimates of fish density-at-length, by species, at each sampling location in each year. Summing these trawl-sample species density-at-length estimates across all trawl samples collected within each sampling stratum in each year (ICES statistical rectangles) and dividing by the number of trawl samples within each stratum per year, gives an estimate of the density of each species and length category within each sampling stratum in each year. Summing these sample stratum density estimates across all sampling strata sampled in each year, and dividing by the number of strata sampled, provides estimates of the average density (denoted N), of each species (denoted S) and length category (denoted l), in each year, across the whole area covered by the survey. It is these last density estimates (denoted Ns,l / km2 where ‘/ km2 indcates per unit area in km2) that are used in the calculation of each survey’s Large Fish Index time-series. However, the Large Fish Index is based on biomass rather than abundance, so abundance densities are converted to biomass density data by applying species weight at length relationships. These relationships are of the form weight = alb where a and b are species-specific parameters.

Deciding the species to be included in the analysis is the first step in calculating each survey’s Large Fish Index time-series. FishBase provided an ‘ecotype’ classification for all species encountered. The Large Fish Index has been designed as an indicator of size composition within demersal fish communities, so species assigned to the Pelagic and Bathypelagic ecotypes were automatically excluded, while species assigned to the Demersal and Bathydemersal ecotypes were automatically included. Species belonging to the Benthopelagic ecotype were generally included, but with the exceptions of Clupea harengus, Dysomma brivirostre, Hyperoplys immaculatus, Ammodytidae (family-level ID code), Salmo (genus-level ID code), Salmo salar and Sarpa salpa, which were all excluded on the basis that these species are relatively poorly sampled by the survey gears.

The appropriate length that defined “large fish” for each survey had next to be determined. Traditionally this has been achieved by fitting 5th-degree polynomial functions to Large Fish Index time-series derived using a range of “large fish” lengths, from 20 cm to LLF 50 cm, where the LFILLF=l is calculated by the equation shown in Figure 2, and selecting the Large Fish Index time-series with the best fit. In five instances the Large Fish Index time-series with the best fit polynomial was not selected to support the assessment because resultant Large Fish Index values were either too high or too low so that either too great a fraction or too small a fraction of fish were classified as ‘large fish’. Where this happened the actual signal in the Large Fish Index I trend was markedly diminished, and an alternative Large Fish Index time-series that addressed this issue was used instead (Table 2). The proportion of total biomass at longer than the “large fish” length becomes too small to derive a usable indicator if this length is set above 50 cm; resulting indicators are too strongly influenced by the ‘noise’ associated with variation in the biomass of fish < 50cm in length.

Figure 2. Equation for calculating the length of “large fish” for the Large Fish Indicator (LFILLF=l). For more details see (for example) Greenstreet and others, 2011.

Table 2. Determination of the length of “large fish” by defining ‘large fish’ for each survey. Values shown are regression (R2) values for 5th-degree polynomial fits each Large Fish Index time-series. Value indicated by * are the best fit for each survey. Values indicated by + are the Large Fish Index time series selected to support assessment where this was not the best 5th-degree polynomial fit.

Survey

“Large Fish” length (cm)

20

25

30

35

40

45

50

CSFraOT4

0.327

0.375+

0.327

0.330

0.329*

0.332

0.307

CSEngBT3

0.213

0.288

0.415

0.449*

0.422

0.355

0.236

CSIreOT4

0.477

0.574

0.724*

0.650

0.551

0.499

0.491

CSNIrOT1

0.473

0.580

0.578

0.711

0.740

0.759*

0.739

CSNIrOT4

0.405

0.520

0.686

0.728

0.743*

0.730

0.665

CSScoOT1

0.622

0.677

0.740

0.783*

0.760

0.697

0.635

CSScoOT4

0.446

0.552

0.678

0.683

0.691*

0.666

0.628

GNSEngBT3

0.680

0.403

0.273

0.310

0.422

0.443

0.532*

GNSFraOT4

0.461

0.426

0.451

0.450

0.512

0.523

0.538*

GNSGerBT3

0.283

0.259

0.297*

0.277

0.284

0.275

0.313+

GNSIntOT1

0.257

0.435

0.583

0.770

0.781

0.803

0.829*

GNSIntOT3

0.441

0.421

0.623

0.753

0.737

0.813

0.891*

GNSNetBT3

0.240

0.447

0.467*

0.275

0.256

0.475+

0.416

Spatial scope: assessment units

A single Large Fish Index time-series was selected for each survey, and individual survey-based assessments were performed. The individual survey-based assessments within each region were then considered to determine an overall assessment outcome for each region.

