Capture accessoire de marsouins communs

D1 - Diversité Biologique

D1.3 - État des populations

Message clé:

On reconnaît que les captures accessoires sont une cause majeure de la mortalité d’origine anthropique du marsouin commun. Presque 4000 marsouins communs, appartenant à une population de plus de 490 000 individus, se noient chaque année, pris dans des filets de pêche, dans les zones évaluées. Les estimations de ces captures accessoires n’inspirent cependant qu’une confiance faible car les données de surveillance sont incomplètes.

Zone Évaluée

Récapitulatif Imprimable

Contexte

La principale cause anthropique de la mortalité des cétacés dans la zone maritime OSPAR est l’enchevêtrement dans des filets de pêche. Les phoques sont également capturés accessoirement mais les connaissances sont insuffisantes pour pouvoir les inclure dans cette évaluation.

Le marsouin commun figure dans la Liste OSPAR des espèces et habitats menacés et/ou en déclin pour la mer du Nord au sens large et les mers Celtiques car certains signes indiquent un déclin de ses populations, sa sensibilité et les menaces que présentent les captures accessoires et le noyage dans des filets de pêche.

Cette évaluation se fonde uniquement sur les toutes dernières informations concernant les nombres de cétacés capturés accessoirement et tués par la pêche, communiquées à la Commission européenne par le Conseil international pour l’exploration de la mer (CIEM). Les Parties contractantes OSPAR n’ont fourni aucune information supplémentaire. Le CIEM estime les nombres de marsouins communs pris dans des filets commerciaux (les filets maillants principalement) dans les unités d’évaluation dérivées par le CIEM figurant dans la Figure 1. Les estimations de captures accessoires sont dérivées des estimations annuelles de l’effort de pêche et des relevés par des observateurs ou une surveillance électronique à distance à bord de navires de pêche commerciale, du nombre de marsouins communs capturés accessoirement.

La fiabilité des données sur l’effort de pêche présente des incertitudes et les données sur les captures accessoires des biais potentiels, donc cette évaluation n’établit pas de comparaison entre les estimations, par le CIEM, des captures accessoires et les valeurs d’évaluation utilisées par l’objectif de qualité écologique (EcoQO) d’OSPAR pour la mer du Nord pour les captures accessoires de marsouins communs.

Image: Marsouin commun mort portant des traces dues à sa capture dans un filet de pêche (avec la permission de Jan Haelters)

Figure 1: Unités d’évaluation (AU) du CIEM pour le marsouin commun (telles que proposées par le CIEM, 2014) – il convient de noter qu’elles sont dérivées au niveau écologique et ne correspondent pas aux Régions OSPAR

The main human-induced cause of mortality of cetaceans in the OSPAR Maritime Area is being caught and entangled in fishing nets (Bjørge et al., 2013; Peltier et al., 2016). There are existing requirements to monitor incidental capture and killing of cetaceans and to ensure through conservation measures that it does not have a significant negative impact on cetacean population size.

Harbour porpoise (Phocoena phocoena) is included in the OSPAR List of Threatened and / or Declining Species and Habitats for the Greater North Sea and Celtic Seas owing to evidence of a decline in populations, their sensitivity and the threat of incidental capture and drowning in fishing nets. The need for reduction of bycatch of cetaceans is also reaffirmed by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) in 2016.

The ICES advice (2015a) used for this assessment is based on output from the ICES Working Group on Bycatch of Protected Species (WGBYC) (ICES, 2015b, 2016b). WGBYC analysed data on incidental catches of cetaceans that Member States are required to collect. Data are primarily collected through observer schemes that should operate for each of the fisheries specified in the Annexes to the European Union Council Regulation (EC) 812/2004. However, coverage of gillnet fisheries, for example, is generally low for several reasons. Under EC Council Regulation 812/2004, concerning protecting whales, dolphins and porpoises against incidental catch, monitoring is mandatory for vessels longer than 15 m, whereas pilot studies and scientific studies are required on vessels smaller than 15 m. Most vessels in the European gillnet fleet are smaller than 15 m and the gillnet fleets of some Member States are very large, comprising hundreds of vessels. In addition, there are obligations for monitoring under the EU Data Collection Multiannual Plan (DC-MAP) as well as Article 12 of the European Union Habitats Directive (92/43/EEC). This provides the opportunity fora regional coordination, for example with respect to data collection methods and the collection of fishing activity variables.

