The status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the abundance of sea trout spawners and parr, this indicator contributes to the overall biodiversity assessment, along with the other biodiversity core indicators.
The core indicator of the Baltic sea trout addresses the Baltic Sea Action Plan's (BSAP) Biodiversity and nature conservation segment's ecological objectives 'Thriving and balanced communities of plants and animals' and 'Viable populations of species'.
The indicator is relevant to the following specific BSAP actions:
The core indicator also addresses the following the qualitative descriptors of the MSFD for determining good environmental status (European Commission 2008):
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';
Descriptor 3: 'Populations of commercially exploited fish and shellfish are within safe biological limits, exhibiting a population age and size distribution that is indicative of a healthy stock'; 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):
Sea trout rivers and brooks are also within the focus of EU Water Framework Directive (WFD) and all actions improving the habitat quality of these watersheds will benefit also the sea trout stocks accordingly.
Sea trout play an important role in maintaining the balance in riverine food webs, both by harvesting invertebrate populations and also serving as an important food source for other predatory species (ICES 2015).
There are around 1,000 sea trout rivers and streams in the Baltic Sea (HELCOM 2011), with an estimated 395 populations of wild sea trout (and 77 mixed populations) in the Baltic Proper, 28 wild populations (and 28 mixed populations) in the Gulf of Bothnia and 85 wild populations (and 16 mixed population) in the Gulf of Finland. Altogether this adds up to 508 wild and 121 mixed sea trout populations in brooks/rivers in the Baltic Sea area (ICES 2015). The migration patterns of different trout populations vary, with the adult trout of some populations spending their entire life cycle in the same river, whereas the adults of other populations carrying out feeding migrations to the coastal areas of the sea where they feed on various invertebrates and small fish. The exact migration patterns of the sea trout are not known, however, they are generally considered to feed in the vicinity of the coastline and to migrate up and down the coast, making them good indicators of food availability in the coastal area. As a predatory fish species, sea trout generally has a structuring role in the ecosystem, mainly via top-down control on lower trophic levels.
selective extraction of species.
Sea trout abundance is affected by commercial and recreational fishing at sea and in rivers. The abundance of spawners returning from feeding migrations in the coastal areas to the rivers is related to the densities of parr in the rivers. The density of sea trout parr also reflects the success of recruitment and depends on other factors such as climate, the size of the river, habitat characteristics and quality and is affected by migration barriers to reproduction areas. This indicator reflects the state of the ecosystem as it is sensitive to river connectivity (effect of dams) and the quality of spawning and rearing habitats.
The main reason for the not good status of sea trout populations in the northern areas of the Baltic Sea is high fishing pressure, particularly by-catch of post smolts in the gillnet fishery. In the Bothnian Bay, Bothnian Sea and the Gulf of Finland young age classes of sea trout are also by-caught in sea fisheries targeting other species, often whitefish. In the Gulf of Finland, the by-catch occurs mainly in gillnets targeting pikeperch. Sea trout is also reported as being by-catch along the Swedish coast in the Bothnian Sea and Bothnian Bay in the commercial coastal salmon trapnet fishery.
The total reported commercial sea trout catch in the Baltic Sea marine area in 2016 was 229 tonnes, which has been approximately the same magnitude as in last five years but about 75% less than in 2004 when the decrease in catch begun (ICES 2017, Relevance figure 1). The estimated recreational catch in marine area was about 500 tonnes in 2015 (only partial estimated available from 2016). In 2016, most of the total Baltic catch (marine + river) was taken by the coastal fishery, equally from the Gulf of Bothnia and the Main Basin (Baltic Proper). The Swedish and Finnish offshore fishery targeting salmon and sea trout in the Baltic Proper was phased out in 2013.
River catch in 2016 was 28 tonnes. Of this, the largest parts were reported from Swedish rivers flowing to the Gulf of Bothnia (22 tonnes, mainly as anglers' catch).
Relevance figure 1. Fishery catches of sea trout in Main Basin (Baltic Proper), Gulf of Bothnia and Gulf of Finland. Data are provided for commercial (left) and recreational (right) catches separately. Note that offshore catches include in some countries and years also coastal catches and that riverine catches have not been reported from all countries (ICES 2017).
In addition to the effects of fisheries on sea trout, the deterioration of habitat quality and damming of rivers affects the populations. Channelizing of rivers has altered the spawning habitats which decreases the number of spawners (ICES 2009). Also dredging, pollution, acidification and siltation of rivers have negative effects on sea trout populations. The magnitude of the different factors influencing sea trout varies locally within a sub-basin.
Predation by cormorants influences the abundance of sea trout both locally and in larger areas (Dieperink et al. 2001; 2002). The predation can be severe in rivers, at river mouths and in coastal areas. An increase in the population size of cormorants has been observed in many countries and the sea trout stock size may have decreased in areas where large cormorant colonies are present, although this needs further investigation (Bzoma 2004; Leopold et al. 1998).
Sea trout rivers and brooks are also in the focus of EU WFD and all actions improving the habitat quality of these watersheds will benefit also the sea trout stocks. Quality improvement of the spawning and rearing habitats will also positively affect the potential production capacities of the rivers.