Results and confidence

According to the current assessment, good status is achieved in the Sound, along the Danish and Swedish coast of the Arcona Basin, the Swedish coast of Bornholm Basin, the Western Gotland Basin, Swedish and Latvian coast of Eastern Gotland Basin, Gulf of Riga, Swedish coast of Northern Baltic Proper and Åland Sea and along the Estonian coast of the Gulf of Finland. All other areas are classified as having failed their threshold values (i.e. poor status), or are not assessed. 

 

Current status and trends in the Baltic sea trout

Of the 720 sea trout river populations, 168 were evaluated as having good environmental status (GES), 142 were evaluated as sub-GES. In 410 rivers population status was uncertain or not evaluated at all. The present status of sea trout populations is alarming in some areas, where only 26% wild and mixed sea trout river populations had estimated smolt production above the 50% threshold value during 2016 (ICES 2017).

A positive development in parr densities since 2012 has been observed in some rivers in Finland (Gulf of Finland), Estonia (Gulf of Finland) and Sweden (Bothnian Sea), reflecting management improvements in these countries.

The ICES Baltic Salmon and Trout Assessment Working Group (ICES WGBAST) has evaluated the status of sea trout populations for 2016 (ICES 2017). The status of populations in the Main Basin (all sub-basins south of the Gulf of Bothnia and Gulf of Finland) is known for 180 rivers with wild populations and unknown for 218 rivers. The status of 85 populations (wild and mixed populations, including tributaries in large systems) is not good (below 50% of the potential smolt production). In several areas in the Baltic Proper a worrying decline of parr densities has been found, although the densities are still at a reasonable level.

In Sweden, densities of parr in rivers entering into The Sound, Arkona Basin and Bornholm Basin (ICES SD 23–25) have remained stable and in the Western Gotland Basin (SD28) they decreased during the 2011-2016 assessment period. In the Bothnian Sea and Bothnian Bay (ICES SD 30, 31) the densities have increased during the same period, but the densities are still very low.

In Estonia, parr densities in rivers entering the Gulf of Finland, Northern Baltic Proper and Gulf of Riga have increased since 2001 in all the spawning rivers that have good or very good habitat quality. However, the Northern Baltic Proper stocks on the islands of Saaremaa and Hiiumaa are at low levels.

In Finland, parr densities have been far below the reference production level in all rivers for several years. There are high annual fluctuations in the observed densities and most of the rivers show densities of less than 1–5 parr per 100 m2. In the Gulf of Finland, the river Ingarskila had parr density of over 80 per 100 m2 in 2009, but the annual variance is very high. There have been improvements in the state of the stock in several rivers in recent years, probably as a result of implementation of new management measures.

In the Russian part of the Gulf of Finland, parr densities are estimated to be at a level of 5–10 parr per 100m2, which is considered low or below optimal.

In Latvia, the rivers Salaca, Gauja and Venta are the three most important sea trout rivers for wild smolt production. The parr density was on average ≤10 parr (0+ and older) per 100 m2 in 2011-2016, which is below optimal. There do not seem to be improvements in the abundance of sea trout in Latvian rivers, however there is much uncertainty in estimates.

In Lithuania, almost all spawning rivers reflect not good status. The average density of juveniles (0+ – 2+) in rivers have fluctuated in the last years, from very high to very low numbers. Surveys were carried out at 75 sites, where the average mean density of juveniles varied from 2.9 to 28.2 per 100 m2 (mean – 12 individuals/100 m2). The main reasons for the present decline are exceedingly high fishing pressure in the sea and coastal fishery as well as illegal fishing in rivers during spawning migration and during the spawning period. The majority of sea trout are caught in coastal areas as a by-catch by gillnets targeting other species.

In Poland, there is only one stream with a wild sea trout stock, 16 with mixed and 8 with reared stocks. The average density of 0+ parr at monitored spawning grounds is usually around 50 per 100 m2, but on some sites can exceed 150 individuals per 100 m2. There have not been great changes in the densities during the last 6 years. The main causes for the not good status of sea trout stocks is the lack of suitable spawning habitats due to dams, water discharge times and gravel extractions. However, poaching, by-catch of smolts in the coastal herring fishery and diseases also negatively affect the stocks.

In Germany, there are nine rivers with natural reproduction (eight of them initiated with stocking). The densities of parr have increased during the last 11 years. Missing habitat score data prevented the estimation of stock status in these rivers, but the status of the stocks is probably mostly not good.