It should be noted that the area around the northern parts of the Kattegat / Belt Seas assessment unit (AU) contains a mix of two different populations; the North Sea population (including Skagerrak and northern Kattegat) and the Belt Seas population (encompassing the southern Kattegat, the Belt Seas, the Sound and the western Baltic; Sveegaard et al., 2105). Surveys to estimate abundance in this region have not aligned survey boundaries with these population units and consequently abundance estimates may represent a mixture of both. The definition of the North Sea and Kattegat and the Belt Seas AUs should be re-evaluated for future assessments.

Harbour porpoise bycatch was highlighted in the OSPAR Quality Status Report (QSR) 2010 by means of the OSPAR ecological quality objective (EcoQO) on bycatch of harbour porpoise in the North Sea. It was not possible to present a quantitative assessment of status in relation to the indicator in 2010 due to the lack of reliable information on bycatch numbers and the need for accurate estimates of harbour porpoise populations and their abundance. The QSR 2010 called for effective minimisation of marine mammal bycatch.

Introduction

Through its Working Group on Bycatch of Protected Species (WGBYC) the International Council for the Exploration of the Sea (ICES) collates and reviews data reported by Member States to the European Commission (under EC Regulation 812/2004), annually. These data also include data collected by monitoring schemes under Article 11 of the European Union Habitats Directive that are used to monitor ‘incidental capture’ (Article 12) and to assess marine mammal conservations status (Article 17). All data are requested by WGBYC in a standard format and collated in a bycatch database.

Observations of bycaught animals are collated over several years to ensure sufficient geographical coverage. The number of observed dead animals is divided by the number of days that fishing activity was observed, in order to produce a ‘bycatch rate’ (see below). Bycatch rate is then multiplied by the number of days fished by vessels in a specific area during the entire year, in order to produce an estimate of total annual bycatch for that section of the fleet.

Species

This indicator assessment focuses on bycatch of harbour porpoise (Phocoena phocoena) only. Of those cetacean species reported as bycatch by European Union Member States, harbour porpoise and short-beaked common dolphin (Delphinus delphis) are the most commonly bycaught species in the OSPAR Maritime Area (e.g. ICES, 2015b, 2016b). These species are the most abundant in the area and consequently, more data on their bycatch are available, compared to other species.

ICES also collated bycatch data on short-beaked common dolphin in parts of the Celtic Seas, Bay of Biscay and Iberian Coast and the Wider Atlantic (ICES, 2016a,b). They calculated bycatch rates for each of 11 fishing gear types (métier level 4, e.g. mid-water otter trawl, set gillnet, bottom pair trawl) in ICES Divisions VII, VIII and IX. Because ICES (2016a,b) did not estimate total annual bycatch (i.e. by multiplying bycatch rates by estimates of fishing effort for each gear type) it is not possible to provide comparable estimates of total annual bycatch for both species in this indicator assessment.

Bycatch Risk Assessment

A Bycatch Risk Assessment approach was adopted at a workshop to Evaluate Aspects of EC Regulation 812/2004 (WKRev812) in 2010 and further elaborated by ICES WGBYC in 2013 (ICES, 2013). The approach aims to “identify sea areas or fisheries that may pose the greatest threat to non-target species in the absence of reliable data that would be needed to quantify the bycatch of that species in a statistically rigorous manner” (ICES, 2015b).

The Bycatch Risk Assessment approach estimates the total annual bycatch of species in a region using the following parameters:

Total bycatch (number of animals caught) = fishing effort × bycatch rate

where:

Bycatch rate (number of animals caught per day) = total number of bycaught animals observed / number of observer days

and,

Fishing effort = number of days at sea (for relevant gear types)

ICES WGBYC pooled information on bycatch rates across years due to the patchiness of coverage of relevant fisheries in any one year (see below).