Baselines

The published study in the Greater North Sea (Greenstreet and others, 2011) established the early 1980s as the reference period for setting the OSPAR Ecological Quality Objective for the Greater North Sea first quarter international bottom trawl survey Large Fish Index. This period was selected on the basis that it was the last time that the International Council for the Exploration of the Sea (ICES) advice for setting fish quotas was to maintain status quo; such as to carry on fishing at prevailing levels because stocks were not considered to be unduly depleted (Greenstreet and others, 2011). Selection of this reference period led to the adoption of a threshold of Large Fish Index above 0.3 as the Ecological Quality Objective for the Greater North Sea (Heslenfeld and Enserink, 2008). In the Celtic Sea study, Shephard and others (2011) noted a period of relatively stable Large Fish Index values during the late 1980s and early 1990s, which included the six highest Large Fish Index values recorded. The stability of the Large Fish Index, combined with these high values, led Shephard and others (2011) to adopt this period as their reference period, and consequently to suggest a threshold of Large Fish Index above 0.4 as a possible Ecological Quality Objective for the Celtic Sea.

Assessment thresholds

Where feasible, the studies discussed above were drawn on to set assessment thresholds for as many of the 13 groundfish survey Large Fish Index time series as possible. In doing so, a number of approaches were applied.

In the Greater North Sea, the principle surveys used to support assessments were the GNSIntOT1 and GNSIntOT3. The GNSIntOT1 is the same survey analysed by Greenstreet and others (2011), so the same reference period of the early 1980s was used. The GNSIntOT1 and GNSIntOT3 surveys were strongly correlated, so the linear regression obtained was used to determine an assessment threshold for the GNSIntOT3 that was equivalent to the assessment threshold for the GNSIntOT1. Absolute assessment thresholds could not be set for the remaining four Greater North Sea surveys, so a recovery trends-based approach was adopted.

In the Celtic Seas, the UK west coast groundfish survey analysed by Shephard and others (2011) ceased in 2004 and so was no longer available for assessment purposes. However, this analysis could be used to inform the assessment thresholds setting for the current extant surveys. The lowest Large Fish Index value reported by Shephard and others (2011) during the reference period was 0.42, so this was adopted as the assessment threshold rather than their suggested value of 0.40. Generally, low values were observed between 1994 and 2004, and within this period the lowest 5-year moving average of 0.138 was observed between 2000 and 2004. The assessment threshold of 0.42 was three times this lowest 5-year moving average value. So, the assessment threshold setting procedure for extant otter trawl surveys in the Celtic Seas region was as follows:

  1. Determine the lowest 5-year moving average Large Fish Index value.
  2. Identify the period that this 5-year low moving average Large Fish Index related to. If between 1994 and 2004, then this was deemed a valid 5 -year low Large Fish Index.
  3. Set the assessment threshold as three times the 5-year low moving average Large Fish Index value.

Where the setting of absolute assessment thresholds was not feasible, a trends-based approach was adopted assuming that the demersal fish community in question had been impacted by fishing and that a recovery trend (increasing Large Fish Index values) was, therefore, necessary to rectify the harm done. Table 3 summarises the assessment threshold setting approach used and the resulting assessment thresholds for each survey.

Table 3. Approaches to setting assessment thresholds (AT) applied to each survey. The 5-year low moving average time-period refers to 1996-2001 except for survey SCFraOT4 for which it is 1996-2000.

Survey

Method used to determine assessment value (AV)

AV

CSFraOT4

Three times valid 5-year low moving average Large Fish Index value (0.222)

0.67

CSEngBT3

Trends-based, recovery trend necessary.

positive trend

CSIreOT4

Trends-based, recovery trend necessary.

positive trend

CSNIrOT1

Three times valid 5-year low moving average Large Fish Index value (0.075)

0.23

CSNIrOT4

Three times valid 5-year low moving average Large Fish Index value (0.159)

0.48

CSScoOT1

Three times valid 5-year low moving average Large Fish Index value (0.216)

0.65

CSScoOT4

Three times valid 5-year low moving average Large Fish Index value (0.152)

0.46

GNSEngBT3

Trends-based, recovery trend necessary.

positive trend

GNSFraOT4

Trends-based, recovery trend necessary.

positive trend

GNSGerBT3

Trends-based, recovery trend necessary.

positive trend

GNSIntOT1

Reference period 1983 with Large Fish Index value of 0.198. Mean Large Fish Index value between 1983 and 1985: 0.195.