In Denmark, approximately 26% of the streams (either small entire streams or parts of larger streams) with original populations of sea trout produce less than 50% of stream capacity. The reasons for this are, in most cases, poor habitat conditions (including heavy sand transport) or migration barriers (including newly established artificial lakes in the lower parts of the streams). The wild sea trout smolt production has, however, increased in the entire country and not least in the streams inside the Baltic area where wild smolt production has increased more than twofold over the last decade and the status of stocks is estimated to achieve good status.

For more information about the state of sea trout stocks, see ICES 2017, HELCOM 2011 and Pedersen et al. 2012.

 

Smolt production and post-smolt survival

The smolt production of rivers in the Russian Kaliningrad and St. Petersburg regions and Latvia, are shown in Results table 1. In Lithuania, it was estimated that in 1999 the rivers produced 323,800 sea trout smolts, but in recent years annual smolt production has dropped to 34,000–46,000 smolts (Results figure 1).

 

Results table 1: Smolt production in Russia and Latvia. Source: Pedersen et al. 2012.

Region/Country Smolt production Potential
Kaliningrad region 3,500 200,000 – 250,000
St. Petersburg region –Northern part 6,000-8,000  
St. Petersburg region –Southern part 4,000  
Latvia 61,000  

 

 Results figure 1.png

Results figure 1. Average annual smolt production in Lithuanian river systems (mean and range) during 2005-2010 and the potential smolt production capacity (green lines). The average of the total annual smolt production was 24,500 individuals.  Source: Pedersen et al. 2012.

 

Tagging studies on post-smolts at sea show a continuous decrease in returns (ICES 2017). Carlin tagging results in the Gulf of Bothnia and Gulf of Finland show that a large and increasing proportion, often the majority, of the sea trout are caught already during their first year at sea. Trout are caught as by-catch in the whitefish fishery, by gillnets and fykenets. Based on tagging data, the proportion of fish caught as undersized during the first sea year is still increasing, even though the total effort of gillnet fishery by professional fishermen has not changed during the past ten years. The recapture rate of sea trout shows a continued decreasing trend for more than 20 years in the Gulf of Bothnia, although it may have levelled off in recent years (Results figure 2). In the Swedish parts of the Gulf of Bothnia, the recapture rate was similar to Finland in the period 1980–2002.

 

Results figure 2.png

Results figure 2. The abundance of sea trout smolts in nine rivers. Source: ICES 2017. Note that only data with consecutive years are connected via lines.

 

Number of sea trout spawners

The number of ascending sea trout spawners is followed only in a few large rivers. Five Swedish rivers in the Bothnian Sea and Bothnian Bay have automatic or manual counting. According to Pedersen et al. (2012) the number of spawners in these rivers were too low to populate all available habitats. In River Piteälven the number has increased continuously, and for some years there was also an increase in Kalixälven, Vindelälven and Byskeälven (see Results figure 3). The increase in the River Piteälven is likely due to the closing of salmon traps in the river estuary. In general, the number of spawners has increased since during 2011-2016.


Results figure 3.png

Results figure 3. Abundance of sea trout spawners in four Swedish rivers. Source: ICES 2017.

 

Even though the number of spawners increased in River Piteälven during the period 2001–2012, the number of spawners observed entering rivers in northern Sweden is still extremely low, especially taking into account the size of the rivers. This is likely due to both low recruitment and elevated mortalities at sea. In addition, anglers' catch, which to some extent indicates the number of spawners, does not suggest any increase in the number of spawners in this area either.

The estimated number of spawners migrating to the Lithuanian Nemunas catchment area varies between 11,500 individuals (in 1992) and 1,800 (in 2003), but on average it is around 4,000 individuals per year.

In the German river Hellbach, a pilot counting of adult spawners estimated nearly 1,600 ascending fish in 2009. In 2010, this number was 500, but that was considered an underestimate due to flood conditions.

 

Confidence of the indicator status evaluation

The estimation of the reference parr density is made using the assessment model in the southern Baltic Sea and based on expert evaluation in the northern Baltic Sea. Both methods are considered to give an accurate enough estimate on the potential maximum parr density to allow for the evaluation of the stock status. Hence, there is no significant regional difference in the confidence of indicator status when it comes to the reference densities.

A counterpart for the reference densities in status criteria is planned to be based on the 4-5 year moving average of parr densities. In some areas (e.g. in Denmark) there are too many rivers to be surveyed annually with available resources. However, most of the rivers are still surveyed at regular intervals and in these a different calculation for the average parr densities can be used. This does not decrease the level of confidence in the evaluation of the state of the stock.

The level of confidence of the assessment is moderate to high