Fishing effort data were collated by the Scientific, Technical and Economic Committee for Fisheries (STECF) and others (see ICES, 2015b, 2016b).

Harbour porpoise Assessment Units

ICES (2014) recommended to OSPAR five assessment units (AUs) for harbour porpoise. The boundaries of these AUs are mostly consistent with ICES Subarea/Division boundaries (Figure 1) with the exception of the boundary between the North Sea and the Kattegat:

North Sea AU: ICES Subarea IV, Division VIId, and part of Division IIIa (Skagerrak) (includes most of the Greater North Sea)

Kattegat and Belt Seas AU: Part of ICES Division IIIa (Kattegat) and Baltic Areas 22 and 23 (includes easternmost part of the Greater North Sea and the Belt Seas, of which the latter lie outside the boundary of the OSPAR Maritime Area)

Western Scotland and Northern Ireland AU: ICES Division VIa and Subdivision VIb2 (includes the northern half of the Celtic Seas)

Celtic Sea and Irish seas AU: ICES Subarea VII with the exception of Division VIId (includes southern half of the Celtic Seas and inshore waters of the French part of the Bay of Biscay and Iberian Coast; note that there are very few harbour porpoises in the deeper parts of the Celtic Seas and the Bay of Biscay and Iberian Coast)

Iberian Peninsula AU: ICES Divisions VIIIc and IXa (includes the Iberian coast in the Bay of Biscay and Iberian Coast).

Harbour porpoises are distributed throughout the Belt, Kattegat and Skagerrak seas, the shelf waters of the Greater North Sea, the Celtic Seas and the Bay of Biscay and Iberian Coast. Three distinct populations occur in this area:

  • the Belt Sea population inhabiting the southern part of Kattegat Sea, the Belt Sea and the southwestern Baltic Sea during summer constitutes a separate population (Sveegaard et al., 2015)
  • the greater North-East Atlantic / North Sea population including the northern Kattegat Sea and the Skagerrak Sea
  • the Iberian coast population (Fontaine et al., 2007).

Thereby the AU of the Kattegat and Belt Seas is likely to contain a mix of two different populations (Sveegaard et al., 2015).

This assessment covers only three of the five AUs: the North Sea (covering most of the Greater North Sea, except the Kattegat); the Kattegat and Belt Seas (covering the Kattegat area of the Greater North Sea); the Celtic Seas and French part of the Bay of Biscay (Figure 1).

Harbour porpoise bycatch estimation

ICES WGBYC collated observer data for net fisheries (Métier level 3 - for fishing métier levels: see https://datacollection.jrc.ec.europa.eu/wordef/fishing-activity-metier) to calculate bycatch rates in three of the five ICES AUs for harbour porpoise (see ICES, 2015b, 2016b). Bycatch rates for the North Sea AU and the Celtic and Irish seas AU were calculated from observer data pooled over the period 2006–2013 (ICES, 2015b). More recent bycatch data collected with Remote Electronic Monitoring (videos on fishing boats) and pooled for the period 2010–2014 were used to estimate bycatch rates in the Kattegat and Belt Seas AU (ICES, 2016b). Within each ICES sub-Division, WGBYC calculated the 95% confidence interval around the estimated bycatch rate (for methodology see ICES, 2015b). Data per sub-Division were then aggregated to calculate an overall bycatch rate for each AU.

Fishing effort by fleets (total number of days at sea) was summarised in each AU: in 2013 for the North Sea, Celtic and Irish Seas and in 2014 for the Kattegat and Belt Seas. Data on fishing effort (i.e. days at sea) were collated across all ‘net’ level 4 métiers: trammel nets, set nets and driftnets, across all seasons and all vessel length categories.

The lower and upper 95% confidence limits of the bycatch rate were then multiplied by fishing effort to generate a range of possible annual bycatch totals for each AU.

