Non-indigenous species have not dispersed naturally into their current environment, but have been transferred there as a result of either intentional or unintentional human activities. Shipping and boating are important vectors for the introduction and spread of non-indigenous species, since the species are easily transported in ballast water tanks or on ship hulls. Up to this date, around 140 non-indigenous species or species with unknown means of arrival (cryptogenic species) have been recorded in the Baltic Sea. Of these, 14 were new introductions for the Baltic Sea in the period 2011–2015.
Harbours and ports are hot spots for the introduction of non-indigenous species, as they may easily find suitable places to settle in shallow water or modified habitats (Lehtiniemi et al. 2015). The non-indigenous species are not dispersed by natural means to the new area, but follow some human-mediated means of transport, so called vectors. The most probable vectors for non-indigenous species into the Baltic Sea are aquaculture and shipping (Galil et al. 2014). The species may attach to the ships hulls, by fouling, or be transported in the ballast water and then be released when the water is exchanged. In addition the opening of connections to different river systems created by canals are probably vectors for dispersal, and many Ponto-Caspian species have found new routes to the Baltic Sea in this way. Salinity levels and temperature may in some cases limit the spread and establishment of non-indigenous species within the Baltic Sea (Holopainen et al. 2016).
After their first introduction to a new sea non-indigenous species may spread further within the new sea area. For instance, the round goby (Neogobius melanostomus), a bottom-dwelling invasive fish, was observed for the first time in the Baltic Sea in 1990. After a few years with low abundance it suddenly increased dramatically, and it is now a dominant species in many areas of the Baltic Sea, with a capacity to change interactions in the benthic food webs (Kotta et al. 2016). This pattern of establishment and consecutive spread is characteristic of invasive species. However, not all non-indigenous species are invasive, and may not spread widely nor become abundant. Established non-indigenous species may influence biodiversity and the ecosystem in a negative or a positive way, or they may have no effect. Even though it is difficult to foresee their effect, a risk assessment could guide the management of non- indigenous species and help to implicate measures at an early stage (Katsanevakis et al. 2014).
The HELCOM core indicator assess the number of new introductions (primary introductions) to the Baltic Sea region. The threshold value is set in relation to the objective that there should be no primary introductions of non-indigenous species due to human activities during a six year assessment period. Thus, the core indicator evaluates the successfulness of management to prevent introductions (Olenin et al. 2016).
Fourteen species have arrived as new non-indigenous species in the Baltic Sea since the year 2011. Hence, the core indicator fails the threshold value (zero new introductions) for good status. The animal species were represented by five small crustaceans, three worms (Annelida), and four species belonging to other animal groups. Two algae were also observed; one diatom and one red alga (Table 4.5.1). The estimate may be seen as a minimum count, as it is difficult to ascertain the absence of a new introduction. The given geographic position for first occurrence may also be influenced by variation in monitoring intensity among sub-basins (Core indicator report: HELCOM 2017v).
During the assessment period an unknown number of previously arrived non-indigenous species have expanded their distribution range to new sub-basins in the Baltic Sea. Since it is often difficult to ascertain if this secondary spread is due to human activities or not, these species are not included in the evaluation of the core indicator. For example, the mud crab (Rhithropanopeus harrisii) was observed as a new species to the Swedish Western Gotland basin in 2014, but given that it was previously observed in Poland, Denmark, Germany and the Russian Kaliningrad coast in the 1950s it is not counted as a new arrival.
|Species||Taxonomic group (phylum or division)||First reported from||Year|
|Beroe ovata||Ctenophora||Great Belt||2011|
|Chaetoceros concavicornis||Ochrophyta (Class Diatomea)||Great Belt||2011|
|Sinelobus c.f.vanhaareni||Crustacea||Arkona Basin||2012|
|Grandidierella japonica||Crustacea||Bay of Mecklenburg||2015|
|Haminoea solitaria||Mollusca||Bay of Mecklenburg||2016|
|Antithamnionella ternifolia||Rhodophyta||Kiel Bay||2014|
|Diadumene lineata||Cnidaria||Kiel Bay||2011|
|Hemigrapsus takanoi||Crustacea||Kiel Bay||2014|
|Tubificoides heterochaetus||Annelida||Gdansk Basin||2013|
|Echinogammarus trichiatus||Crustacea||Bornholm Basin||2014|
|Garveia franciscana||Cnidaria||Kiel Bay||2014|
|Proasellus coxalis||Crustacea||Bornholm Basin||2011|
|Laonomesp.||Annelida||Gulf of Riga||2013|
Human mediated introductions of species to the Baltic Sea has also occurred in the past. A reconstruction of previous events suggest that the rate of introduction of non-indigenous species has increased in recent decades (Ojaveer et al. 2016). Introduction rates during the first and second decade of the 2000s seem to be of the same order of magnitude (Figure 4.5.1). Importantly, the likelihood of observing new introductions is dependent on the monitoring effort, and increases with increasing monitoring effort.
Non-indigenous species pose a threat to the marine environment as they may induce changes in the structure and dynamics of the ecosystem. The impacts are complex and may be hard to distinguish from impacts of other pressures. Economic impacts may occur due to loss of fishing possibilities, expenses incurred by industries to clean intake or outflow pipes, and biofouling. Public health impacts can arise from the introduction of pathogens or toxic algae (Zaiko et al. 2011). In general, however, the impacts of non- indigenous species in marine ecosystems are poorly documented (Ojaveer et al. 2016).
Once a non-indigenous species has become established and spread to a wide area then eradication is not a viable management option. Hence, management should primarily aim to prevent further introductions, along with minimizing the negative effects of the already introduced non-indigenous species.
The entry into force of the Ballast water management convention of the International Maritime Organization in September 2017 and its further ratifications can be expected to decrease the pressure and risk of new introductions of non-indigenous species and other harmful organisms to the Baltic Sea. To date the HELCOM countries Germany, Russia, Denmark, Sweden and Finland have all ratified the convention.