In total 46 eels were sampled in August – September 2006. All eels were sampled with eel fykenet. All specimens were determined as females. Average total length and weight for caught eels differed by site. In lake Ķīšezers it was 81,3 cm (69,4-91,7 cm) and 979,8 g (521-1330 g), in Mērsrags – 77,9 cm (60,3-90,7 cm) and 1009,8 g (416-1560 g). All of them were yellow eels (Table _ and _). 40 of the sampled fish were naturally recruited, 6 were restocked eels that came from some freshwater bodies.
In summer 2006 30 perch were sampled in Jūrkalne on Open Baltic Sea coast. The average length and weight of the sampled fish was 27,5 ± 2,6 cm (24,4-34,3 cm) and 312,2 ± 96,1 g (198-567 g) (Table _). 19 perch in the sample were femailes, 11 were males making the sex ratio approximately 2:1.
Results
In total 46 eels collected in Latvian waters in 2006 were analyzed for otolyth Sr:Ca ratios.
Among them 40 eels showed the pattern of naturally-recruited eel and 6 eels showed the pattern of stocked eels. The mean otolyth Sr:Ca ratios of the naturally-recruited eels and stocked eels were shown in the Fig. 1 and 2, respectively. The mean otolyth Sr:Ca ratios of 40 naturally-recruited eels gradually decrease from approximately 6 × 10-3 (around 200 m from the otolyth core) after the glass eel stage to approximately 3 × 10-3 at catch. The profiles of otolyth Sr:Ca ratios of naturally-recruited eels collected in Latvia were very similar to that found in Lithuania since the naturally-recruited eels all experienced the same migrating routes from high saline North Sea, semi-saline Baltic Sea to very low saline Latvian coasts. The naturally-recruited eels collected in Latvia, as that collected in Lithuania, also showed diversified migratory patterns. 30 naturally-recruited eels evidently invaded the freshwater once to several times in their life (so called inter-habitat shifters). Typical Sr:Ca ratio pattern of such specimens is shown in Figure 4. 15 eels were collected in lake Ķīšezers which is connected to river Daugava and thus has short connection with Riga Gulf. Therefore it is reasonable that no “sea eel” pattern was evident in the sample from this area. However 10 specimens from eels collected in Mērsrags could be considered as sea eels that have never entered freshwater at all. Typical Sr:Ca ratio for such specimens is shown in Fig.5. Although in the previous study in Lithuania the Sr:Ca ratio for seawater residents was >3.23 × 10-3 in Riga Gulf salinity is lower so the ratio also could be lower.
4 of the stocked eels, showed consistent low otolyth Sr:Ca ratio from the elver stage to yellow or silver eel stage (Fig. 2 and 3), which was different from that found in the eels collected in the Baltic Sea of Lithuanian territorial waters. The consistent low otolyth Sr:Ca ratios without any higher otolyth Sr:Ca ratios indicates that the stocked eels completely reside in the
freshwater environments and there is no indication for the out-migration to the Baltic Sea.
However 2 specimens according to their Sr:Ca ratio, have entered coastal waters after their release. One of them was collected in Mērsrags, second one – in lake Ķīšezers. All of the stocked eels were released probably in some midland lakes, which are far from the sea coast and connected to sea with rivers. However, downstream migration from lakes where eels has been stocked is restricted by eel weirs or hydroelectric power station dams. Therefore
appearance of stocked eels observed in the sampling sites can be explained by escapement from Daugava basin upstream lakes via eel weirs during non traditional sea dwelling
migration timing or even possibly trough hydroelectric power station turbines. Although the last one possibility is considered to be dependent from turbine characteristics and the mode of operation of power plant as well as from size of eel (EPRI, 2001; Hadderingh and Baker, 1998) there is no investigations and evidences of such migration in Daugava River.
Additionally the possibility of eel migration along the sea coastline from other river basins
should be considered. The possible origins of the stocked eels could be revealed only after careful studies of restocking activities performed in Latvia during previous decades.
0 4 8 20
16
12
0 150 300 450 600 750 900 1050 1200 1350 1500 1650 Distance from core (μm)
Sr/Ca (× 10-3 )
Figure 1. Temporal changes of mean otolith Sr:Ca ratios from 19 naturally-recruited European eels (Anguilla anguilla) collected in the Latvian waters. Y error bar represents the standard error of mean.
0 4 8 20
16
12
0 150 300 450 600 750 900 1050 1200 1350 1500 1650 1800 Distance from core (μm)
Sr/Ca (× 10-3 )
Figure 2. Temporal changes of mean otolith Sr:Ca ratios from 12 stocked European eels (Anguilla anguilla) collected in the Latvian waters. Y error bar represents the standard error of mean.
0 4 8 12 16 20 24
0 200 400 600 800 1000 1200 1400 1600 1800 Distance from core (μm)
1000 (Sr/Ca)
Freshwater signature due to the stocking program
Figure 3. Temporal changes of the otolyth Sr:Ca ratios from a typical stocked European eels (Anguilla anguilla) (No. 156) collected in the Latvian waters. The consistent low otolyth Sr:Ca ratios without relative high otolyth Sr:Ca ratio indicated that the eel completely reside in the freshwater environment and did not out migrate to the Baltic Sea. Y error bar represents the standard error of mean.
0 4 8 12 16 20
0 200 400 600 800 1000 1200 1400
Distance from core (μm)
1000 (Sr/Ca) 1
2
3
4
Figure 4. Temporal changes of Sr:Ca ratio for a typical inter-habitat shifter, 1 and 3 indicating freshwater residence, 2 and 4 – sea residence
0 4 8 12 16 20
0 150 300 450 600 750 900 1050 1200 1350 1500 1650 1800 Distance from core (μm)
Sr/Ca × 10-3
Figure 5. Temporal changes of Sr:Ca ratio for typical sea eel specimen from Latvian sample.
Table 5. Biological characteristics (mean ± SD) of the European eels collected in the lake Ķīšezers. N: sample size.
Mean (±SD) and range Origin of the eels Sampling period Developmental stage N
Total length (cm) Body weight (g) Age (year)
Naturally-recruited eel September Yellow eel 12 81.1 ± 7.3
(69.4 - 91.7)
980.8 ± 250.9 (521 – 1330)
12.8 ± 1.4 (11 – 15)
Stocked eel September Yellow eel 4 81.8 ± 6.7
(73.8 - 89.2) 976.8 ± 281.5
(712 – 1230) 14.3 ± 3.3 (11 – 18)
Table 6. Biological characteristics (mean ± SD) of the European eels collected in Mērsrags, Latvian coastal zone. N: sample size.
Mean (±SD) and range Origin of the eels Sampling period Developmental stage N
Total length (cm) Body weight (g) Age (year) Naturally-recruited eel August –
September Yellow eel 28 78.8 ± 6.2
(66.2 – 90.7) 1038.7 ± 258.3
(543 – 1560) 12.2 ± 1.8 (9 – 16) Stocked eel August –
September Yellow eel 2 No.210 60,3
No.224 65,7
No.210 416 No.224 650
15 -
Table 7. Biological characteristics (mean ± SD) of the perch collected in Jūrkalne, open Baltic Sea coast. N: sample size
Mean (±SD) and range
Sampling site Sampling period N
Total length (cm) Body weight (g)
Jurkalne, Open Baltic Sea August 30 27.5 ± 2.6
(24.4 – 34.3)
312.2 ± 96.1 (198 – 567)