The effort data and the observations of harbour porpoise bycatch cover a wide range of vessel types and fishing areas. In their estimation of bycatch rates and fishing effort, WGBYC stated that “no account was taken of any spatial heterogeneity or of differences in mesh sizes or other important gear characteristics. There is therefore an implicit assumption that the summarized observations are representative of the nature and diversity of the gillnet fisheries within each assessment region, and this is not likely to be true. For this reason a range of numbers has been used to highlight the uncertainty in the overall bycatch estimate within each assessment unit. This approach does not address several potential biases. An examination of these will require detail of the fleet structure and how the observations are stratified.” (ICES, 2015b).

Abundance estimates

To provide context, the estimates of total bycatch of harbour porpoise in each AU were compared against the best abundance estimate. The best estimates of abundance for harbour porpoise in the Greater North Sea is taken from preliminary analysis of the results from the Survey of small Cetaceans in the European Atlantic and North Sea (SCANS-III) survey conducted in July 2016 (Hammond et al., in prep). The best abundance estimate for the Celtic and Irish Seas AU is taken from the SCANS-II survey conducted in 2005 (Hammond et al., 2013). The estimate from SCANS III for the Celtic and Irish Seas AU is not complete.

Ireland was not involved in SCANS III as it undertook a comprehensive national survey (ObSERVE Programme) in the period 2015–2016. In the Kattegat and Belt Seas AU the estimate from SCANS III was used rather than the estimate from 2012 produced by Viquerat et al. (2014), while the estimates are very similar, the most recent estimate was chosen.

Résultats

Le Conseil international pour l’exploration de la mer (CIEM) a estimé la mortalité par capture accessoire de marsouins communs dans trois de ses cinq unités d’évaluation (AU): le Kattegat et les mers Belt, la mer du Nord et les mers Celtiques et d’Irlande. Les résultats figurent dans le tableau 1. Les données sur les captures accessoires de marsouins communs, communiquées par les observateurs à bord de navires de pêche, sont insuffisantes pour l’AU de la péninsule ibérique et le risque de captures accessoires dans l’AU de l’ouest de l’Ecosse est très faible. Ces résultats ne sont donc pas présents dans cette évaluation.

Le CIEM exprime le total estimé des captures accessoires de marsouins communs sous la forme de limites de confiance inférieures ou supérieures de 95% plutôt que sous la forme d’une estimation unique. Le CIEM a utilisé les écarts de confiance pour mieux refléter l’incertitude que présentent les estimations de l’ensemble des captures accessoires au sein de chaque AU. Le CIEM considère que cette incertitude est due aux facteurs suivants:

  • les données sur l’effort de pêche (exprimées en nombre de jours en mer) sont probablement sous-estimées car les activités des navires de pêche plus petits (en particulier d’une longueur <10 m), des navires de plaisance et de pêche à partir de la plage ne sont pas représentées. Ceci entraînerait des sous-estimations des captures accessoires;
  • les taux de captures accessoires pourraient être surestimés car la majorité des enregistrements de captures accessoires sont recueillis par des observateurs à bord de grands navires (>15 m) utilisant plus d’engins que les navires plus petits et dont le risque de capture des cétacés pourrait dont être plus élevé;
  • les données sur l’effort de pêche et les enregistrements de captures accessoires recueillis par des observateurs à bord de navires couvrent tout un éventail de types de navires et d’engins de pêche (c’est-a-dire trémails, filets maillants, filets dérivants). On n’a tenu compte d’aucune hétérogénéité spatiale (c’est-à-dire éparsité) ou d’aucune différence de maillage, de longueur des filets ou autre caractéristique importante des engins. Le CIEM indique que « on présume implicitement ici que les observations résumées sont représentatives de la nature et de la diversité de la pêche au filet maillant au sein de chaque région d’évaluation et il est peu probable que ce soit le cas ».

Le CIEM conclut que son approche appliquée à l’estimation des captures accessoires « n’aborde pas plusieurs biais potentiels. Des informations détaillées sur la structure de la flotte et sur la méthode de stratification des observations seront nécessaires pour pouvoir étudier ces biais ».

La méthode utilisée pour cette évaluation inspire une confiance modérée/faible, la disponibilité des données inspire une confiance faible.

