Rivers in assessment unit 5 (Eastern Main Basin, SD 26 and 28)

Estonian rivers

The River Pärnu flows into the Gulf of Riga and is the only Estonian salmon river in the Main Basin. The first obstacle for salmon migrating in the river is the Sindi dam, located 14 km from the river mouth. The fish ladder at the dam has not been effective due to its small size and the location of the entrance. The quality of spawning areas above the dam is relatively good, and parr abundancy is associated with poor accessibility.

Electrofishing surveys on the spawning and nursery ground below the dam have been per-formed since 1996; the number of ind./100 sqm has been very low during the whole period (Table 3.1.5.1 and Figure 3.1.5.1). No salmon parr were found in 2003, 2004, 2007, 2008, 2010 and 2011.

In 2018, the 0+ parr density below Sindi dam was 1.4 ind./100 sqm. The habitat quality below the dam is poor, and that is the main cause for the low parr density. Since 2013, electrofishing is also carried out upstream from the Sindi dam. Above the dam salmon parr have been found only in some years, and densities have been very low. In 2017, however, average 0+ parr density (four sites electrofished) was 26 parr/100 m². In 2018, 13 sites were electrofished upstream the dam;

salmon parr were found at only two of these, with an average density of 0.1 parr/100 m². In 2019, average 0+ parr density was 6.5 ind./100 sqm and increased in 2020 to 8.1 ind./100 sqm.

In autumn 2018, removal of the Sindi dam started, and ascending salmon were able to pass the dam in November same year. As salmon now has free access to all spawning grounds, the pop-ulation should be able to recover. A juvenile supplemental release programme was also initiated in 2012 aimed at assisting population recovery. The first juvenile salmon were released in 2013, and as pointed out initially in this section, under present conditions with large numbers of juve-niles being stocked every year, Pärnu should be considered as a mixed river.

Latvian rivers

There are seven wild salmon rivers in Latvia, mainly flowing into the Gulf of Riga. Some rivers have been annually stocked with hatchery-reared parr and smolts, and salmon in these rivers thus consist of a mixture of wild and reared fish. In 2018, salmon parr were found at 31 sites (15 rivers) sampled by electrofishing. Parr densities are presented in Table 3.1.5.1 and Figure 3.1.5.2.

The wild salmon population in river Salaca has been monitored by smolt trapping since 1964 and by parr electrofishing since 1993. From 2000, no releases of artificially reared salmon have been carried out. High water level in Salaca River during the monitoring week, may have af-fected the electrofishing results in 2020 when eleven sites were electrofished in the river Salaca and its tributaries. All sites in the main river hold 0+ age salmon parr. The 0+ salmon parr were present in the Salaca tributaries - Jaunupe, Svētupe and also Korģe which is considered a sea trout river. Average density of 0+ salmon parr in the whole river Salaca basin (including tribu-taries) was greater than in 2019 - 80,2 ind./100 sqm and density of older salmon parr was 0,9 ind./100 sqm. Smolt trap in the Salaca river was operated between April 21st and June 4th 2020.

There were a few days when smolt trap was not set due to hydrological conditions or strong flow of woody debris. Data for such days were interpolated. Highest salmon and sea trout smolt run amount was registered on 10th of April. In total 904 salmon and 552 sea trout smolts were caught and 266 salmon and 134 sea trout smolts were marked using streamer tags for trap efficiency estimation. Smolt trap efficiency in Salaca River ranged from 5 to 20,6 % (on average 14% for salmon smolts and 10,7% for sea trout). Total salmon smolt production was calculated from the numbers of smolts captured and the trap efficiency. Total smolt run in 2020 was estimated to be 12,8 (±4,2) thousand salmon and 4,8 (±1,3) thousand sea trout smolts that migrated from Salaca river to the Gulf of Riga.

Disregarding the recommendations of the Institute BIOR to start works not earlier than on June 1st, in 2020 during the smolt migration time, dredging works of the Salacgriva Port were carried out. The works were performed in the period from 10th of May to 20th of August, using a self-propelled type dredger. Dredging works were carried out during daylight hours below smolt trap. It could have a negative effect on smolt run, but the exact magnitude of the impact is un-known. The damage to fish resources was estimated at EUR 3836.91 to be compensated by the port authority.

At the beginning of August 2020, the counting of ascending salmon was started in Salaca river 3 km upstream from the river mouth using Riverwatcher (Vaki Ltd) fish counter installed in the resistance board weir. Due to technical problems with Riverwatcher, counting was stopped at the end of the August, and it was not possible to continue even after receiving the replacement part from Vaki - there was a unexpectedly large increase in water level. During the one month operation period 15 ascending salmon females, two salmon males and four sea trout females were registered.

In 2021, it is planned to move the fish counter ~2 km upstream from previous site to more calm and shallower site, which unfortunately excludes the salmon population of Jaunupe River.

