Non-indigenous species

Non-indigenous species are species that have spread or been transferred as a result of human activities, reaching environments in which they previously did not naturally occur. Shipping and aquaculture 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. To date, around 140 non-indigenous species or new species with unknown origin (cryptogenic species) have been recorded in the Baltic Sea. Of these, twelve were new introductions for the Baltic Sea in the period 2011–2016.

Harbours and ports are hot spots for the introduction of non-indigenous species as they offer extended periods during which ships are stationary, and often offer suitable places for species to settle in shallow water or modified habitats (Lehtiniemi et al. 2015). Non-indigenous species are usually not dispersed by natural means, but arrive in their new environments via some form of human-mediated transport, so called vectors. The most probable vectors for non-indigenous species into the Baltic Sea are aquaculture and shipping (Galil et al. 2014). These species commonly attach to the ships hulls (so called biofouling) or are transported in ballast water and then released when the water is exchanged. Furthermore, the opening of connections to different river systems created by canals are important vectors for dispersal, and many Ponto-Caspian species have found new routes to the Baltic Sea in this way. Although the Baltic Sea contains numerous non-indigenous species, 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 area, non-indigenous species may spread further. The rate of spread is often determined by species specific factors, such as environmental tolerance or reproductive rates. For example, the round goby (Neogobius melanostomus), a bottom-dwelling invasive fish originating in the Black Sea and Caspian Sea, was observed for the first time in the Baltic Sea in 1990. After a few years with low abundance, the species 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 web (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 different ways, and their effects are often difficult to foresee. Risk assessments are important to guide the management of non-indigenous species and help to implicate measures at an early stage (Katsanevakis et al. 2014).

The HELCOM core indicator assesses the number of new introductions (primary introductions) to the Baltic Sea region for the given assessment period (2011-2016). The threshold value is zero, as it 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 (Core indicator report: HELCOM 2018af). Thus, the core indicator evaluates the successfulness of management to prevent introductions (Olenin et al. 2016).

Assessment result

Twelve species have arrived as new non-indigenous species in the Baltic Sea between 2011 and 2016.  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 three 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, and the presence of designated monitoring strategies differs greatly between the sub-basins (Core indicator report: HELCOM 2018af).

During the assessment period, an unknown number of previously arrived non-indigenous species have also expanded their distribution range to new sub-basins in the Baltic Sea. It is often difficult to ascertain if this secondary spread is due to human activities or not. Secondary spread is not included in the evaluation of the core indicator, which only includes first time introductions. 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 in the Baltic Sea for this assessment period.

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.

Figure 4.5.1. Number of new non-indigenous species in the Baltic Sea. Upper graph: Estimated number of new observed non-indigenous species in Baltic Sea per decade. The bars indicate the number of invasions per time period. The red part of the last bar denotes observations from 2011 onwards. Lower graph: The same data set shown as cumulative numbers since the 1900s. Based on data from the data based ‘AquaNIS’, as used in Ojaveer et al. (2016).

Impacts and recovery

Non-indigenous species pose a threat to the marine environment as they may induce changes in the structure and dynamics of the ecosystem. For example, the distribution and abundance of the round goby is a reality to be dealt with in many parts of the Baltic Sea. How this fish, as well as other non-indigenous species, will affect the food web and the ecosystem is important to comprehend so that potential changes can be foreseen.

The impacts of single, let alone multiple, non-indigenous species are complex and may in some cases be hard to distinguish from the impacts of other pressures. Economic impacts occur due to loss of fishing possibilities, expense to industries for cleaning intake pipes, and to remove biofouling, for example. Public health impacts can arise from the introduction of pathogens or toxic algae (Zaiko et al. 2011). However, even though the risks are generally known, it is often hard to predict the impacts of non-indigenous species in marine ecosystems, as these are poorly documented (Ojaveer et al. 2016).

Once a non-indigenous species has become established and spread to a wide area, eradication is not a viable management option.  Full recovery in the sense of returning back to a previous state is not possible. 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 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 Denmark, Estonia, Finland, Germany,  Russia, and Sweden have ratified the convention. Increased attention will be placed on the development of measures to address biofouling as a vector in the introduction of non-indigenous species.