Répartition des tailles dans les communautés halieutiques

D4 - Réseau trophique marin

D4.2 - Proportion des espèces sélectionnées au sommet du réseau trophique

Message clé:

La longueur typique permet de mesurer la structure des tailles des communautés halieutiques et d’élasmobranches et elle diminue lorsque la pêche exerce des pressions importantes. La longueur typique du poisson démersal évalué, bien que faible par rapport aux années 1980, se rétablit dans l’ensemble de la zone maritime OSPAR depuis 2010. Le poisson pélagique révèle dans l’ensemble des fluctuations mais aucune tendance. A l’échelle locale, on relève des divergences avec cette tendance.

Contexte

L’objectif stratégique d’OSPAR, en ce qui concerne la biodiversité et les écosystèmes, est de stopper et de prévenir une perte supplémentaire de la biodiversité, de protéger et de conserver les écosystèmes et de rétablir, lorsque possible, les écosystèmes qui ont subi des impacts préjudiciables des activités de l’homme.

L’indicateur de la longueur typique est l’un de trois indicateurs du réseau trophique utilisés actuellement par OSPAR. Il représente la longueur moyenne du poisson (poisson osseux et élasmobranches) et fournit des informations sur la répartition des tailles dans les communautés halieutiques. On calcule l’indicateur à partir des données sur les captures d’espèces échantillonnées dans le cadre d’études scientifiques et exprimées en centimètres. Les communautés sont représentées par des assemblages trophiques basés sur leur habitat (groupes halieutiques): à savoir, les assemblages démersaux (c’est-à-dire espèces vivant sur le sol marin ou à proximité) et les assemblages pélagiques (c’est-à-dire espèces vivant dans la colonne d’eau).

La mortalité par la pêche limite la structure par âge des populations halieutiques, réduisant la proportion d’individus plus gros (Figure 1). On s’attend à un déclin graduel et régulier de la longueur typique en réaction à des pressions élevées exercées par la pêche. En effet la structure des tailles des assemblages halieutiques intègre les impacts des pressions exercées par la pêche durant de longues périodes. Les simulations de modèles démontrent que dans les réseaux trophiques, lorsque les interactions entre les prédateurs et les proies l’emportent sur d’autres interactions, les espèces de grande taille à des niveaux trophiques élevés (position des espèces dans le réseau trophique) sont extrêmement sensibles à la perte de diversité à des niveaux trophiques plus bas.

Figure 1: Loup de l’Atlantique de grande taille (avec la permission de Jim Ellis)

Background (Extended)

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.

Assessment Method

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:

where M_iis the body mass (standardised to kilogrammes per unit area fished) of the i-th fish with length L_i (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 ctoFigure 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 = aL^b, 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
Wider Atlantic	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.

Résultats

Les résultats de cette évaluation (Figure 2) s’appliquent au niveau de la communauté et n’identifient aucune espèce en particulier.

Mer du Nord au sens large

L’assemblage halieutique démersal évalué se rétablit à l’échelle de la mer du Nord au sens large dans son ensemble grâce aux augmentations récentes de l’indicateur de la longueur typique dans certaines sous-divisions: Orkney / Shetland, Kattegat / Skagerrak et la côte du Royaume-Uni dans la Manche. Le niveau actuel est cependant faible par rapport à la structure des tailles relevées dans les années 1980. Des zones du sud-est et du centre-ouest de la mer du Nord continuent à causer des préoccupations, les déclins à long terme ayant atteint les niveaux les plus bas relevés. L’assemblage halieutique pélagique révèle dans l’ensemble des fluctuations mais aucune tendance, à l’exception d’une diminution à long terme atteignant un niveau minimal au sud-est de la mer du Nord.

