This pre-core indicator and its threshold values are yet to be commonly agreed in HELCOM. The indictor is included as a test indicator for the purposes of the 'State of the Baltic Sea' report, and the results are to be considered as intermediate.
The level of pressures affecting the status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. This indicator provides an indicator-based evaluation of the numbers drowned mammals and waterbirds in fishing gear, and this information should be considered together with other biodiversity core indicator evaluations in order to achieve an overall assessment of the status of biodiversity, particularly once further developed.
The core indicator on number of drowned mammals and waterbirds in fishing gear addresses the Baltic Sea Action Plan's (BSAP) Biodiversity and nature conservation segment's ecological objectives 'Viable populations of species' and 'Thriving and balanced communities of plants and animals' as well as the Eutrophication segment's ecological objective 'Natural distribution and occurrence of plants and animals'. It also addresses the following specific target:
'By 2015 by-catch of harbour porpoise, seals, water birds and non-target fish species has been significantly reduced with the aim to reach by-catch rates close to zero'.
In the BSAP, it was further agreed to set up a reporting system and database for harbour porpoise incidental by-catch, and competent fisheries authorities were urged to minimize the incidental by-catch of harbour porpoises.
The core indicator also addresses the following qualitative descriptors of the MSFD for determining Good Environmental Status (European Commission 2008a):
Descriptor 1: 'Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions' and
Descriptor 4: 'All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity',
and the following criteria of the Commission Decision (European Commission 2010):
For the three seal species occurring in the Baltic Sea, the HELCOM Recommendation (27-28/2) adopted in 2006 relating to seals recommends:
Presently, management objectives for all protected species are unclear at the EU level (ICES 2013a). While broad commitments have been made to achieve Good Environmental Status (GES) under the EU Marine Strategy Framework Directive (MSFD), and to Favourable Conservation Status (FCS) under the Habitats Directive, translating these goals into specific targets on incidental by-catch limits is as yet unspecified by the EU.
The EU Habitats Directive lists harbour porpoise as a strictly protected species (Annex IV). The harbour porpoise and the three seal species are listed in Annex II, meaning that they are to be protected by the means of the Natura 2000 network. Article 12, paragraph 4 of the Habitats Directive states that Member States shall establish a system to monitor the incidental capture and killing of the animal species listed in Annex IV (a) (European Commission 1992). In the light of the information gathered, Member States shall take further research or conservation measures as required to ensure that incidental capture and killing does not have a significant negative impact on the species concerned. Member States of the EU are further obliged to develop national programmes for monitoring fisheries, including on board monitoring, under Article 3 of Council Regulation 199/2008, Commission Regulation 665/2008 and the Annex of Commission Decision 2010/93/EU (European Commission 2008b, 2008c, 2009a). These plans include detailed data on fleet capacity and fishing effort by metier and fishing area.
The EU Birds Directive aims to protect, inter alia, habitats of endangered and migratory birds to ensure their conservation in the Europe (European Commission 2009b). This not only refers to birds needing special conservation measures (Article 4 (1)) and listed in Annex I (black-throated diver, red-throated diver, Slavonian grebe, Steller's eider, smew), but also to all migratory species (Article 4 (2)). Therefore, all waterbird species breeding, wintering and staging during migration in the Baltic Sea are covered by this Directive.
EU legislation clearly requires Member States to take measures prohibiting deliberate killing or capture by any method (Article 5 Birds Directive; Article 12 Habitats Directive) which also includes the mere acceptance of the possibility of killing or capture (Case C‑221/04 Commission v Spain  ECR I‑4515, paragraph 71). Further, the Habitats Directive requires that incidental capture or killing of cetaceans is monitored, and that it should not have a significant negative impact on the species.
As a voluntary instrument within the framework of EU and international environmental and fishery legislation and conventions, the EU Commission has adopted an 'Action Plan for reducing incidental by-catches of seabirds in fishing gears' (European Commission 2012). It aspires to provide a management framework to minimise incidental by-catch as much as possible in line with the objectives of the reformed EU Common Fisheries Policy (CFP), i.e. to cover all components of the ecosystem. Among others, proposed action includes the monitoring of seabird incidental by-catch with a minimum coverage of 10% of the fisheries and mitigation measures.
The Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) aims to achieve and maintain a favourable conservation status of small cetaceans. Six of the nine Baltic Sea countries are Parties to the Convention (Denmark, Germany, Sweden, Poland, Lithuania and Finland).
All waterbird species occurring in the Baltic Sea are subject of the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA).
There was a substantial increase in gillnet fisheries in parts of the region, while in other parts gillnet fishing has declined.
