Size Composition in Fish Communities

D4 - Marine Food Webs

D4.2 - Proportion of selected species at the top of food webs

The Typical Length indicator measures the size-structure of fish and elasmobranch communities and it decreases under high fishing pressure. Although low compared to the 1980s, Typical Length for the assessed demersal fish has been recovering across the OSPAR Maritime Area since 2010. Pelagic fish generally show fluctuations but no trend. Locally, there are deviations from these patterns.

Area Assessed

Printable Summary

Background

OSPAR’s strategic objective with respect to biodiversity and ecosystems is to halt and prevent further loss in biodiversity, protect and conserve ecosystems and to restore, where practicable, ecosystems, which have been adversely impacted by human activities.

The Typical Length indicator is one of three food-web indicators currently used by OSPAR. It represents the average length of fish (bony fish and elasmobranchs) and provides information on the size composition within communities of fish. The indicator is calculated using catch data from species sampled by scientific surveys and given in units of centimetres. Communities are represented by habitat-based feeding assemblages (groups of fish): namely, demersal assemblages (i.e. species living on or near the sea floor) and pelagic assemblages (i.e. species living in the water column).

Fishing mortality constrains the age structure of fish populations, reducing the proportion of larger individuals (Figure 1). A gradual, steady decline in Typical Length is expected in response to high fishing pressure. This is because the size structure of the fish assemblage integrates the impacts of fishing pressure over long periods of time. Model simulations demonstrate that in food webs where predator-prey interactions dominate over other interactions, large species at high trophic levels (the position of the species within the food web) are highly sensitive to loss of diversity at lower trophic levels.

Figure 1: A large-bodied Atlantic Wolffish (courtesy of Jim Ellis)

Fishing mortality constrains the age structure of fish communities, reducing the proportion of larger / older individuals. Fishing is also size-selective, preferentially removing larger / older fish, and therefore fundamentally affects fish community size composition. So far three indicators relating to fish size have been developed to assess impacts of fishing on healthy fish communities and the foodweb, considering similar but different parameters.

The distribution of biomass over body size (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 et al. (2007) found that body size was related to trophic level in fish in the North Sea at the community level (see also Reum et al., 2015). Barnes et al. (2010) demonstrated the relationship between fish size and trophic transfer efficiency. Riede et al. (2011) demonstrated that log-mean body size was significantly related to trophic level in marine invertebrates, and ectotherm and endotherm vertebrates using data on multiple ecosystems. Model simulations (Rossberg et al., 2008) have demonstrated that in food webs 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 fish body size 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 et al., 2013). Processes related to rising sea temperature also serve to reduce body size of fish (Daufresne et al., 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 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 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 Typical Length (TyL) is the weighted geometric mean length of fish, with weights given by the standardised catch rate of individuals in an area and defined as follows:

Formula a. Typical Length indicator

where Mi is the body mass (standardised to kilogrammes per unit area fished) of the i-th fish with length Li (in units of centimetres) 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 surveys are conducted at regular intervals (e.g. annually) in the same area with a standard fishing gear. Sufficiency of available sample sizes can be judged using re-sampling techniques (Shephard et al., 2012, Lynam and Rossberg, 2017). 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 survey specific. Where available, catchability estimates can be used to attempt to correct for this component of the systematic measurement error (e.g. Fraser et al., 2007). However, such estimates are sparse in the scientific literature and prone to great uncertainty. In future, the recent catchability corrections estimated by Walker et al. (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 great (ICES, 2014b). For simplicity, Typical Length is defined with reference to a particular sampling design with a varying limitation to the size range sampled by fishing gear. For each survey, this indicator is calculated for sub-divisions that represent different habitats and communities, where possible.

The data are collected under the national programmes and the Data Collection Framework (EC, 665/2008). Currently, the most important data sources for Typical Length are those groundfish surveys that are conducted through the International Council for the Exploration of the Sea (ICES). The International Bottom Trawl Survey (IBTS) programme in the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast 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 (Figure a) and acoustic surveys, supplemented with pelagic trawling, are more suitable for pelagic species such as mackerel (Figure b) and time series of Typical Length from such surveys may be preferable should sufficient length sampling of fish be made.

Figure a: Common sole (Solea solea) (courtesy of Hans Hillewaert)

Figure b: Atlantic Mackerel (Scomber scombrus) (courtesy of Jacek Lesinowski)

Data Used and Quality Assurance

The assessment draws on raw data from the ICES database of groundfish surveys (DATRAS, www.ices.dk/marine-data/data-portals/Pages/DATRAS.aspx). These data have been quality controlled by OSPAR as part of this assessment process to generate a data product for assessment purposes. Time series of Typical Length for fish and elasmobranchs are derived from each available groundfish survey, where the community is separated into demersal and pelagic habitat-based feeding assemblages.

Time series of Typical Length by assemblage were determined for 20 surveys carried out across four separate sub-regions: the Greater North Sea, Celtic Seas, Bay of Biscay and Iberian Coast, and Wider Atlantic (Table a). Ecological sub-divisions were determined for the Greater 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 in 2013, and building upon work in the EU VECTORS 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 other OSPAR regions, the strata from the survey design were considered appropriate to represent the ecological sub-divisions. The survey areas and sub-divisions are shown in Figure c to Figure r. Details on the hauls (samples) for each sub-division in different years has been presented in Table b to Table x.

Standard data collected on these surveys consists of numbers of each species of fish sampled in each sample, measured to defined length categories (i.e. so a fish with a recorded length of 14 cm would be between 14.00 cm and 14.99 cm in length). By dividing the species catch numbers-at-length by the area swept by the trawl on each sampling occasion, these 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 have to be converted to biomass density data by applying species weight (w) at length relationships (of the form w = aLb, where a and b are species-specific parameters). Density estimates per length category per species based on biomass (kg per km2) are referred to below as catch-per-unit-area (CPUA).

