4. Pressures 4.5. Non-indigenous species State of the Baltic Sea Second HELCOM holistic assessment 2011-2016
Assessment result
Twelve species have arrived as new non-indige-nous 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 crus-taceans, 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 num-ber of previously arrived non-indigenous species have also expanded their distribution range to new sub-basins in the Baltic Sea. It is often diffi-cult 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 have also occurred in the past. A re-construction of previous events suggest that the rate of introduction of non-indigenous species has increased in recent decades (Ojaveer et al. 2016). In-troduction rates during the first and second decade of the 2000s seem to be of the same order of magni-tude (Figure 4.5.1). However, it is important to note that 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 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).
4. Pressures 4.5. Non-indigenous species State of the Baltic Sea Second HELCOM holistic assessment 2011-2016
Impacts and recovery
Non-indigenous species pose a threat to the ma-rine 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 oth-er 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 im-pacts of other pressures. Economic imim-pacts occur due to loss of fishing possibilities, expense to in-dustries 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 es-tablished 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 pri-marily aim to prevent further introductions, along with minimizing the negative effects of the already introduced non-indigenous species.
The entry into force of the IMO 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-in-digenous species and other harmful organisms to the Baltic Sea. To date, the HELCOM countries Denmark, Estonia, Finland, Germany, Lithuania, Russia and Sweden have ratified the convention.
Increased attention will be placed on the develop-ment of measures to address biofouling as a vector in the introduction of non-indigenous species.
The round goby (Neogobius melanostomus) originates from the Black Sea and Caspian Sea.
© Zilvinas Putys
State of the Baltic Sea Second HELCOM holistic assessment 2011-2016
4.6. Species removal by fishing and hunting
Commercially exploited fish
The Baltic Sea fisheries target both marine and freshwater species, but the most important species for the commercial fisheries are marine (Box 4.6.1).
Cod, herring and sprat represent about 95 % of the total catch in biomass terms. The fish is used for human consumption, but industrial use represents a large share, as oil, fish meal or animal fodder, depending on the market conditions. Other im-portant commercial species are plaice, flounder, dab, brill, turbot, along with the migratory spe-cies salmon, and sea trout. Common commercial species with freshwater origin include pike, perch, pikeperch, vendace, and whitefish.
The Baltic Sea fisheries also catch eel, classified as a widely distributed species with a population that extends over several marine regions but which has declined dramatically (see also Box 5.3.1 in Chapter 5.3). Recreational fishing mainly targets the same stocks as commercial fisheries. Incidental by-catch-es of birds and mammals in connection to the fisher-ies are evaluated in Chapters 5.4 and 5.5.
The overall objective of the Baltic Sea fisheries is to ensure economically, environmentally and social-ly sustainable use of fisheries resources in alignment with the ecosystem-based approach. Long term management plans for the internationally managed fish stocks aim to ensure that these are capable of producing a maximum sustainable yield (MSY), as mainly being regulated by the exploitation rate (EC 2016). The status evaluation presented here was based on fisheries management advice provided by the International Council for the Exploration of the Sea (ICES 2017b-f). Two aspects: fishing mortality and spawning stock biomass, were evaluated sep-arately for each stock. Status was evaluated against the condition that the average assessment ratio during 2011-2016 should achieve the reference val-ues for both fishing mortality and spawning stock biomass (see also Box 4.6.2).