In river Venta, wild salmon parr were found above the Rumba waterfall because of a high water level in the autumn of 2017. In 2020 only 4 ind./100 sqm 0+ and 0.1 ind./100 sqm 1+ and older parr were caught in river Venta. Average parr production has negative trend due to high water temperatures and low water level in recent summers.

In river Gauja, 2020 wild salmon 0+ parr production decreased (1.8 ind./100 sqm) compared to in 2019 (6.2 ind./100 sqm). In Amata, which is a tributary to Gauja, salmon 0+ parr production increased to 9.2 ind./100 sqm compared to previous year when the densities were (0.9 ind./100 sqm).

In 2020, wild salmon parr were also found in the small Gulf of the Riga rivers Vitrupe, Aģe and Pēterupe. Age structures of parr in these rivers testify that salmon reproduction does not occur in every year. Parr production seems to be most stable an on a higher level in Aģe.

Wild 0+ salmon parr were also detected in Užava, Irbe, Tebra and Durbe river (Saka river basin) and in some of their tributaries. Older salmon parr were not present in these rivers. In the Durbe river after habitat mapping two sampling stations were established in representative rapid sec-tions.

In 2018, habitat mapping was initiated to re-evaluate productive habitat sizes in Latvian rivers.

According to the first results from river Bārta, the total area of riffles suitable for salmon spawn-ing and nursery constituted only 0.6 ha in the river section from the Latvian-Lithuanian border to Lake Liepājas, which is many times less than the 10 ha estimated earlier. None of the mapped riffles were evaluated to have high or good quality, 67% of the habitats had moderate quality, whereas the remaining ones had poor quality. Problems with habitat siltation and overgrowing are common in the river.

In 2019, habitat re-assessment was carried out in the Irbe, Užava river and Saka river basin. In the Irbe river deposition of sand and silt in rapids suitable for salmon reproduction is visible problem. Rapids and riffles suitable for salmon spawning and nursery constitute 0.21 ha instead of 10 ha assumed previously. Habitat mapping in Užava river show that canalisation in 1960s has left considerable effect on available habitats in this river. Total available and suitable habitats constitute only 0.59 ha (0.46 ha with good quality). The size of the reproduction area was previ-ously thought to be 5 ha. In the Saka river basin, upper parts of Tebra 2.4 ha of suitable habitats for salmon spawning and nursery areas was found. Previous estimate was 20 ha.

Lithuanian rivers

Lithuanian salmon rivers are listed in the Annex 2. Salmon inhabits 12 tributaries in the Nemu-nas river basin and river B. Šventoji that flows directly into the Baltic Sea. Purely natural salmon population inhabits only the Nemunas tributary Žeimena and its tributaries Mera and Saria.

The index river Žeimena has never been stocked with artificially reared salmonids. Its tributary Mera is a typical sea trout river, and therefore has the salmon production been very low all the time. Mixed populations are found in the B. Šventoji (river that flows directly in to the Baltic Sea) and the following tributaries of river Nemunas; Neris, Šventoji, Vilnia, Dubysa, Siesartis,

Širvinta, Virinta, Minija, Vokė. Reared populations occur in the Nemunas tributary river Jūra and some smaller tributaries. In these rivers, salmon releases have been made regularly for sev-eral years.

Electrofishing is the main monitoring method for evaluation of occurrence and densities of 0+

and older salmon parr. Parr densities in Lithuanian rivers are presented in Table 3.1.5.2 and Fig-ures 3.1.5.3 and 3.1.5.4. The abundance of salmon parr depends on hydrological conditions, spawning success, and protection of spawning grounds.

In 2020, the average density of salmon 0+ parr in the index river Žeimena increased to 11.7 ind./100 sqm and the densities of older parr was 0.1 ind./100 sqm. The 2020 density is above the mean values for the whole survey period. Parr density in Neris in 2020 stayed on a highest ob-served level. Average 0+ parr density was 11 ind./100 m² and older parr density was 0.2 ind./100 m² (Table 3.1.5.2).

The correlation between salmon juvenile density and water temperature during July, the warm-est month of the year, has been invwarm-estigated in two rivers characterized by different thermal regimes; Neris (r = -0,530, p =0,035) and Žeimena (r = -0,555, p =0,021). It was found that during a period of several years, water temperatures in July varied within a range of a few degrees (19.1°C on average). However, in 2010 the water temperature reached 22.6°C, which could have had a lethal impact on some of the weaker juveniles in the river. In that year, the parr density was also estimated to be the lowest in Žeimena recorded so far; only 0.2 ind./100 sqm. The average tem-perature during July in Neris is 20.9°C. Temtem-peratures above the ‘stress level’ (>22°C) were seen seven times during a period of 17 years; in 2001, 2002, 2006, 2010, 2012, 2014 and 2018. These results illustrate that the thermal regime is a very important determinant for salmon production in Lithuanian rivers. Other concerns include pollution, and that rivers are of lowland type with scarce parr rearing habitats. Finally, quite high mortality rates are expected due to predation;

densities of several predators are significantly higher than in more northern Baltic salmon rivers.