These trawl-sample density-at-length estimates are 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 Greater North Sea and generally depth-based strata elsewhere).

Table a: Groundfish surveys, region in which they operate, and period over which they were undertaken
Sub-region Survey Accronym1 Survey period
Greater North Sea GNSEngBT3 1990–2015
GNSFraOT4 1988–2015
GNSGerBT3 2002–2015
GNSIntOT1 1983–2016
GNSIntOT3 1998–2016
GNSNetBT3 1999–2015
Celtic Seas CSFraOT42 1997–2015
CSEngBT3 1993–2015
CSIreOT4 2003–2015
CSNIrOT1 1992–2015
CSNIrOT4 1992–2015
CSScoOT1 1985–2016
CSScoOT4 1995–2015
Bay of Biscay and Iberian Coast BBIC(n)SpaOT4 1990–2014
BBIC(s)SpaOT1 1993–2014
BBIC(s)SpaOT4 1997–2014
BBICPorOT4 2002–2014
CSBBFraOT42 1997–2015
Wider Atlantic WAScoOT3 1999–2015
WASpaOT3 2001–2014

1. Survey acronym convention: first two to four capitalised letters indicate the European Union Marine Strategy Framework Directive (MSFD) sub-region (BBIC: Bay of Biscay and Iberian Coast; CS: Celtic Seas; GNS: Greater North Sea; WA: Wider Atlantic). Next capitalised and lower case letters signify the country involved (Spa: Spain; Por: Portugal; Fra: France; Eng: England; Ire: Republic of Ireland; NIr: Northern Ireland; Sco: Scotland; Ger: Germany; Int: International; Net: The Netherlands).

International refers to the two international groundfish surveys carried out in the Greater North Sea under the auspices of ICES. In the Bay of Biscay and Iberian Coast sub-region, Spanish surveys are further delimited by (n) for surveys operating in the northern Iberian coast area and (s) for surveys operating in the southern Iberian coast area).

Next two capitalised letters indicate the type of survey (OT: otter trawl; BT: beam trawl). Final number indicates the season in which the survey is primarily undertaken (1: January to March; 3: July to September; 4: October to December).

2. This is a single survey that operates across both the Celtic Seas and the Bay of Biscay and Iberian Coast sub-regions, from the southern coast of the Republic of Ireland and down the western Atlantic coast of France. For assessment purposes this single survey was split into its two sub-regional components.

Data Treatment

Surveys with rectangular sampling grids (GNSIntOT1, GNSIntOT3, GNSNetBT3, GNSGerBT3, GNSFraOT4)

Catch per unit swept area (CPUA) data (kg / km2) from multiple hauls are averaged by species for each rectangular grid cell using the strata below; in the Greater North Sea these are ICES statistical rectangles, in the eastern English Channel a mini-grid (0.25° by 0.25°) is used by GNSFraOT4. The resulting rectangle-based CPUA estimates are multiplied by the area (km2) of their rectangles (using a Lambert equal area projection) to give species biomass-at-length (now measured in kg per rectangle). Sub-divisional strata level (not GNSFraOT4) estimates of biomass-at-length are given by the sum of the rectangle-based biomass-at-length estimates and corrected by a scaling factor = 1 / (proportion of the area of sub-division monitored in the survey year) (units are now tonnes per sub-division). The scaling factor correction ensures that the weighting of the strata relative to each other in each year is not altered by the sampling levels. Sub-divisional estimates of Typical Length are calculated at this point for investigating local responses of each assemblage.

Regional estimates of biomass-at-length are estimated from the sum of sub-divisions (or in the case of GNSFraOT4 by the rectangle-based estimates). Typical Length is calculated from these data to give a survey level assessment within each region.

Figure c: Greater North Sea surveys with rectangular grids and sub-divisions used (note only GNSIntOT1 and GNSIntOT3 cover every sub-division appropriately)

The colours differentiate the different spatial sub-divisions used.

Table b: Hauls (samples) from in the GNSIntOT1 survey generating data used in the assessment
Year Central West Kattegat & Skaggerak North East Orkney& Shetland South East South West Total
1983 64 33 62 35 140 42 376
1984 69 36 82 56 159 51 453
1985 81 31 94 65 179 61 511
1986 70 38 91 65 201 57 522
1987 88 43 94 61 187 58 531
1988 65 35 87 55 114 45 401
1989 64 42 90 38 147 43 424
1990 68 41 85 43 101 38 376
1991 69 37 99 46 123 47 421
1992 63 42 77 38 79 39 338
1993 53 40 75 49 110 40 367
1994 71 43 79 37 81 46 357
1995 64 44 76 28 79 41 332
1996 61 45 71 40 70 37 324
1997 63 42 80 38 88 43 354
1998 69 41 76 46 113 51 396
1999 69 42 73 37 83 47 351
2000 69 41 80 42 98 47 377
2001 75 42 74 42 125 64 422
2002 82 43 73 44 107 63 412
2003 81 42 73 46 99 65 406
2004 69 42 71 44 89 52 367
2005 73 46 72 46 93 52 382
2006 73 41 73 45 93 49 374
2007 63 42 67 40 90 46 348
2008 65 42 68 47 99 43 364
2009 64 42 76 45 98 44 369
2010 70 41 71 47 98 48 375
2011 67 39 68 44 98 49 365
2012 66 42 68 47 88 46 357
2013 65 42 66 47 84 49 353
2014 61 39 43 36 81 45 305
2015 64 42 66 47 82 48 349
2016 64 44 63 41 79 43 334
Table c: Hauls (samples) from in the GNSIntOT3 survey generating data used in the assessment
Year Central West Kattegat & Skaggerak North East Orkney & Shetland South East South west Total
1998 54 40 54 31 58 31 268
1999 71 42 77 50 77 35 352
2000 68 75 46 82 38 309
2001 58 41 77 41 74 37 328
2002 60 42 75 47 73 33 330
2003 60 42 70 42 70 35 319
2004 66 42 74 48 71 35 336
2005 54 42 80 43 73 35 327
2006 60 41 69 43 72 35 320
2007 55 40 69 46 66 35 311
2008 54 40 69 39 79 34 315
2009 54 41 45 35 63 34 272
2010 58 41 71 46 60 32 308
2011 58 43 73 44 66 34 318
2012 56 41 70 38 64 35 304
2013 56 40 75 46 54 33 304
2014 58 42 75 42 63 34 314
2015 60 43 74 46 69 35 327
2016 66 45 87 51 69 39 357

