Harbour Porpoise Bycatch

D1 - Biological Diversity

D1.3 - Population condition

Bycatch is recognised as a major cause of human-induced mortality of harbour porpoise. Nearly 4000 harbour porpoises of a total population in excess of 490 000 are drowned in fishing nets annually in the areas assessed. However, there is low confidence in these bycatch estimates due to incomplete monitoring data.

Area Assessed

Printable Summary

Background

The main human-induced cause of mortality of cetaceans in the OSPAR Maritime Area is being caught and entangled in fishing nets. Seals are also bycaught but there is insufficient knowledge to include them in this assessment.

Harbour porpoise has been 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.

This assessment is taken solely from the latest advice on the numbers of cetaceans that are incidentally caught and killed by fishing, provided to the European Commission by the International Council for the Exploration of the Sea (ICES). No additional information has been provided by OSPAR Contracting Parties. ICES estimated the numbers of harbour porpoise caught in commercial nets (mainly set gillnets) in the ICES derived Assessment Units shown in Figure 1.  The bycatch estimates are derived from estimates of annual fishing effort and counts of bycaught harbour porpoises made by observers or remote electronic monitoring on commercial fishing vessels.

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.

Dead harbour porpoise with net marks from being caught in a fishing net (courtesy of Jan Haelters)

Figure 1: Harbour porpoise ICES Assessment Units (AUs) (as proposed by ICES, 2014) –

note that these are ecologically derived and do not align with the OSPAR Regions

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.

Results

Bycatch mortality in harbour porpoise have been estimated by the International Council for the Exploration of the Sea (ICES) in three of their five assessment units (AUs): the Kattegat and Belt Seas, the North Sea, and the Celtic and Irish Seas. The results are shown in Table 1. Data on harbour porpoise bycatch from observers on fishing vessels were insufficient for the Iberian Peninsula AU and the risk of bycatch in the West of Scotland AU is very low. As a result these are not represented in this assessment.

ICES expressed the estimated total harbour porpoise bycatch in the form of lower and upper 95% confidence limits rather than as a single estimate. The confidence intervals were used by ICES to better reflect uncertainty in the estimates of overall bycatch within each AU. ICES considers this uncertainty to result from the following factors:

  • The data on fishing effort (in number of days at sea) are likely to be underestimated because 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;

  • The bycatch rates may be overestimated because the majority of bycatch records were collected by observers on large vessels (>15 m) that use more gear than smaller vessels and may have higher likelihood of catching cetaceans;

  • The data on fishing effort and the bycatch records from observers on vessels cover a wide range of vessel types and fishing gear types (i.e. trammel nets, set gillnets, driftnets). No account was taken of any spatial heterogeneity (i.e. patchiness) or of any differences in mesh size, net length or other important gear characteristics. ICES point out “there is 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”.

ICES concluded that their approach to estimating bycatch “does not address several potential biases. An examination of these will require detail of the fleet structure and how the observations are stratified”.

Table 1. Harbour porpoise bycatch mortality from fishing nets in each Assessment Unit, compared against the best estimate of abundance
AU Kattegat and Belt Seas North Sea Celtic and Irish Seas
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)ᵃ 345,400 (cv 0.18)(2016)ᵃ 107,300 (cv 0.3) (2005)ᵇ
Annual bycatch as a percentage of the best abundance¹ estimate 0.39-0.62% 0.36-0.58% 1.06-1.37%

 

Table 1 Notes: 

Data were insufficient for the Iberian Peninsula AU and the risk of bycatch in the West of Scotland AU is very low. These have not been included in the assessment

2016 Data from preliminary analysis of the results from SCANS-III survey; b2005 data from SCANS-II survey because SCANS III estimates are  not yet complete in the Celtic and Irish Seas AU (Source: ICES).

Note1: 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. Furthermore, variation in “Best abundance estimate” is not included in the calculation of annual bycatch rate.

Confidence has been rated as moderate / low for the method of this assessment and low for data availability.

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

In 2013 up to 2,000 harbour porpoise died as a result of entanglement in commercial nets in the ICES defined North Sea Assessment Unit (AU), out of a total abundance estimate of 345,400. In the same year, an estimated 1,500 individuals died in the Irish and Celtic Seas AU, out of a total abundance estimate of 107,300. In 2014, a further 260 harbour porpoise were estimated to have died in the Kattegat and Belt Seas AU, out of a total abundance estimate of 42,300.

Bycatch estimates provided by the International Council for the Exploration of the Sea (ICES) represent the best available estimates given the underlying data. More accurate bycatch rates could be obtained by observing bycatch on a sample of vessels that represents the wider fishing fleet in terms of fishing gear type, vessels size and distribution of fishing activity over space and time. The current bycatch estimates are derived from observing only 0.28% of the fishing effort for the fishing gear types classified as ‘nets’; a higher observer coverage in dedicated surveys would also improve the reliability of future estimates of bycatch rate.

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.

Knowledge Gaps

This indicator assessment has not used an assessment value. The Agreement on the Conservation of Small Cetaceans in the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) recommends  that ‘total anthropogenic removal’ of harbour porpoise should not exceed 1.7% of the best available estimate of abundance, with the precautionary objective of reducing bycatch to less than 1% and ultimately 0%.

The use of ‘Net meter per day’ could provide a more accurate record of fishing effort than ‘days at sea’, especially in the case of net types (e.g. set gillnets) that are more likely to catch harbour porpoise than mobile gear (e.g. trawls).

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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