In Baltic Sea fisheries, the use of anchored gill nets substantially increased in the 1990s, and because of the change in stock age composition of cod also in the late 1990's and early 2000 (ICES 2016b), intensifying the conflict between certain fisheries and bird and mammal species. Waterbirds diving during foraging in order to catch demersal or pelagic fish (divers, grebes, cormorants, mergansers, alcids) and benthic invertebrates (ducks), respectively, are prone to become entangled in various types of nets and to die by drowning. In addition to hunting (Mooij 2005) and oiling (Larsson & Tydén 2005), drowning in fishing gear is a quantitatively important source of mortality for waterbirds living in the Baltic.
The intensification in use of anchored gill nets in the coastal waters of Estonia, Latvia and Lithuania has substantially increased the risk of drowning for the indicator species in the last decades (Žydelis et al. 1990). In other areas, such as Polish, Swedish and Danish waters, fishing efforts have decreased in recent years, (Katarzyna Kaminska, Ida Carlén & Finn Larsen, pers. comm.).
In the wide range of parameters that influence the population dynamics of birds in general, waterbirds belong to those species with high longevity and low reproductive rates. They are therefore vulnerable to the loss, especially of adult individuals, as it takes a relatively long time to compensate for such losses (Bernotat & Dierschke 2016). For waterbirds living in the Baltic Sea, the mismatch between the loss of individuals and the effort to replace them is most pronounced in alcids, whereas ducks may compensate more easily owing to higher reproductive rates and lower ages of first breeding. However, other factors promoting or impeding population growth rates may override this pattern. For example, changing population sizes are at least partly influenced by favourable food supply (increase of alcids; Österblom et al. 2006), reduced mussel stocks (common eider; Laursen & Møller 2014) or low reproductive success (long-tailed duck; Hario et al. 2009).
The same applies to harbour porpoise and seals, which are top predators in the Baltic Sea marine food web and which, due to their population dynamics, are vulnerable to additive mortality (Bernotat & Dierschke 2016). Incidental mortality that exceeds the potential rate of increase will drive a population to extinction. It is thus necessary to keep the sum of all anthropogenic mortality, including incidental by-catch, below a critical value. From the conservation perspective, immediate management consequences are needed if this critical value is exceeded. In 1991, the Scientific Committee of the International Whaling Commission recommended that incidental mortality should not exceed half of the potential rate of increase (IWC 1991). Furthermore, incidental mortality greater than one fourth of the potential rate of increase should be considered cause for concern (IWC 1996). The figure for the potential rate of population increase for harbour porpoises used for population model simulations by ASCOBANS and the IWC is 4% per annum based on their known life history parameters. Given the high levels of environmental contaminants, including heavy metals and PCBs, of harbour porpoises in the Baltic Sea and impaired immune function (e.g. Siebert et al. 1999, Beineke et al. 2005, 2007a,b, Ciesielski et al. 2006) and the correlation between lower PCB burdens and reproductive failure in UK harbour porpoises (Murphy et al. 2015), the reproductive rate of harbour porpoises in the Baltic Sea may be lower than this generically used rate of increase.
For harbour porpoises, the incidental by-catch risk is highest in various types of gillnets: set gill nets (gear type: GNS), entangling nets (trammel nets, GTR) and driftnets (GND) (ICES 2013a). The latter are banned in the Baltic Sea, but some hybrid nets such as 'semi-driftnets' which are fixed on one end of the net with the other end drifting around this anchor are of special concern (Skora & Kuklik 2003). In a number of cases, fisheries have tried to circumvent driftnet restrictions of the EU Common Fisheries Policy (CFP) through minor technical modification (Caddell 2010). Due to their properties (one end freely drifting around an anchor), semi-driftnets which are locally used in Poland may thus attract close attention from the Commission in future years, if they remain widely used on a commercial scale (Caddell 2010). These nets have been reported as GND until 2007, and now (after the ban of GND) are considered GNS (Hel Marine Station, pers. comm.). The Proposal for a Regulation on the conservation of fishery resources and the protection of marine ecosystems through technical measures (COM(2016)134 final-2016/0074, 11.3.2016) is still preliminary as work is still in progress. Therefore the Commission has not finally decided a definition of set-nets in Art. 6(21) which accounts for the hybrid nature of semi-driftnets .
The mean longevity of harbour porpoises is only 8-10 years (Read & Hohn 1995; Lockyer & Kinze 2003; Bjørge & Tolley 2009). Stranding data show that only 5% of porpoises live beyond 12 years (Lockyer & Kinze 2003). Sexual maturity is reached late, at the age of 3 to 5 years (Sørensen & Kinze 1994; Adelung et al. 1997; Benke et al. 1998; Lockyer & Kinze 2003). Based on this, it is estimated that a female with a longevity of about 10 to 12 years can deliver only 4 to 6 calves during its life span (Lockyer & Kinze 2003), which would only allow for slow recovery.