Figure d: Spatial coverage by the Netherlands groundfish survey

The colours differentiate the different spatial sub-divisions used.

Table d: Hauls (samples) from in the GNSNetBT3 survey generating data used in the assessment
Year Central West North East Orkney & Shetland South East South West Total
1999 21 11 2 93 18 145
2000 21 12 2 92 19 146
2001 21 12 4 83 10 130
2002 22 11 2 93 19 147
2003 21 10 3 94 22 150
2004 21 10 4 97 19 151
2005 22 11 4 100 22 159
2006 22 11 4 91 16 144
2007 22 10 4 94 16 146
2008 21 12 3 82 14 132
2009 21 12 3 88 15 139
2010 22 6 3 64 14 109
2011 21 12 4 73 15 125
2012 22 11 4 94 21 152
2013 22 12 3 86 14 137
2014 23 11 3 65 15 117
2015 22 12 4 91 17 146

Figure e: Spatial coverage by the German groundfish survey

The colours differentiate the different spatial sub-divisions used.

Table e: Hauls (samples) from in the GNSGerBT3 survey generating data used in the assessment
Year North East South East South West Total
2002 13 28 3 44
2003 13 28 2 43
2004 13 38 2 53
2005 5 38 2 45
2007 13 32 2 47
2008 13 29 2 44
2009 14 40 2 56
2010 13 39 2 54
2011 13 40 2 55
2012 12 38 2 52
2013 12 40 2 54
2014 2 26 2 30
2015 13 40 2 55

Figure f: Spatial coverage by the French channel otter trawl survey

The colours differentiate the different spatial sub-divisions used.

Table f: GNSFraOT4 samples used in the assessment
Year Total Year Total
1988 66 2002 88
1989 61 2003 90
1990 69 2004 83
1991 75 2005 101
1992 54 2006 94
1993 59 2007 85
1994 80 2008 92
1995 79 2009 90
1996 50 2010 77
1997 80 2011 93
1998 73 2012 83
1999 89 2013 84
2000 89 2014 94
2001 97 2015 72
Surveys with irregular depth banded strata (i.e. all surveys other that those with rectangular sampling grids above)

Catch per unit swept area (CPUA) data (kg per km2) from multiple hauls averaged by species for each survey strata. Sub-divisional estimates of biomass-at-length are subsequently given by CPUA multiplied by area of the survey strata (km2, using a Lambert equal area projection). Sub-divisional estimates of Typical Length are calculated at this point for investigating local responses of each assemblage.

Regional sea estimates of biomass-at-length are estimated from the sum of sub-divisional estimates. Typical Length is calculated here for regional sea assessment.

Figure g: Depth strata for GNSEngBT4

The colours differentiate the different sub-divisions used.

Table g: GNSEngBT4 samples used in the assessment
Year French coast <25 m Mid-Channel UK coast <25 m Total
1990 25 22 19 66
1991 30 21 19 70
1992 28 30 17 75
1993 28 25 19 72
1994 29 26 19 74
1995 29 26 21 76
1996 30 28 18 76
1997 29 24 20 73
1998 30 28 19 77
1999 26 23 22 71
2000 30 21 21 72
2001 29 29 33 91
2002 28 28 28 84
2003 29 24 20 73
2004 26 24 20 70
2005 22 17 19 58
2006 26 27 17 70
2007 26 23 21 70
2008 24 26 19 69
2009 25 25 21 71
2010 23 20 20 63
2011 24 25 16 65
2012 22 20 18 60
2013 23 24 17 64
2014 21 26 18 65
2015 23 22 16 61

Figure h: Depth strata for Irish Sea surveys (CSEngBT3, CSNIrOT1 and CSNIrOT4) note only CSEng BT3 includes St George’s Channel

The colours differentiate the different sub-divisions used.