0.20

GNSIntOT3

Correlated with survey GNSIntOT1, with Large Fish Index equal to 1.098 times that of GNSIntOT1.

0.22

GNSNetBT3

Trends-based, recovery trend necessary.

positive trend

Results

Findings from the 2012 UK Initial Assessment

The Large Fish Index was considered in the Initial UK Assessment (HM Government, 2012), however, this latest assessment provides a retrospective view based on improved datasets which extend the time series end-point from 2011 to 2016.

Latest findings

Status assessment

Surveys in the Celtic Seas only covered relatively small parts of that region. The assessment, therefore, relied on the compilation of individual survey-based assessment outcomes. The proportion of large demersal fish meets the assessment threshold in the northern half of the region (Figure 3 plots e and f), but not in other parts of the Celtic Seas region. Recovery is evident in the Irish Sea and Large Fish Index assessment thresholds could be achieved there by 2020 (Figure 3 plots c and d). In the south and west of this region, evidence of a recovery in Large Fish Index values is unclear (Figure 3 plots a, b and f), and at best suggests that it could take a decade or more before the assessment threshold is reached unless current levels of pressure are reduced.

Figure 3. Trends in the large fish index in thirteen individual groundfish surveysin the Greater North Sea and Celtic Seas using the optimal length value determined to define large fish for each survey. Plots are colour coded to reflect the region (yellow –Celtic Seas; blue - Greater North Sea).

The assessment of the Greater North Sea which relied primarily on two surveys (Figure 3, plots j and k), conducted at different times of the year, suggest that the proportion of large fish does not currently meet assessment thresholds (see Figures 4 and 5). However, both surveys show recovery and that size composition has recovered strongly from its most disturbed condition at the turn of the century. Extrapolation of current recovery trends in these two surveys suggests that Large Fish Index assessment thresholds should be met before the next Marine Strategy Framework Directive assessment in 2024. These results are generally supported by the other surveys.

Figure 4. Large Fish Index time series derived from the first quarter international otter trawl survey in the Greater North Sea (Figure 1 plot j, (January to March)) showing annual data, data used to determine the recovery trend, the linear trend fitted to the recovery period data, and the Large Fish Index assessment threshold.

Figure 5. Large Fish Index (LFI) time series derived from the third quarter international otter trawl survey in the Greater North Sea (Figure 3 Plot k, (July to September)) showing annual data used to determine the recovery trend, linear trend fitted to the recovery period data and the Large Fish Index assessment threshold.

There is medium/to high confidence in the assessment methodology and high confidence in the data availability.

Trend assessment

Trends in the Large Fish Index in each of the 13 individual groundfish surveys that were assessed are shown in Figure 3.

Further information

Assessment outcome summary

Celtic Seas

The demersal fish community size composition in the Celtic Seas is currently at an acceptable status in the north of the region and showing strong recovery in the east which is potentially sufficient to meet Large Fish Index assessment thresholds by 2019. In the south and west of the Celtic Seas, assessment thresholds are not met and evidence for recovery is not convincing. At best it is likely to require a decade or more before assessment thresholds are achieved in these areas. Further restriction of fishing activity may be necessary for the south and west, and elsewhere there is little scope for any expansion in fishing activity levels.

Greater North Sea

The assessment was principally dependent on two otter trawl surveys (surveys GNSIntOT1, GNSIntOT3) in the region, but was generally supported by the other surveys. Demersal fish community size composition in the Greater North Sea is currently below assessment thresholds. However, strong recovery trends are evident, and extrapolation of these trends infers that assessment thresholds could be met by 2022. Time lags involved in the Large Fish Index response to changing fishing pressure could imply that the necessary measures have been implemented.

Celtic Seas individual survey assessments

Only one continuing survey Large Fish Index time-series, derived from the CSScoOT1 survey, correlated closely with the now extinct UK West Coast Groundfish Surveys Large Fish Index time-series analysed by Shephard and others (2011) that was used to derive the original Large Fish Index for the Celtic Seas. This was the only survey with a start date similar to that of the UK West Coast Groundfish Surveys, thus the CSScoOT1 time-series fully encompassed that of the UK West Coast Groundfish Surveys. All other surveys started six to 17 years later than the UK West Coast Groundfish Surveys, and because the UK West Coast Groundfish Surveys ceased approximately ten years before the end of the extant survey times series, in many cases the period of overlap was limited, reducing the likelihood of close correlations. Assuming the UK West Coast Groundfish Surveys assessment threshold of 0.42, the linear regression with the CSScoOT1 survey suggested an equivalent assessment threshold of 0.43 for the CSScoOT1 survey (Figure 6).