Tableau 1. Mortalité du marsouin commun par capture accessoires dans des filets de pêche dans chaque unité d’évaluation (AU), par rapport à la meilleure estimation de l’abondance.
AU Kattegat et mers Belt Mer du Nord Mers Celtiques et d’Irlande
Captures accessoires totales estimées LC 95% (an) 165–263 (2014) 1235–1990 (2013) 1137–1472 (2013)
Meilleure estimation de l’abondance (an) 42 300 (23 368 – 76 658) (2016)a 345 400 (246 526 – 495 752) (2016) a 106 382 (57 689 – 196 176) (2005)b
Captures accessoires annuelles exprimées en pourcentage de la meilleure estimation de l’abondance1 0,39–0,62% 0,36–0,58% 1,06–1,37%

 

Les données sur l’AU de la Péninsule ibérique sont insuffisantes et le risque de capture accessoire à l’ouest de l’Ecosse est très faible. Ces AU ne sont pas prises en compte dans cette évaluation

a Données de 2016 provenant d’une analyse préliminaire des résultats de l’étude du SCANS-III;

b Données de 2005 provenant de l’étude du SCANS-II car les estimations du SCANS III pour l’AU des mers Celtiques et d’Irlande ne sont pas encore complètes (Source: CIEM).

Note1: Les données sur l’effort de pêche (exprimées en nombre de jours en mer) sont probablement sous-estimées car les activités des navires commerciaux plus petits (en particulier d’une longueur <10 m), des navires de plaisance et de pêche à partir de la plage ne sont pas représentées. Ceci entraînerait des sous-estimations des captures accessoires. De plus, la variation de la « meilleure estimation de l’abondance » n’est pas comprise dans le calcul du taux annuel de captures accessoires.

Table a shows the values that were used by ICES to calculate bycatch rates presented in Table 1.

Table a. Harbour porpoise bycatch mortality from fishing netsa in each Assessment Unit (AU), compared against the best estimate of abundance. Observations of bycatch on fishing vessels were pooled over the time periods shown owing to the patchy coverage of relevant fisheries in any one year. Bycatch rate = number of bycaught harbour porpoise observed divided by number of observer days. Estimated total annual bycatch of harbour porpoise (in the year shown) = fishing effort (in the year shown) multiplied by bycatch rate.
AU Kattegat and Belt Seas North Sea Celtic and Irish Seas
Years over which bycatch data were collated 2010 - 2014 2006 - 2013 2006 - 2013
Number of observer days 1195 1398 3207
Number of bycaught animals observed 25 50 147
Bycatch rate – number of animals caught per dayᵇ 0.016-0.025 0.028-0.045 0.035-0.045
Fishing effort days-at-sea (year) 10,625 (2014) 44,165 (2013) 32,930 (2013)
Estimated total porpoise bycatch 95% CLs (year) 165-263 (2014) 1,235 - 1,990 (2013 1,137 - 1,472 (2013)
Best abundance estimate (year) 42,300 (cv 0.3) (2016)ᶜ 345,400 (cv 0.18) (2016)ᶜ 107,300 (cv 0.3) (2005)ᵈ
Annual bycatch as a percentage of the best abundance¹ estimate 0.39-0.62% 0.36-0.58% 1.06-1.37%

aData on fishing effort (i.e. days at sea) were collated across all ‘net’ level 4 métiers: trammel nets, set nets, and driftnets, across all seasons and all vessel length categories (ICES, 2015b, 2016b).

bBycatch rates are rounded to the nearest three decimal places; total bycatch was estimated using bycatch rates that were not rounded (Source: ICES).

c2016 data from preliminary analysis of the results from SCANS-III survey.

d2005 data from the Survey of small Cetaceans in the European Atlantic and North Sea (SCANS-II survey) because SCANS III estimates  are not yet complete in the Celtic and Irish seas AU. Note that estimated total porpoise bycatch has been calculated using unrounded figures for bycatch rate.