Table h: CSEngBT3 samples used in the assessment
Year Irish Coast, <50 m Isle of Man, 50–100 m St George's Channel <100 m Eastern Irish Sea, <50 m Total
1993 9 10 19 49 87
1994 6 14 15 30 65
1995 6 14 15 30 65
1996 6 14 14 32 66
1997 6 14 16 30 66
1998 6 13 15 30 64
1999 6 13 15 30 64
2000 6 14 13 29 62
2001 6 12 15 30 63
2002 6 13 16 30 65
2003 6 12 14 30 62
2004 6 12 16 30 64
2005 6 13 15 29 63
2006 6 12 15 30 63
2007 6 12 15 30 63
2008 6 13 12 29 60
2009 6 12 15 30 63
2010 6 12 16 30 64
2011 6 12 16 29 63
2012 6 14 16 29 65
2013 6 14 16 29 65
2014 6 14 14 29 63
2015 6 13 15 30 64
Table i: CSNIrOT1 samples used in the assessment
Year Irish Coast, < 50 m Isle of Man, 50–100 m Eastern Irish Sea, <50 m Total
1992 11 14 10 35
1993 18 16 10 45
1994 19 13 8 40
1995 19 15 8 42
1996 18 11 9 38
1997 19 14 7 40
1998 19 16 9 44
1999 19 15 9 43
2000 19 16 11 46
2001 19 17 10 46
2002 21 16 11 50
2003 19 16 11 49
2004 18 15 11 44
2005 19 11 8 38
2006 19 14 11 44
2007 19 16 11 46
2008 19 16 11 47
2009 19 15 11 47
2010 19 15 11 48
2011 18 15 11 47
2012 19 15 11 47
2013 19 16 11 49
2014 19 16 11 49
2015 19 16 11 49
Table j: CSNIrOT4 samples used in the assessment
Year Irish Coast, < 50 m Isle of Man, 50–100 m Eastern Irish Sea, <50 m Total
1993 18 16 10 44
1994 19 16 7 42
1995 18 7 9 34
1996 19 16 9 44
1997 19 16 9 44
1998 19 17 9 45
1999 18 17 9 44
2000 19 11 9 39
2001 19 15 11 48
2002 18 16 11 48
2003 19 15 11 48
2004 19 16 11 49
2005 18 16 11 48
2006 18 16 11 45
2007 19 16 11 47
2009 19 16 11 49
2010 19 16 11 49
2011 19 14 11 46
2012 19 16 11 49
2013 19 16 11 49
1992 24 12 8 44
2014 19 16 11 49
2015 19 16 10 49

Figure i: Depth strata for CSIreOT4 survey

The colours differentiate the different sub-divisions used.

Table k: CSIreOT4 samples used in the assessment
Year VIa_Coast VIa_Deep VIa_Medium VIIb_Coast VIIb_Deep VIIb_Medium VIIg_Coast VIIg_Medium VIIj_Coast VIIj_Deep VIIj_Medium
2003 11 12 13 8 11 6 7 22 3 14 8
2004 14 11 12 9 9 6 11 24 4 13 5
2005 10 9 12 4 13 7 7 21 5 15 9
2006 19 11 12 7 9 8 10 26 4 17 8
2007 14 11 12 6 9 8 10 30 4 18 6
2008 14 8 16 6 10 5 13 26 3 15 8
2009 21 12 12 6 11 8 12 23 3 12 8
2010 14 13 13 6 13 8 14 34 4 19 10
2011 16 8 15 8 7 6 15 33 5 19 9
2012 15 7 17 12 9 6 16 32 3 23 7
2013 14 10 17 7 12 8 17 31 4 22 6
2014 15 11 12 5 13 7 17 33 4 19 7
2015 15 10 16 6 6 5 15 26 3 17 5
Table l: CSIreOT4 hauls which were not included in the assessment (as they were not in the Celtic Sea region)
Year VIa_Slope VIIb_Slope VIIj_Slope
2003 1 1
2004 1 2
2005 1 13 7
2006 5 16 9
2007 4 14 9
2008 4 17 10
2009 3 18 7
2010 3 12 4
2011 3 3
2012 1 10 6
2013 2 12 5
2014 2 11 4
2015 3 9 4

Figure j: Depth-based strata for CSScoOT1

The colours differentiate the different sub-divisions used.

Table m: CSScoOT1 samples used in the assessment (red1 was excluded from further analysis due to poor sampling and because it is outside of the assessed region)
Year blue1_lam blue2_lam clyde_lam green1_lam green2_lam lightblue_lam pink_lam red2_lam red3_lam windsock_lam
1985 3 11 1 16 1 6 2 5 8 1
1986 2 8 1 5 1 4 2 1 4 1
1987 2 10 1 8 1 4 2 2 5 4
1988 2 9 1 7 1 5 2 3 3 3
1989 2 9 1 6 4 2 2 6 3
1990 2 10 1 7 5 2 1 3 3
1991 1 13 1 11 1 5 2 2 4 3
1992 2 8 1 5 1 4 1 3 6 1
1993 2 10 1 6 1 5 2 3 4
1994 3 11 1 7 1 4 2 1 5 1
1995 3 10 1 7 1 4 2 1 6 1
1996 4 9 1 6 1 2 2 2 6 1
1997 3 8 1 8 1 3 2 1 5
1998 3 7 1 7 1 4 1 1 6 1
1999 2 12 2 8 1 5 2 2 6
2000 3 14 1 7 1 4 2 2 6 1
2001 3 7 1 7 1 4 2 2 6 1
2002 3 9 2 7 1 4 2 2 6 1
2003 3 12 1 8 3 4 2 2 5 2
2004 2 10 1 7 1 5 2 2 6 2
2005 4 7 1 8 2 5 2 3 7 2
2006 5 8 1 11 1 5 2 2 6 2
2007 4 10 1 12 2 5 2 5 5 4
2008 3 8 1 9 2 7 2 4 7 2
2009 4 7 1 9 2 6 2 4 3 2
2010 5 7 1 11 2 5 2 4 6 2
2011 3 4 2 8 1 4 4 3 6 2
2012 5 6 2 13 2 7 3 2 4 1
2013 4 8 3 12 2 6 6 2 6 3
2014 5 5 2 11 5 3 2 7 1
2015 5 5 9 3 4 4 2 7 3
2016 5 5 2 14 2 5 3 3 6 2

Figure k: Depth-based strata for CSScoOT4

The colours differentiate the different sub-divisions used.