Figure 6. Correlation between the UK West Coast Groundfish Surveys and CSIntOT1 Large Fish Index time series.

Figure 6. Correlation between the UK West Coast Groundfish Surveys and CSIntOT1 Large Fish Index time series.

Five of the six extant otter trawl surveys operating in the Celtic Seas had a five-year average low Large Fish Index period within the 11 years between 1994 to 2004 and so were considered suitable for application of the three-times multiplier to derive assessment thresholds. The last Large Fish Index values in the time series only exceeded these assessment thresholds derived in this way in the CSScoOT1 and CSScoOT4 surveys, both of which operate in the northern part of the region (Table 4). The final Large Fish Index values in the time series meet their respective assessment thresholds in only two surveys, CSScoOT1 and CSScoOT4, suggesting that management objectives for demersal fish community size composition have only been achieved in the northern part of the Celtic Seas.

Table 4. Assessment outcomes for five otter trawl surveys with valid 5year low moving average Large Fish Index periods and so eligible for assessment against assessment thresholds derived as 3-times the 5-year low moving average Large Fish Index value.

Survey

5-year low period

5-year low valid (yes/no)

5-year low Large Fish Index

Assessment Value

Final year Large fish Index

CSIreOT4

2006-2010

no

0.333

none

not applicable

CSNIrOT1

1996-2001

yes

0.075

0.23

0.194

CSNIrOT4

1994-1998

yes

0.159

0.48

0.381

CSScoOT1

2000-2004

yes

0.216

0.65

0.655

CSScoOT4

2000-2004

yes

0.152

0.46

0.499

CSFraOT4

1999-2003

yes

0.222

0.67

0.257

Recovery periods were defined as starting the year after the lowest Large Fish Index value was observed. For four surveys, linear trends fitted to the recovery period Large Fish Index data suggested that assessment thresholds would be met prior to the next Marine Strategy Framework Directive assessment phase in 2024, and for three surveys, assessment thresholds would be met prior to 2020 (Figure 7). These surveys are in effect two pairs, where each pair covers a different part of the Celtic Seas region. But within each pair, the surveys are separated temporally suggesting that assessment outcomes are independent of survey seasonality, corroborating similar evidence obtained in the Greater North Sea. The four surveys ultimately meeting assessment thresholds cover the area to the north and east of Ireland. The surveys failing to meet assessment thresholds cover the area to the south and west of Ireland. Survey Large Fish Index trends in the last 15 to 20 years suggest that assessment thresholds for the demersal fish community will be achieved within the next decade in the north and east of the Celtic Seas but not the south and west.

Figure 7. Large Fish Index time series derived from the CSScoOT1, CSScoOT4 CSNIrOT1, CSNIrOT4 CSFraOT4 and CSIreOT4 surveys showing annual data, fitted 5th-degree polynomial, data used to determine the recovery trend, linear trend fitted to the recovery period data and the Large Fish Index assessment threshold where available.

One beam trawl survey, CSEngBT3, operates in the southern part of the Celtic Seas region. The survey commenced too recently to include a suitable reference period for the provision of guidance as to an appropriate assessment threshold. Because this is a beam trawl survey, use of the three-times multiplier on the five-year average lowest Large Fish Index period derived from Shephard and others (2011) was not considered appropriate. A trends-based recovery target would appear to have been met (Figure 8), and as such, this survey also tends to corroborate the recent positive trend observed in the CSIreOT4 survey. However, like CSIreOT4, the recovery rate is slow, with an annual Large Fish Index increase of just 0.0056. Given the lowest observed Large Fish Index value in the time series of 0.25 in 2000, this recovery rate suggests that several decades might be required before any substantive increase in the size composition of the demersal fish community sampled by this survey is achieved. Analysis of the only beam trawl survey operating in the Celtic Seas does not alter the earlier conclusion. Demersal fish size composition in the southern part of the Celtic Seas is not likely to meet assessment thresholds and is only recovering slowly.

Figure 8. Large Fish Index time-series derived from the CSEngBT3 survey showing annual data, fitted 5th-degree polynomial, data used to determine the recovery trend and the linear trend fitted to the recovery period data.

