Habitat and Biology
Depth zone: Shelf (50 m - 200 m). Vertical distribution: Demersal/Benthic.
Sea trout is an anadromous form of brown trout (Salmo trutta L.). Sea trout usually live in the same water system as resident brown trout, and they can be genetically isolated from each other or breed together and genetically belong to the same population. The species is naturally distributed in northern and western Europe from the White Sea to northern Spain, including the entire Baltic Sea area. Populations are often partially migratory, i.e. one part of the population leaves the river for feeding in the sea (predominantly females migrate), while the other part stays in the river as residents. Sea trout spawn in rivers and smaller streams, often in the upper reaches or in smaller tributaries, where the nursery areas of trout are also found. They live their first (1–5) years as parr in the stream, leaving the stream as smolts for a feeding migration at sea that lasts for ½ up to 5 years, after which they return to their natal stream for spawning. Spawning may be repeated several times.Environmental influence on the stock
Jurisdictional distribution: Shared between nations
Considered a single stock: No
The nominal sea trout catch from the Baltic Sea was 479 tonnes in 2011, which is 199 tonnes (30%) less than in 2010 (Table 18.104.22.168). Most of the stocks in the Baltic Sea migrate in the coastal area within about 150 km of the home river and are therefore exploited locally, but longer migrations also occur in all areas, particularly in Poland, southern Sweden, and Denmark. The fish that migrate only short distances are mainly exploited in coastal and river fisheries, but long-migrants are also taken in offshore fisheries. The majority of sea trout catch is from mixed-stock fisheries.
The Main Basin is the most important area for professional sea trout catches, with the catch in this area constituting more than 60% of the total catch in 2011. Catches in the Main Basin have decreased from 1023 tonnes in 2002 to a minimum of 262 tonnes in 2008. After two years of catches around 500 tonnes, the catch fell again in 2011 to 293 tonnes. The catch of sea trout by Poland may be heavily overestimated due to possible misreporting of salmon as trout.
Around half of the total Baltic catch was taken by the coastal fishery, mainly in the Gulf of Bothnia and slightly less in the Main Basin. About one third was caught by the offshore fishery, almost exclusively by Polish vessels. River catch was 92 t in 2011. The largest part of this (41 t) was reported from Swedish rivers flowing to the Gulf of Bothnia, mainly as anglers’ catch, and from Polish rivers (39 t) as commercial catch and brood-stock fishery.
Catch in the recreational fishery is known with little accuracy and only part of it is included in the nominal catch presented above. Information has been gathered in Sweden, Finland, and Denmark in recent years, and the annual estimated catch for the Gulf of Bothnia could be as high as 400 to 500 tonnes. In Denmark, the total recreational catch in Subdivisions 22–25 in 2010 was estimated to be 346 tonnes. In spite of figures being incomplete, the share caught in the recreational fishery constitutes a significant part of the total catch, in some areas by far the largest share.
In the Gulf of Bothnia and Gulf of Finland, the fishery targeting sea trout is very limited. Instead, sea trout are caught as bycatch in fisheries targeting whitefish, pikeperch, and perch. This fishery has had a high effort level in the past ten years in Finland. A significant part of this fishery is recreational.
Effects of the fisheries on the ecosystem
|Catch distribution ||Total catch (2011) is 0.479 kt, where 100% are landings.. |
The effects of sea trout fisheries probably have a minor influence on the marine ecosystem function, but as some trout populations are at low levels fisheries in these cases have affected the biodiversity of the ecosystems, particularly in rivers. There is limited knowledge on the magnitude of these effects.
Assessment ModelQuality consideration
Electrofishing survey data were not available from all countries, and there is a general need for more electrofishing data to increase precision in the analyses.
The catch estimates for recreational fishing are incomplete or totally missing for several countries. The unknown share of the recreational catch can be considerable. Sea trout that migrate offshore are to a large extent taken as bycatch in the offshore salmon fishery. Salmon catches are to some extent misreported as trout in this fishery and improvement of control measures is therefore desirable to prevent misreporting. According to an estimate in the Baltic salmon assessment model, the misreporting of salmon as sea trout in the Polish offshore fishery in 2011 could have been around 43 000 individuals. Assuming an average weight of salmon of 4.57 kg, this means that the Polish sea trout catch could be overestimated by around 197 tonnes, which constitutes up to about 40% of the total sea trout catch.
Inclusion of trout in the EU Data Collection Framework should be considered. Collection of sea trout data from rivers is important, similarly to the salmon situation.Scientific basis
The assessment of sea trout stocks is based on densities of parr in rivers, estimated from electrofishing survey data. The observed densities have been compared to reference densities derived from good habitats, adjusted for climate and river width. In addition, data on numbers of ascending spawners, recapture rates of tagged sea trout, smolt counting in rivers, and catches in rivers are used in the monitoring of sea trout stocks, when such information is available.
|Working group report ||WGBAST |
Biological State and Trend
Source of information
The above excerpts are from the first two pages of the ICES advice, the supporting information to this advice can be read in full at the following reference:
ICES. Sea trout in Subdivisions 22–32 (Baltic Sea). Report of the ICES Advisory Committee, 2012. ICES Advice, May 2012. http://www.ices.dk/committe/acom/comwork/report/2012/2012/trt-bal.pdf