Note 1: The data on fishing effort (in number of days at sea) are likely to be underestimated as effort from smaller commercial vessels (particularly <10 m in length), from recreational vessels and from fisheries from the beach is not represented. This would lead to underestimates in bycatch.

Confidence Assessment

The methodology for estimating levels of bycatch is often used to assess this indicator and has been used in published ICES assessments and is therefore rated as moderate. The methodology for setting the threshold was not applied in this assessment because it is acknowledged to require further development  and is therefore rated as low. This results in an overall rating of moderate/low.

The assessment is undertaken using limited data with poor spatial coverage within the areas assessed. Data on certain aspects of the fishing fleet are also missing. The confidence rating is low.

Conclusion

Jusqu'à 2000 marsouins sont morts en 2013, pris dans des filets de pêche commerciale dans l’unité d’évaluation (AU) de la mer du Nord, définie par le CIEM, l’abondance totale étant estimée à 345 400. On estime que la même année, 1500 individus sont morts dans l’AU des mers Celtiques et d’Irlande, l’abondance totale étant estimée à 107 300. On estime que 260 autres marsouins communs sont morts en 2014 dans l’AU du Kattegat et des mers Belt, l’abondance totale étant estimée à 42 300.

Les estimations des captures accessoires fournies par le Conseil international pour l’exploration de la mer (CIEM) représentent les meilleures estimations disponibles, étant donné les données sous-jacentes. On pourrait obtenir de meilleurs taux de captures accessoires en observant les captures accessoires à bord d’un échantillon de navires représentant la flotte de pêche au sens large du point de vue du type d’engin de pêche, de la taille des navires et de la répartition des activités de pêche dans l’espace et dans le temps. Les estimations actuelles des captures accessoires sont dérivées de l’observation de 0,28% seulement de l’effort de pêche pour les types d’engins de pêche considérés « à filet »; une meilleure couverture par les observateurs dans des études spécifiques permettrait également d’améliorer la fiabilité des estimations futures du taux de captures accessoires.

This bycatch assessment is for harbour porpoise (Phocoena phocoena) only. Bycatch estimates provided by the International Council for the Exploration of the Sea (ICES) represent the best available estimates given the underlying data. They could be more reliably compared against assessment values, if their accuracy was improved. This would require more accurate estimates of bycatch rate and fishing effort.

The International Council for the Exploration of the Sea (ICES, 2016a,b) has also collated bycatch data for short-beaked common dolphin (Delphinus delphis) in ICES Divisions VII, VIII and IX, which include parts of the Celtic Seas, Bay of Biscay and Iberian coast, and Wider Atlantic. A total of 2,509 common dolphin were recorded killed during 13,746 observer-days. ICES (2016a) cautioned that the total observed common dolphin bycatch is “based upon information that is not complete. It is not known if these estimates are biased (nor the direction of any bias), so this advice cannot yet be regarded as definitive”. This lack of confidence in the data meant that ICES (2016a,b) did not estimate total annual bycatch (i.e. by multiplying bycatch rates by estimates of fishing effort for each gear type). Hence, the preliminary results for common dolphin in this OSPAR assessment could not be compared with bycatch estimates for harbour porpoise.

Lacunes des Connaissances

L’évaluation de cet indicateur n’a pas utilisé de valeur d’évaluation. L’Accord sur la conservation des petits cétacés de la mer Baltique, du nord est de l'Atlantique et des mers d'Irlande et du Nord (ASCOBANS) recommande que « l’élimination anthropique totale » du marsouin commun ne devrait pas dépasser 1,7% de la meilleure estimation disponible de l’abondance, l’objectif de précaution étant de réduire les captures accessoires pour qu’elles représentent moins de 1% et en dernier ressort 0%.

L’utilisation du principe « mètres de filet par jour » pourrait fournir un enregistrement plus précis de l’effort de pêche que le principe « nombre de jours en mer », en particulier dans le cas des types de filets (par exemple filets maillants) qui risquent plus de capturer des marsouins communs que des engins mobiles (par exemple chaluts).