Greater North Sea individual survey assessments

GNSIntOT1 survey

  • Length of “large fish” value set at 50cm
  • 1983 Large Fish Index value is 0.1986 and average the average Large Fish Index value over the 3-years period 1983 to 1985 is 0.1951
  • Assessment threshold is therefore 0.2
  • Large Fish Index value in 2016 is 0.164 and average Large Fish Index value over the period 2014-2016 is 0.171
  • The Greater North Sea Large Fish Index is currently below its assessment threshold
  • Large Fish Index low point (0.044) occurred in 2001
  • Strong recovery from 2002 onwards with Large Fish Index increasing by 0.073 per year on average
  • Linear trend fitted to recovery period suggests the 0.2 assessment threshold should be met by 2020 (Figure 9)
  • Residuals from this trend in the last 3-years of the time series are all positive suggesting a high level of confidence that the assessment threshold will be achieved by 2024
  • This conclusion is consistent with the predictions of Greenstreet and others (2011)

Figure 9. Large Fish Index time series derived from the GNSIntOT1 (January to March) survey showing annual data, fitted 5th- degree polynomial, data used to determine the recovery trend, linear trend fitted to the recovery period data and the Large Fish Index assessment threshold.

GNSIntOT3

  • The assessment based on the GNSIntQ3 survey corroborates the GNSIntOT1 assessment outcome
  • Length of “large fish” value set at 50 cm
  • The GNSIntOT3 Large Fish Index time-series correlated with GNSIntOT1 LFI time-series with a 1-year offset (Figure 10), so that quarte 1 in a given year is approximately equivalent to quarter three of the previous year. This relationship implies that both surveys are influenced by the same recruitment event. This gives a relationship of LFIGNSIntOT3 = 1.098LFIGNSIntOT1, which implies a GNSIntOT3 Large Fish Index assessment threshold of 0.22;
  • The Large Fish Index value in 2016 is 0.211 and the average value over period 2014 to 2016 is 0.198
  • The Greater North Sea Large Fish Index is currently below its assessment thresholds
  • The Large Fish Index low point of 0.041 occurred in 2000;
  • Strong recovery from 2001 onwards with the Large Fish Index increasing by 0.098 per year on average
  • Linear trend fitted to recovery period suggests the 0.2 assessment threshold should be met by 2020 (Figure 11)
  • Residuals from this trend in the last 3-years of time series are mostly positive suggesting a high level of confidence that assessment thresholds will be achieved by 2018
  • The Large Fish Index from 2015 (0.191) is less than that of 2016 (0.211), which because of the 1-year offset, suggests that the GNSIntOT1 value for 2017 value will also increase over the its value.

Figure 10. Correlation between the GNSIntOT1 and GNSIntOT3 Survey Large Fish Index time series.

Figure 10. Correlation between the GNSIntOT1 and GNSIntOT3 Survey Large Fish Index time series.

Figure 11. Large Fish Index time series derived from the GNSIntOT3 (July to September) survey showing annual data, fitted 5th- degree polynomial, data used to determine the recovery trend, linear trend fitted to the recovery period data and the Large Fish Index assessment threshold.

Other Greater North Sea surveys

Other than between the two near-regional scale international bottom trawl surveys, the correlation between Large Fish Index time-series derived from groundfish surveys operating in the Greater North Sea was low. A number of factors contributed to this, including:

  • Variation in spatial coverage between surveys;
  • Surveys operating in different parts of the region (for example, the GNSFraOT4 survey is principally in the English Channel);
  • Variation in the extent of area covered (. for example, the GNSGerBT3 only covers 20 ICES statistical rectangles);
  • Data concerns with the last 2 years (or first 26 years) of the GNSFraOT4 survey.

The principle Greater North Sea Assessment, therefore, relies on the GNSIntOT1 and GNSIntOT3 surveys.

Excluding the last two years of the GNSFraOT4 survey, the GNSFraOT4 and GNSEngBT3 surveys show lowest Large Fish Index values in the mid-1990s with substantial increases to the current time. The GNSGerBT3 and GNSNetBT3 surveys indicate variation in Large Fish Index, but show no substantial improvement in recent years over Large Fish Index values from the early 2000s.

Overall Outcome Summary

Demersal fish community size composition in the Greater North Sea is currently below assessment thresholds. The two surveys supporting the principle assessment both suggest that size composition has recovered strongly from its most disturbed condition at the turn of the century. Extrapolation of current recovery trends in these two surveys suggests that Large Fish Index assessment thresholds should be met before the next Marine Strategy Framework Directive assessment in 2024. Time lags involved in the Large Fish Index response to changing fishing pressure and imply that the necessary management to meet these assessment thresholds is already in place. There would thus no need for further restriction of fishing activity. However, because assessment thresholds have not been achieved, neither is there any evidence to support a case for any relaxation of the current fisheries management regime.