Table n: CSScoOT4 samples used in the assessment (the years 1995 and 1996 are excluded from further analysis due to poor sampling)
Year blue1_lam blue2_lam clyde_lam gray_lam green1_lam green2_lam
1995 2 2 2
1996 3 7 3
1997 3 2 2 3 6 4
1998 2 2 2 3 5 4
1999 3 2 2 3 6 5
2000 4 2 2 3 7 3
2001 5 1 2 3 7 3
2002 5 1 2 3 10 3
2003 5 2 2 2 11 5
2004 5 2 2 3 10 3
2005 5 1 2 3 11 5
2006 5 2 2 11 5
2007 6 2 2 3 15 4
2008 5 1 2 3 11 4
2009 5 1 2 3 11 4
2011 3 2 4 4
2012 3 1 3 9 5
2013 4 5 1
2014 2 1 2 3 8 3
2015 3 1 1 3 10 2
Table o: CSScoOT4 samples also used in the assessment (1995 and 1996 excluded from further analysis due to poor sampling)
Year green3_lam green4_lam lightblue_lam red1_lam red2_lam windsock_lam
1995 3 1 2 6
1996 3 1 2 9 1
1997 4 1 4 3 8 2
1998 2 1 4 2 9 1
1999 1 1 5 1 8 2
2000 4 1 6 4 12 2
2001 5 1 6 8 9 3
2002 6 1 7 6 10 7
2003 5 1 7 5 9 4
2004 5 1 7 6 11 5
2005 6 2 7 5 11 4
2006 5 1 2 7 10 4
2007 5 2 7 7 15 5
2008 4 3 4 7 11 3
2009 7 3 6 7 14 3
2011 3 2 4 8 13 5
2012 8 2 3 8 15 3
2013 1 7 1 4
2014 5 1 7 10 11 4
2015 6 2 4 6 11 5

Figure l: Northern Celtic Sea strata for CSFraOT4 survey (data located above 48°N only from larger survey area) The colours differentiate the different sub-divisions used.

The colours differentiate the different sub-divisions used.

Table p: CSFraOT4 samples used in the assessment
Year Cc3e Cc4e Cc4w Cc5 Cn2 Cn3 Cs4 Cs5
1997 5 2 9 3 2 2 13 3
1998 9 1 9 3 2 4 12 6
1999 7 4 8 2 3 4 11 4
2000 6 2 6 1 2 4 12 6
2001 6 8 16 3 3 5 16 6
2002 4 5 14 4 4 4 16 7
2003 6 6 12 3 4 7 15 6
2004 7 6 9 2 4 5 14 5
2005 6 6 6 3 4 8 14 4
2006 5 6 9 4 4 4 9 3
2007 8 6 11 2 4 5 16 4
2008 6 8 10 3 4 6 13 4
2009 2 6 9 3 3 6 13 4
2010 4 1 11 3 3 4 12 5
2011 4 11 7 4 5 7 13 5
2012 2 5 8 1 4 5 10 4
2013 4 6 7 4 4 5 17 5
2014 7 4 12 3 4 5 12 6
2015 3 2 12 4 4 6 12 6
Table q: CSFraOT4 samples not used in the assessment (Cc6, Cs6, Cc7 and Cs7 are excluded from further analysis because they are not in the assessed region and are all poorly sampled)
Year Cs6 Cs7 Cn2e Cc3w Cc6 Cc7
1997 2 1 3 1
1998 2 3
1999 2 2 1 3 2 2
2000 2 2
2001 2 2 2 1 2 2
2002 2 3 1 3 2 2
2003 4 1 2 1 3 1
2004 1 3 3 2
2005 4 2 1 1 4 1
2006 1 2 1 3 3 1
2007 3 2 2 2 2 2
2008 2 5 3 2 2
2009 2 2 2 2 2 2
2010 2 2 2 3 3 2
2011 4 3 2 3 1 2
2012 4 2 3 2 2 2
2013 2 3 1 3 2
2014 3 5 3 4 2 2
2015 2 3 1 5 2 1

Bay of Biscay and Iberian Coast

Figure m: Depth-based strata for CSBBFraOT4 (southern area, south of 48°N)

The colours differentiate the different sub-divisions used.

Table r: CSBBFraOT4 samples used in the assessment
Year Gn1 Gn2 Gn3 Gn4 Gn57 Gs1 Gs2 Gs3 Gs4 Gs5 Gs67
1997 4 11 15 18 7 3 5 4 2 3 3
1998 1 8 10 23 3 2 6 3 3 3 2
1999 1 3 9 18 6 2 5 3 1 1 6
2000 2 4 18 19 7 2 3 3 3 5
2001 6 18 22 6 2 2 3 3 2 4
2002 3 3 18 18 7 2 6 2 3 3 4
2003 2 3 14 19 8 3 4 3 3 2 4
2004 2 2 19 19 7 3 3 3 2 2 5
2005 1 6 15 17 9 1 5 3 3 1 5
2006 2 4 15 16 7 3 3 3 2 2 5
2007 3 3 17 17 7 3 4 4 1 1 6
2008 2 3 16 19 8 2 5 3 3 3 3
2009 3 3 17 20 7 2 4 3 1 4 3
2010 2 5 18 18 7 2 4 3 1 2 6
2011 2 3 15 21 7 3 4 3 2 1 6
2012 2 4 16 16 7 3 4 3 1 4 4
2013 3 5 13 22 7 4 3 3 2 3 4
2014 3 5 16 20 5 2 5 3 2 3 4
2015 2 6 17 20 7 2 4 4 3 3 4

Figure n: Depth-based strata for BBICnSpaOT4

The colours differentiate the different sub-divisions used.

Table s: BBICnSpaOT4 samples used in the assessment (note only data for the eastern strata available in the data product)
Year AB PA
1990 6 14
1991 11
1992 2
1993 13
1994 14 9
1995 15 6
1996 14 5
1997 4
1998 15 7
1999 15 9
2000 13 7
2001 14 6
2002 13 7
2003 13 6
2004 7 11
2005 14 9
2006 14 11
2007 14 10
2008 13 12
2009 13 11
2010 14 11
2011 14 9
2012 14 9
2013 15 11
2014 14 13

Figure o: Depth-based strata for BBICPorOT4

The colours differentiate the different sub-divisions used.