Marine mammal bycatch monitoring

During their review of bycatch reports by European Union Member States, the International Council for the Exploration of the Sea (ICES) found that monitoring of bycatch by observers on fishing vessels varies over space and time (ICES, 2015b). Monitoring of bycatch in set gillnets, trammel nets and drift nets (fishing métier level 4), the most lethal gear to harbour porpoise (Phocoena phocoena), is mandatory only on vessels over 15 m in length; yet the majority of vessels using these gear types in the North East Atlantic measure less than 15 m. There are also substantial amounts of recreational (i.e. non-commercial) netting that takes place in some countries that are not monitored for bycatch and effort. This results in bycatch estimates with unquantified bias and generally wide confidence intervals, which diminishes the confidence in reported impacts on population size.

ICES (2015a) concluded that more effective monitoring of bycatch, fishing effort and population sizes of cetaceans would be required to provide a quantitative assessment of the impact of bycatch on harbour porpoise and other cetacean species.

In 2016, Parties under the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) adopted a resolution on the monitoring and mitigation of small cetacean bycatch. This aims to ensure that monitoring programmes are robust and able to generate estimates of cetacean bycatch for all relevant fisheries (this may include different vessel sizes, observers, remote electronic monitoring, rapid bycatch assessment methods and other measures as appropriate). The resolution also addresses the development, implementation and evaluation of appropriate technical and other measures to mitigate cetacean bycatch (and may include alternative fishing methods that are ecologically sustainable, pingers not audible to seals and alerting devices proven to be effective for appropriate mitigation, or gear exchange schemes aimed at reducing bycatch). European Union regulations on cetacean bycatch (EC/812/2004) require the use of acoustic deterrents but only on certain commercial fishing vessels operating in specified areas and times of year.

To provide total bycatch estimates whatever the vessel length and métier, a new method using strandings and their likely origin at sea was implemented to assess small cetacean bycatch (see Peltier et al., 2016). Use of different complementary data sources constitutes an efficient way to estimate cetacean bycatch. For example: observer programmes on large vessels to understand the specificity of interactions between mega-vertebrates and fisheries; rapid by-catch assessment carried out for evaluating cetacean bycatch and specific interactions on smaller vessels and artisanal fisheries (Poonian et al., 2008; Moore et al., 2010; Goetz et al., 2015; Oliveira et al., 2015); and the interpretation of stranding data, in order to evaluate the impact of fisheries on cetaceans at the population scale. It should be noted that remote electronic monitoring using onboard cameras provides a reliable alternative to observers. This method applies to large- and medium-sized vessels (Kindt-Larsen et al., 2012; ICES, 2015b, 2016b).

Common dolphin (Delphinus delphis) bycatch estimates were calculated from strandings since 1990 (Peltier et al., 2016). Estimations are in progress for harbour porpoises in the Greater North Sea, south of the Celtic Seas and the Bay of Biscay. Such total bycatch estimates from strandings could therefore be included during the further development of this indicator.

Assessment values

Owing to uncertainly concerning the reliability of the fishing effort data and the potential for biases in the bycatch data, this assessment does not compare the ICES bycatch estimates with assessment values used by the OSPAR North Sea Ecological Quality Objective (EcoQO) for bycatch of harbour porpoise. ICES has suggested that fishing effort could be more accurately recorded using measures of ‘net meter per day’. This, or a similar metric could more precisely record fishing effort than ‘days at sea’, for net types (e.g. set gillnets) that are more likely to catch harbour porpoise than mobile gear (e.g. trawls). Information on net length and soak time are rarely reported in fishing effort statistics.