Confidence assessment

There is medium/high confidence in the assessment methodology, and the majority of the assessment method has been published in peer-reviewed literature and previously used in some regions. However, there are some new elements on assessment threshold setting that have been developed specifically for this assessment. There is high confidence in the data availability, no significant data gaps and sufficient spatial coverage.

Conclusions

The UK target has only been met in the northern part of the Celtic Seas. The assessments show that the recovery in the proportion of large fish in the demersal fish community reported in the Greater North Sea in the Greater North Sea Ecological Quality Objective report (OSPAR Commission, 2009) and the UK Initial Assessment (HM Government, 2012) has continued for the period up to 2015. Recovery is also evident in a large part of the Celtic Seas region. Assessment thresholds are met only in the northern part of the Celtic Seas. Elsewhere assessment thresholds could be achieved by 2022 if the current pressures levels are not increased.

Some exceptions were observed, notably in the south and west of the Celtic Seas, where evidence of recovery was lacking, or the recovery rate was so slow that it might take more than a decade before demersal fish size composition achieves the assessment threshold, unless current levels of pressure are reduced. The time-lagged response of the Large Fish Index to variation in fishing pressure means that where assessment thresholds are met, or where they are not met but recovery is underway, measures necessary to restore demersal fish size composition are probably already in place. Additional measures may be required where assessment thresholds are not met, and evidence of recovery trends is either absent or weak. However, since Large Fish Index assessment thresholds have generally yet to be achieved over most of the area assessed, any relaxation of management would be premature.

Knowledge gaps

Knowledge gaps for this assessment are:

  1. The lack of empirical data (including historical data) or appropriate models to inform Large Fish Index assessment thresholds.
  2. The lack of a single universal protocol to determine the optimal length value defining ‘large fish’ for all surveys.
  3. The effects of warming sea temperature on Large Fish Index assessment thresholds and demersal fish size composition recovery rates.
  4. Full understanding of relationships between all human pressures and their impact on the full demersal fish community, not just the commercial stocks.

Future development of the indicator should explore the potential of spatial sub-divisional assessments.

Further information

The availability of survey data extending early enough back in time to be considered suitable as reference periods makes the use of an empirical approach to set assessment thresholds for the majority of the currently extant groundfish surveys extremely difficult. To support assessment in the future, a modelling approach for determining suitable Large Fish Index assessment thresholds will be necessary. Simple statistical models incorporating the relationship between variation in average community level fishing mortality and the Large Fish Index I response have proved useful in the past in demonstrating that the management measures necessary to achieve Large Fish Index assessment thresholds are already in place (Greenstreet and others, 2011; 2012; Shephard and others, 2011; Modica and others, 2014). However, a more rigorous approach to assessment threshold setting is likely to be required. Several community models that have been used to explore the nature of the response of the Large Fish Index to variation in fishing pressure on demersal fish communities, but in many instances these involve ‘simulated’ communities (Fung and others, 2013; Shephard and others, 2013); that do not exist and which can shed little light on specific assessment thresholds for specific survey-derived Large Fish Index time-series. What is required are models that are parameterised to represent specific fish communities, sampled by particular surveys, and subjected to specified patterns of fishing activity, such as the model developed for the North Sea demersal fish community by Spiers and others (2016).

The three major studies that have been principally responsible for the development and application of the Large Fish Index across the Northeast Atlantic Region all fitted a 5th-degree polynomial function to a range of Large Fish Index series, each using a different length value to define ‘large fish’ in order to find the large fish length that produced the Large Fish Index series with the highest signal to noise ratio (the Large Fish Index series with the best fit; Greenstreet and others, 2011; Shephard and others, 2011; Modica and others, 2014). However, for several of the surveys analysed to support the current assessment, this approach was considered unsatisfactory. Rather than finding the optimum signal to noise ratio, the Large Fish Index series best fitted by a 5th-degree polynomial function appeared to have had almost all the signal eliminated along with the noise. In these cases, a more subjective approach to selecting the optimum Large Fish Index series had to be adopted. To support future assessment, effort should be directed towards developing a single protocol for determining the optimum length of “large fish” that can be applied universally.