Table t(i): BBICPorOT4 samples used in the assessment (17, 21, 37 and 42 and all data pre-2005 are excluded from further analysis due to poor sampling)
Year 0 1 2 3 5 8 9 10 11 12 13 14 15 16 17 18 19
2002 2 1 4 2 2 2 3 2 2 1 2 2 2 2 1 1
2005 2 3 4 1 2 2 4 3 6 2 2 3 3 2 2 2
2006 2 3 4 1 3 2 4 5 4 2 1 3 3 1 2 2
2007 3 2 4 1 3 3 6 5 4 1 2 3 3 1 3 1
2008 4 1 3 1 2 2 4 5 3 1 2 3 3 1 1 2
2009 4 1 6 1 2 3 5 7 2 1 2 3 2 2 1 2 2
2010 3 3 3 1 2 2 5 8 3 1 1 2 3 1 1 2
2011 4 1 4 1 3 5 4 3 1 2 3 2 2 2 2
2013 2 3 5 1 3 2 8 4 3 1 1 3 2 2 1 2
2014 1 3 5 1 2 5 5 3 2 1 3 3 1 2 2
Table t(ii): BBICPorOT4 samples used in the assessment (17, 21, 37 and 42 and all data pre-2005 are excluded from further analysis due to poor sampling) (continuation)
Year 20 21 22 23 24 25 26 27 29 30 31 32 33 36 37 38 39 40 42
2002 1 1 3 3 2 6 2 1 4 3 2 1 3 3
2005 1 1 2 3 2 5 5 2 3 1 5 1 1 3 4 4 1
2006 2 1 3 4 5 4 1 3 1 5 2 1 1 6 3 1
2007 2 1 1 3 3 3 5 5 2 4 4 3 1 2 6 3
2008 2 1 3 3 3 4 4 2 4 1 6 2 1 3 5 1
2009 1 1 4 2 2 6 3 1 4 5 2 2 4 6 2
2010 3 1 1 2 3 2 5 6 2 4 1 3 2 1 1 4 1 1
2011 3 1 1 2 2 2 5 5 2 3 4 2 2 2 2 3 1
2013 3 2 1 2 2 5 5 2 4 1 3 3 1 1 2 6 3 1
2014 3 1 2 2 7 3 3 3 4 2 1 1 4 2 1

Figure p: Depth-based strata for BBICsSpaOT1 and BBICsSpaOT4

The colours differentiate the different sub-divisions used.

Table u: BBICsSpaOT1 samples used in the assessment (A and E are excluded from further analysis due to poor sampling)
Year A B C D E Total
1993 2 5 5 5 7 24
1994 3 8 1 4 3 19
1995 2 12 3 3 20
1997 3 8 6 2 2 21
1998 11 5 4 20
1999 2 10 7 8 1 28
2000 15 7 9 1 32
2001 9 7 8 7 31
2002 12 7 9 6 34
2004 3 12 5 5 5 30
2005 12 6 8 6 32
2006 1 11 7 7 6 32
2007 4 15 4 8 31
2008 4 11 5 6 5 31
2009 3 10 4 7 6 30
2010 4 11 4 8 27
2011 1 9 10 6 6 32
2012 3 8 5 5 2 23
2013 4 10 7 7 5 33
2014 4 10 6 7 5 32
Table v: BBICsSpaOT4 samples used in the assessment (A and E are excluded from further analysis due to poor sampling)
Year A B C D E Total
1997 3 4 5 5 17
1998 3 10 4 3 5 25
1999 14 7 4 2 27
2000 3 5 5 7 20
2001 8 8 9 4 29
2002 4 9 8 6 4 31
2003 4 13 7 7 2 33
2004 4 13 6 7 3 33
2005 12 6 10 5 33
2006 12 7 9 5 33
2007 10 5 8 3 26
2008 4 9 5 7 6 31
2009 4 12 5 11 3 35
2010 13 8 9 5 35
2011 10 8 9 4 31
2012 3 11 7 6 27
2014 11 9 10 4 34

Wider Atlantic

Figure q: Depth-based strata for WAScoOT3

The colours differentiate the different sub-divisions used.

Table w: WAScoOT3 samples used in the assessment (the outer area ‘mylightblue_lam’ was excluded from further analysis due to poor sampling)
Year myblue_lam mygreen_lam myred_lam mylightblue_lam Total
1999 4 31 6 41
2001 3 35 6 44
2002 2 25 2 29
2003 4 49 7 60
2005 2 32 4 38
2006 1 27 4 32
2007 4 32 6 42
2008 33 4 37
2009 4 34 3 41
2011 8 19 5 5 37
2012 6 18 4 3 31
2013 8 15 5 2 30
2014 11 21 4 2 38
2015 9 21 4 5 39

Figure r: Depth-based strata for WASpaOT3

The colours differentiate the different sub-divisions used.

Table x: WASpaOT3samples used in the assessment (the outer area ‘mylightblue_lam’ was excluded from further analysis due to poor sampling)
Year 12 13 22 23 11a 11b Total
2001 16 8 18 20 2 10 74
2002 18 5 17 20 6 10 76
2003 20 7 15 14 5 11 72
2004 16 4 15 11 5 10 61
2005 18 5 16 15 5 9 68
2006 19 6 15 15 5 10 70
2007 19 5 17 16 6 9 72
2008 19 5 16 14 7 7 68
2009 20 5 14 17 6 9 71
2010 17 9 17 16 6 5 70
2011 19 8 17 12 5 10 71
2012 18 7 15 16 4 11 71
2013 19 5 13 20 6 9 72
2014 19 7 14 17 7 8 72

 

Overall Assessment Basis

Where multiple surveys were available for assessment, key surveys were prioritised for assessment given the length of time series available and spatial coverage. If these measures were equal between surveys, then whichever surveyed the greatest biomass by assemblage was selected for indicator assessment. 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 length of 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 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. CSNIrOT1 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.