Mers Celtiques

Les études révèlent des signes mitigés au sein des mers Celtiques quant à la longueur typique de l’assemblage halieutique démersal mais des études réalisées au nord suggèrent un certain rétablissement par rapport aux faibles longueurs précédentes, des augmentations étant relevées à l’ouest de l’Ecosse. Des diminutions sont cependant évidentes dans les eaux du rebord continental à l’ouest. On retrouve le même tableau ailleurs, on relève des diminutions près de la côte irlandaise de la mer d’Irlande et dans la zone du Clyde mais des augmentations au sud de l’Irlande, à l’île de Man, dans la mer des Hébrides et dans le Minch. L’assemblage halieutique pélagique ne révèle dans l’ensemble aucune modification à long terme au niveau sous-régional. On relève cependant des augmentations au sud de l’Irlande et des diminutions dans certaines zones septentrionales, notamment dans la mer des Hébrides et les zones côtières de la mer d’Irlande et à l’ouest de l’Irlande.

Golfe de Gascogne et côte ibérique

La longueur typique de l’assemblage halieutique démersal a augmenté dans cette région à la suite d’augmentations à long terme dans les sous-divisions septentrionales des eaux du plateau à l’ouest de la France et dans les eaux côtières de la mer de Cadix. Nombre de sous-divisions à l’ouest du Portugal ont également révélé des augmentations, contrairement aux diminutions dans certaines zones du sud. L’assemblage halieutique pélagique ne révèle dans l’ensemble aucune modification à long terme. On a cependant relevé des diminutions atteignant un niveau bas par rapport à la structure des tailles relevée antérieurement dans les sous-divisions septentrionales des eaux du plateau à l’ouest de la France.

Atlantique au large

La longueur typique de l’assemblage halieutique démersal a augmenté dans le banc de Porcupine et le banc de Rockall. On a relevé des fluctuations sans modification à long terme de la structure des tailles des assemblages halieutiques pélagiques, alors que récemment (six dernières années) on a relevé une augmentation linéaire pour le banc de Porcupine.

La méthode utilisée dans cette évaluation inspire une confiance modérée / faible et la disponibilité des données inspire une confiance élevée.

Figure 2: Profil spatial de l’indicateur de la longueur typique et séries temporelles pour les études fondamentales
La longueur typique du poisson et des élasmobranches est divisée en assemblages démersaux (à gauche) et en assemblages pélagiques (à droite) dans le cadre des sous-divisions d’études fondamentales, lorsque des données sont disponibles. La longueur de la période pour laquelle des modifications à long terme sont définies dépend des données disponibles pour l’étude, toutes les périodes considérées étant de plus de dix ans.

Results (Extended)

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

Des diminutions à long terme de la longueur typique, entre les années 1980 et les années 2000, dans la mer du Nord au sens large et des années 1990 à 2005 dans les mers Celtiques, signifient que la détérioration de la structure des tailles des communautés halieutiques est telle que des poissons plus petits sont désormais dominants dans des communautés. On a relevé dans l’ensemble une augmentation dans l’Atlantique au large et le golfe de Gascogne et la côte ibérique, depuis 2010.

Cependant, l’indicateur des assemblages halieutiques démersaux demeure souvent à une valeur relativement basse, mais depuis 2010 un rétablissement de la longueur typique du poisson et des élasmobranches démersaux dans la mer du Nord au sens large et les mers Celtiques, semble se produire dans l’ensemble ou tout au moins dans quelques sous-divisions. L’assemblage halieutique pélagique ne révèle aucune modification à long terme dans la plus grande partie de la zone maritime OSPAR.

Conclusion (Extended)

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.

Lacunes des connaissances

Il y a lieu d’entreprendre des travaux supplémentaires pour évaluer des lignes de base et des valeurs d’évaluation adéquates pour cet indicateur. En effet il est fort probable que toute ligne de base historique pour les communautés halieutiques et d’élasmobranches représente un état ayant subi des impacts. Il faudra identifier des valeurs d’évaluation, de préférence grâce à une modélisation à espèces multiples.

Knowledge Gaps (Extended)

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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