A further knowledge gap relates to the effects of climate change on the Large Fish Index. Demersal fish size composition can only recover from a perturbed state through natural population dynamics processes operating at a population level and cascading through communities. Communities of large-bodied species will only regain a more natural size composition through the production of relatively large recruitment cohorts that are subsequently allowed the time to grow. In many organisms, individuals in colder environments grow more slowly but eventually become larger as adults. This widespread pattern is embodied by two well-established rules: Bergmann's rule, which describes the association between temperature and body size in natural environments, and which gives rise to gradients of increasing body size with latitude, and the temperature-size rule, which describes reaction norms relating temperature to body size in laboratory experiments (Angilletta and Dunham, 2003). In recent years, rising sea temperatures in the North Sea, associated with climate change have been linked to an increase in demersal fish species richness (Hiddink and ter Hofstede, 2008; ter Hofstede and others, 2010; Simpson and others, 2011) attributed to an influx of, or an increase in the abundance of, species with a southern Lusitanian biogeographic affinity (Beare and others, 2004a; 2004b; 2004c). Because of Bergmann’s rule, these Lusitanian species tend to be smaller bodied than their northern Boreal counterparts (Genner and others, 2004; 2010). Increased abundance of smaller-bodied Lusitanian species associated with rising sea temperature holds implications for Large Fish Index assessment thresholds, particularly in the more northerly regions of the north-east Atlantic (Queirós and others, 2018). In the North Sea, cod and saithe accounted for approximately 60% of the biomass of ‘large’ fish (Greenstreet and others, 2011). The decline in the cod stock through the 1980s was responsible for a large fraction of the coincidental reduction in the Large Fish Index. Recent and future recovery of the Large Fish Index I could depend heavily on future cod and saithe biomass trajectories. Both species are Boreal, and population size changes and distribution shifts have been documented, particularly for cod, that might suggest that the future prognosis for both species in the North Sea might not be too optimistic. In particular, both species have experienced marked and prolonged reductions in recruit production linked to increases in sea temperature, hindering recovery in their stock biomass, and potentially slowing recovery in the Large Fish Index.

Finally, to advise on the most effective management measures to achieve management objectives for the Large Fish Index, relationships underpinning the Large Fish Index response to variation in the human activities that impact demersal fish communities need to be well understood. All three studies responsible for the development of the Large Fish Index in the north-east Atlantic examined the relationship between the Large Fish Index and a metric of community averaged fishing mortality. In reality, this mortality metric was a weighted average of the annual fishing mortality on the few assessed demersal fish stocks in each study area. To date, few studies have attempted to estimate fishing mortality rates on non-commercial species (Piet and others, 2009), but such information could contribute much to a better understanding of how fishing affects the Large Fish Index.

References

Angilletta MJ Jr, Dunham AE (2003) ‘The temperature-size rule in ectotherms: simple evolutionary explanations may not be general’ American Naturalist, 162:332-342 (viewed on 22 November 2018)

Beare D, Burns F, Greig A, Jones EG, Peach K, Kienzle M, McKenzie E, Reid DG (2004a) ‘Longterm increases in the prevalence of North Sea fish having southern biogeographic affinities’ Marine Ecology Progress Series, 284:269-278 (viewed on 22 November 2018)

Beare DJ, Burns F, Peach K, Portilla E, Greig A, McKenzie E, Reid DG (2004b) ‘An increase in the abundance of anchovies and sardines in the north-western North Sea since 1995’ Global Change Biology 10:1209-1213 (viewed on 22 November 2018)

Beare DJ, Burns F, Peach K, Reid DG (2004c) ‘Red mullet migration into the northern North Sea during late winter’ Journal of Sea Research 53:205-212 (viewed on 22 November  2018)

Fung T, Farnsworth KD, Reid DG, Rossberg AG (2012) ‘Recent data suggest no further recovery in North Sea Large Fish Indicator’ ICES Journal of Marine Science 69:235-239 (viewed on 22 November 2018)

Fung T, Farnsworth KD, Shephard S, Reid DG, Rossberg AG (2013) ‘Why the size structure of marine communities can require decades to recover from fishing’ Marine Ecology Progress Series 484:155-171 (viewed on 22 November 2018)

Genner MJ, Sims DW, Southward AJ, Budd GC, Masterson P, McHugh M, Rendle P, Southall EJ, Wearmouth VJ, Hawkins SJ (2010) ‘Body size-dependent responses of a marine fish assemblage to climate change and fishing over a century-long scale’ Global Change Biology 16:517-527 (viewed on 22 November 2018)