Bay of Biscay and Iberian Coast

BBICsSpaOT1 for both demersal and pelagic assemblages was preferred over BBICsSpaOT4 given the length of the survey. CSBBFraOT4, BBICPorOT4 and BBICnSpaOT4 did not overlap with any other surveys.

 

Time-Series Assessment

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 long-term trend in each time series (sub-division and survey level) was modelled through the application of a LOESS smoother (i.e. locally weighted scatterplot smoothing) with a simple ‘fixed span’ of one decade.

Breakpoint analyses uses data to define stable underlying periods (see Probst and Stelzenmüller, 2015). The method makes it possible to say whether there is a significant change in the time series state over time, namely whether the recent period is not significantly different from the historically observed period. The method avoids the arbitrary choice of reference periods for assessment (i.e. how many years to use to calculate an average) which can lead to subjective assessments. The shorter the period chosen, the more likely it is that noise in the data or natural fluctuations in the system are being compared against each other. However, too long a period and it could be that actual changes in state are averaged out. The minimum detectable period is defined in this analysis as three years. The analysis uses two statistical approaches: First applying the ‘supremum F test’ to establish whether a non-stationary time series or a constant period for the entire time series is more suitable. If the former, then breakpoint analysis is applied to find periods of at least three years duration.

Populations should have a size structure indicative of sustainable populations and should occur at levels that ensure long-term sustainability in line with prevailing conditions. There should be no significant adverse change in the function of different trophic assemblage levels due to human activities. Appropriate baselines for both demersal and pelagic assemblages are not currently available to determine assessment thresholds. The current assessment uses a time-series approach to identify long-term changes in state and further investigation is required to identify if reductions in the size structure of assemblages is due to human activities, food web interactions or prevailing climatic conditions.

Results

The results of this assessment (Figure 2) apply at the community level and do not identify particular species.

Greater North Sea

The assessed demersal fish assemblage is recovering at the scale of the Greater North Sea as a whole due to recent increases in typical length indicator in some sub-divisions: Orkney / Shetland, Kattegat / Skagerrak and the United Kingdom coast in the English Channel. However, the current level is low relative to observed size structure in the early 1980s. Areas of concern, with long-term declines to lowest observed levels remain in the south-eastern and central-western North Sea. The pelagic fish assemblage generally shows fluctuations without trend, with the exception of a long-term decrease to a minimum level in the south-eastern North Sea.

Celtic Seas

Although the surveys showed mixed signals within the Celtic Seas for the typical length of the demersal fish assemblage, surveys in the north suggest some recovery from previous low states with increases to the west of Scotland. 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 and in the Clyde area, but increases to the south of Ireland, Isle of Man, Sea of the Hebrides, and The Minch. The pelagic fish assemblage generally shows no long-term change at the sub-regional level. However, increases are seen to the south of Ireland and decreases in some northerly areas including the Sea of the Hebrides and in coastal areas in the Irish Sea and to the west of Ireland.

Bay of Biscay and Iberian Coast

The typical length of the demersal fish assemblage has increased in this region due to long-term increases in northerly sub-divisions in shelf waters to the west of France and in the coastal area of the Sea of Cadiz. Many sub-divisions to the west of Portugal have also shown increases, in contrast to decreases in some areas to the south. The pelagic fish assemblage generally showed no long-term change. However, decreases to a low state relative to previously observed size structure were identified in northerly sub-divisions in shelf waters to the west of France.

Wider Atlantic

The typical length of the demersal fish assemblage has increased at the Porcupine Bank and the Rockall Bank. While fluctuations without long-term change in size structure have been shown in the pelagic fish assemblages, in the recent period (last six years) a linear increase has been shown for the Porcupine Bank.

There is moderate / low confidence in the method for this assessment and high confidence for data availability

Figure 2: Spatial pattern of Typical Length indicator and time series for key surveys

Typical Length for fish and elasmobranchs is separated into demersal assemblages (left) and pelagic assemblages (right) for sub-divisions for key surveys, where data are available. The duration of the period for which long-term change is defined is dependent on the survey data available, all time periods considered are over ten years long.

Figure s to Figure ay show time series for each survey sub-division. The label for the different sub-divisions is above the plot. For each figure the plot labelled ‘sea’ is a time series of the aggregated survey data for demersal and pelagic assemblages. Each mini-heading shows the p value for the supremum F test which demonstrates whether a significant long-term change is evident (the changes are shown by red dashed lines if significant, or a grey dashed line is used to show a mean level for the whole time series). Annual estimates are shown by blue circles with a fitted LOESS smooth plot (black line) with an estimate of spread shown (± 1 standard deviation). The solid horizontal blue line shows the 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.

Greater North Sea

Figure s: Time series of Typical Length for each sub-division of the GNSIntOT1 survey (Demersal fish species)

Figure t: Time series of Typical Length for each sub-division of the GNSIntOT1 survey (Pelagic fish)

Figure u: Time series of Typical Length for each sub-division of the GNSIntOT3 survey (Demersal fish)

Figure v: Time series of Typical Length for each sub-division of the GNSIntOT3 survey (Pelagic fish)

Figure w: Time series of Typical length for each sub-division of the GNSNetBT3 survey (Demersal fish)

Figure x: Time series of Typical Length for each sub-division of the GNSGerBT3 survey (Demersal fish)

Figure y: Time series of Typical Length for each sub-division of the GNSEngBT3 survey (Demersal fish)

Figure z: Time series of Typical Length for each sub-division of the GNSFraOT4 survey (Pelagic fish)

Celtic Seas

Figure aa: Time series of Typical Length for each sub-division of the CSEngBT3 survey (Demersal fish)

Figure ab: Time series of Typical Length for each sub-division of the CSIreOT4 survey (Demersal fish)