ASCOBANS currently recommends that ‘total anthropogenic removal’ of harbour porpoises should not exceed more than 1.7% of the best available estimate of abundance (‘environmental limit’). This would ensure that population size is maintained at or restored to at least 80% of carrying capacity. If available evidence suggests that a population is severely reduced, or in the case of species other than the harbour porpoise, or where there is significant uncertainty in parameters such as population size or bycatch levels, then ‘unacceptable interaction’ may involve an anthropogenic removal of much less than 1.7%. ‘Total anthropogenic removal’ includes mortality resulting from all pressures caused by human activities, such as ship strikes and pollution as well as bycatch. An ASCOBANS resolution explicitly linked to harbour porpoise bycatch states that "total anthropogenic removal is reduced by the Parties to below the threshold of ‘unacceptable interactions’ with the precautionary objective to reduce bycatch to less than 1% of the best available abundance estimate and the general aim to minimise bycatch (i.e. to ultimately reduce to zero)” (ASCOBANS, 2006, 2016). Indirect mortality due to other pressures is more difficult to identify and measure, compared to mortality caused by bycatch. Overall, the number of cetaceans being killed by human activities could be more than the number estimated as bycatch.

The ASCOBANS value of 1.7% of the best available abundance estimate is derived from work undertaken by a working group convened by the International Whaling Commission and ASCOBANS (IWC, 2000). However, there has been much debate surrounding the nature of the population model used to arrive at this value. This was a very simple deterministic population dynamics model, which assumed a ‘biological’ population with independent population dynamics and no uncertainties in any parameter. Although assessment units (AUs) have been defined for harbour porpoise, there is little evidence to support the assumption that the population dynamics of animals within each AU are independent. Consequently, where the population dynamics are not independent, the management limits calculated on the basis of independent biological populations are unlikely to be appropriate. An alternative to such an approach is the bycatch management procedures (adapted Catch Limit Algorithm (CLA) approach) developed under the Small Cetaceans in the European Atlantic and North Sea; SCANS-II and Cetacean Offshore Distribution and Abundance in the European Atlantic; CODA projects (Winship, 2009).

In 2009, ICES advised the European Commissionthat a Catch Limit Algorithm approach is the most appropriate method to set limits on the bycatch of harbour porpoises or common dolphins. In order to use this (or any other) approach, specific conservation objectives must first be specified. In both species improved information on bycatch and the biology of the species would improve the procedure.” (ICES, 2009). In 2010, ICES again advised the European Commission that “ICES advised in 2009 of the need for explicit conservation and management objectives for managing interactions between fisheries and marine mammal populations. This advice has not been acted upon. Lacking these objectives, ICES is unable to properly consider the impacts of these interactions in its management advice” (ICES, 2010). The ICES Working Group on Marine Mammal Ecology noted again (ICES, 2013) that this advice still had not been acted upon and, to aid such decisions, suggested that ASCOBANS be asked to consider the policy decisions required for the setting of safe bycatch limits. This discussion has started within ASCOBANS, with two related workshops having been held in 2015 (ASCOBANS 2015a,b), and at least one more planned.

Overlap of harbour porpoise populations in the Kattegat

The northern part of the Kattegat and Belt Seas AU is occupied by harbour porpoise from two genetically distinct populations – the North Sea population and the Kattegat/Belt Seas population (Sveegaard et al., 2015). Abundance estimates in the North Sea AU will underestimate the size of the North Sea population and abundance estimates in the Kattegat and Belt Seas AU will overestimate the size of the Kattegat and Belt Seas population. For the North Sea population, this error is probably less significant than for the much smaller Kattegat and Belt Seas population. Animals from both populations are at risk of being bycaught in each other’s AUs. Reported bycatch numbers in the northern Kattegat cannot be reliably assigned to one population or the other. Tissue samples to be taken from bycaught animals and advanced genetic techniques would make it possible to assign bycatch in the Kattegat and eastern North Sea to one of the two populations present.

ASCOBANS 2015a. ASCOBANS Expert Workshop on the Requirements of Legislation to Address Monitoring and Mitigation of Small Cetacean Bycatch, January 2015, Bonn, Germany (http://www.ascobans.org/sites/default/files/document/AC22_Inf_4.1.a_Report_ExpertWorkshop_EUBycatchLegislation.pdf)

ASCOBANS 2015b. ASCOBANS Workshop on Further Development of Management Procedures for Defining the Thresholds of Unacceptable Interactions, July 2015, London, United Kingdom (http://www.ascobans.org/sites/default/files/document/AC22_Inf_4.1.c_WS_UnacceptableInteractions_I_2015_Report.pdf)

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