Genner MJ, Sims DW, Wearmouth VJ, Southall EJ, Southward AJ, Henderson PA, Hawkins SJ (2004) ‘Regional climate warming drives long‐term community changes of British marine fish’ Proceedings of the Royal Society of London Series B Biological Sciences, 271:655-661 (viewed on 22 November 2018)

Greenstreet SPR, Rogers SI, Rice JC, Piet GJ, Guirey EJ, Fraser HM, Fryer RJ (2011) ‘Development of the EcoQO for fish communities in the North Sea’ ICES Journal of Marine Science 68:1-11 (viewed on 22 November 2018)

Greenstreet SPR, Rogers SI, Rice JC, Piet GJ, Guirey EJ, Fraser HM, Fryer RJ (2012) ‘A reassessment of trends in the North Sea Large Fish Indicator and a re-evaluation of earlier conclusions’ ICES Journal of Marine Science 69:343-345 (viewed on 22 November 2018)

Heslenfeld P, Enserink EL (2008) ‘OSPAR Ecological Quality Objectives: the utility of health indicators for the North Sea’ ICES Journal of Marine Science 65:1392-1397 (viewed 22 November)

Hiddink JG, ter Hofstede R (2008) ‘Climate-induced increases in species richness of marine fish’ Global Change Biology 14:453-460 (viewed on 22 November 2014)

HM Government (2012) ‘Marine Strategy Part One: UK Initial Assessment and Good Environmental Status’ (viewed on 5 July 2018)

HM Government (2015) ‘Marine Strategy Part Three: UK Programme of Measures’ December 2015  (viewed on 5 July 2018)

Modica L, Velasco F, Preciado I, Soto M, Greenstreet SPR (2014) ‘Development of the large fish indicator and associated target for a Northeast Atlantic fish community’ ICES Journal of Marine Science 71:2403-2415 (viewed on 22 November 2018)

OSPAR Commission (2009) ‘Evaluation of the OSPAR system of Ecological Quality Objectives for the North Sea (update 2010)’ Publication Number 406/2009 (viewed on 22 November 2018)

OSPAR Commission (2017) ‘Intermediate Assessment 2017’ (viewed on 21 September 2018)

Piet GJ, van Hal R, Greenstreet SPR (2009) ‘Modelling the direct impact of bottom trawling on the North Sea fish community to derive estimates of fishing mortality for non-target fish species’ ICES Journal of Marine Science, 66:1985-1998 (viewed on 22 November 2018)

Queirós A.M., Fernandes J., Genevier L., Lynam C.P. 2018. ‘Climate change alters fish community size‐structure, requiring adaptive policy targets’ Fish and Fisheries 19(4):613-621 (viewed on 22 November 2018)

Simpson SD, Jennings S, Johnson MP, Blanchard JL, Schon P-J, Sims DW, Genner MJ (2011) ‘Continental shelf-wide response of a fish assemblage to rapid warming of the sea’ Current Biology 21:565-1570 (viewed on 22 November 2018)

Shephard S, Reid DG, Greenstreet SPR (2011) ‘Interpreting the Large Fish Indicator for the Celtic Sea’ ICES Journal of Marine Science 68:1963-1972 (viewed on 22 November 2018)

Shephard S, Fung T, Rossberg AG, Farnsworth K, Reid DG, Greenstreet SPR, Warnes S (2013) ‘Modelling recovery of Celtic Sea demersal fish community size-structure’ Fisheries Research 140:91-95 (viewed on 22 November 2018)

Spiers DC, Greenstreet SPR, Heath MR (2016) ‘Modelling the effects of fishing on the North Sea fish community size composition’ Ecological Modelling 321:35-45 (viewed on 22 November 2018)

ter Hofstede R, Hiddink JG, Rijnsdorp AD (2010) ‘Regional warming changes fish species richness in the eastern North Atlantic Ocean’ Marine Ecology Progress Series 414:1-9 (viewed on 22 November 2018)

Acknowledgements

Assessment Metadata

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

Recommended reference for this indicator assessment

Greenstreet, S.P.R.1, Moriarty, M.1 & Lynam, C.P.2 2018. Proportion of large fish (Large Fish Index)*. UK Marine Online Assessment Tool, available at: https://moat.cefas.co.uk/biodiversity-food-webs-and-marine-protected-areas/fish/large-fish-index/

* Adapted from OSPAR Intermediate Assessment 2017 on Proportion of Large Fish

1Marine Scotland

2Centre for Environment, Fisheries and Aquaculture Science