Figure ac: Time series of Typical Length for each sub-division of the CSIreOT4 survey (Pelagic fish)

Figure ad: Time series of Typical Length for each sub-division of the CSNIrOT1 survey (Demersal fish)

Figure ae: Time series of Typical Length for each sub-division of the CSNIrOT1 survey (Pelagic fish)

Figure af: Time series of Typical Length for each sub-division of the CSNIrOT4 survey (Demersal fish)

Figure ag: Time series of Typical Length for each sub-division of the CSNIrOT4 survey (Pelagic fish)

Figure ah: Time series of Typical Length for each sub-division of the CSScoOT1 survey (Demersal fish)

Figure ai: Time series of Typical Length for each sub-division of the CSScoOT1 survey (Pelagic fish)

Figure aj: Time series of Typical Length for each sub-division of the CSScoOT4 survey (Pelagic fish)

Figure ak: Time series of Typical Length for each sub-division of the CSFraOT4 survey (Demersal fish).

Figure al: Time series of Typical Length for each sub-division of the CSFraOT4 survey (Pelagic fish)

Bay of Biscay and the Iberian coast

Figure am: Time series of Typical Length for the CSBBFraOT4 survey (Demersal fish)

Figure an: Time series of Typical Length for each sub-division of the CSBBFraOT4 data (Pelagic fish)

Figure ao: Time series of Typical Length for each sub-division of the BBIC(n)SpaOT4 data (Demersal fish)

Figure ap: Time series of Typical Length for each sub-division of the BBIC(n)SpaOT4 survey (Pelagic fish)

Figure aq: Time series of Typical Length for each sub-division of the BBIC(s)SpaOT1 survey (Demersal fish)

Figure ar: Time series of Typical Length for each sub-division of the BBIC(s)SpaOT1 survey (Pelagic fish)

Figure as: Time series of Typical Length for each sub-division of the BBIC(s)SpaOT4 survey (Demersal fish)

Figure at: Time series of Typical Length for each sub-division of the BBIC(s)SpaOT4 survey (Pelagic fish)

Figure au: Time series of Typical Length for each sub-division of the BBICPorOT4 survey (Demersal fish)

Wider Atlantic

Figure av: Time series of Typical Length for each sub-division of the WAScoOT3 survey (Demersal fish)

Figure aw: Time series of Typical Length for each sub-division of the WAScoOT3 survey (Pelagic fish)

Figure ax: Time series of Typical Length for each sub-division of the WASpaOT3 survey (Demersal fish)

Figure ay: Time series of Typical Length for each sub-division of the WASpaOT3 survey (Pelagic fish)

Assessment of Confidence

The method has been developed specifically for this assessment. There is consensus within the scientific community regarding this methodology, however further methodological development is required therefore, it has been rated as moderate / low.

There are no significant data gaps and there is sufficient spatial coverage, confidence for data availability is rated as high.

Conclusion

Long-term decreases in Typical Length, between the 1980s and 2000s in the Greater North Sea and from the 1990s to 2005 in the Celtic Seas, imply that the size structure of fish communities deteriorated such that communities are now more dominated by small-bodied fish. In the Wider Atlantic and Bay of Biscay and Iberian Coast, an overall increase has been observed since 2010.

However, while the indicator in demersal fish assemblages is often still at a relatively low value, recovery since 2010 appears to be underway in the Typical Length of demersal fish and elasmobranchs in the Greater North Sea and Celtic Seas, overall or at least in some sub-divisions. The pelagic fish assemblage shows no long-term change in much of the OSPAR Maritime Area.

In the fish and elasmobranch community, Typical Length responds to changes in the dynamics of 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 geographic 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 the indicator for specific benthic and water column habitats and the often-local impacts of pressures.

Within the Greater North Sea there are clear sub-divisional differences, with demersal and pelagic assemblages in the northerly areas now 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 fisheries may have contributed to this depletion, it is unclear whether rising sea temperatures have led to increases in small-bodied fish (i.e. young fish and / or small species). In the Bay of Biscay, where declines are seen in the size structure of pelagic fish, increases are evident in demersal fish.

In addition to the key surveys for each assessment region, additional survey information was assessed which generally confirmed the overall conclusions.

The increase in the typical length of demersal fish since 2010 in the Greater North Sea, evident in the International Bottom Trawl Survey (IBTS) quarter one (Q1) survey, was also shown in the quarter three (Q3) survey (GNSIntOT1 and GNSIntOT3), while the more spatially restricted groundfish surveys showed no significant change (GNSNetBT3, GNSGerBT3, GNSEngBT3). For pelagic fish, no change was evident in the two IBTS surveys in the Greater North Sea.

Within the Celtic Seas, increases in the 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 in the Irish Sea in two surveys (CSNIrOT1 and CSNIrOT4 since 2010) with no change in a third (CSEngBT3), to the south and west of Ireland increases were evident in one survey (CSIreOT4 since 2010). The pelagic fish generally showed no significant changes in typical length, with the exception of a decrease in the Irish Sea in one survey (CSNIrOT4 in 1998).

For the Bay of Biscay and Iberian coast, the typical length of demersal fish increased in two of the five available surveys (CSBBFraOT4 since 2004 and BBICnSpaOT4 since 1998) and no overall changes in the typical length of pelagic fish were detected.

An overall increase since 2002 in the typical length of demersal fish in the Wider Atlantic were significant in both assessed surveys (WAScoOT3 and WASpaOT3). No change in the typical length of pelagic fish was evident in WAScoOT3 but an increase from 2012 was evident in the WASpaOT3 survey.

Knowledge Gaps

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

The setting of assessment values for this indicator should consider their relation to the European Union’s 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 driving improvements in the size structure of the demersal fish community 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 species composition is changing. 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 historic values, if possible.

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