Characterization of an Atlantic salmon Salmo salar stream at the southern limit of its eastern Atlantic distribution

Abstract

Ten reaches of an Atlantic stream located in north-west Spain were sampled intensively during one summer to characterize the conditions where Atlantic salmon Salmo salar have been re-introduced along the stream. Fish species richness and diversity showed a downstream increase, which was mainly attributed to the higher number of cyprinid species found in the lower reaches. Moreover, except for brown trout Salmo trutta that appeared to be the most ubiquitous species, the densities of the other species was higher in the lower than in the upper stream reaches. Redundancy analysis showed that the pattern of fish assemblages observed along the studied stream was mainly related to the expected gradient observed in the levels of dissolved oxygen, discharge and mean current speed. There was a significant differentiation between midstream and downstream reaches, both in terms of the composition of their fish assemblages and the freshwater habitat. This study emphasizes the importance of describing the variations in fish assemblages and habitat characteristics along a river to explore its relation to potential changes in the survival of fish populations. In particular, the development of habitat-fish relationships may be a useful tool for water managers to assess the effects of development and restoration projects on the habitat of S. salar.

Characterization of the Water Quality Status on a Stretch of River Lérez around a Small Hydroelectric Power Station

“…Figure 4 show seasonal variation of the temperature. In river Tea, the mean annual water temperature registered was 14.0 °C, with maximum values in August-September (19.8 °C) and minimum between November and January (9.3 °C) [42]; thus, more extreme values than those registered by us in the San Xusto SHP station, probably due to the more extreme air temperatures in river Tea. According to Directive 2006/44/EC [24], thermal discharges must not cause the temperature downstream of the point of thermal discharge to exceed 21.5 °C in salmonid waters.…”

“…The geological substrate of the watershed is the main factor constraining the physicochemical characteristics of unpolluted watercourses in Galicia [10]. In river Tea, Pontevedra Province, mean annual pH observed was 6.18 [42], while in Galician rivers with low mineralization via marine influence mean values of 6.6 (range = 5.2-7.1) were observed [22]. In the stretch of river Lérez studied pH values were lower to those observed by these authors.…”

“…It was higher in autumn-winter (11.93 mg/L; SD = 1.11; n = 26), coinciding with maximum flow and minimum temperature, than in spring-summer (mean = 10.39 mg/L; SD = 0.99; n = 24) (Mann-Whitney U = 624.00; p < 0.001; n = 50), being that dissolved oxygen is inversely related to temperature and directly related with the water flow [13]. A lower value of annual mean DO was observed in river Tea: 9.6 (±0.2) mg/L with maximum values between January and February (11.0 ± 0.2 mg/L) and minimum between August and September (7.9 ± 0.4 mg/L) [42]. DO value was minimum in sampling point 1 (before the dam) as it was expected, and maximum in point 4 (downstream the tailrace) and point 2 (downstream the dam), although differences in dissolved oxygen between points were not statistically different (Table 2, Figure 5).…”

“…DO value was minimum in sampling point 1 (before the dam) as it was expected, and maximum in point 4 (downstream the tailrace) and point 2 (downstream the dam), although differences in dissolved oxygen between points were not statistically different (Table 2, Figure 5). DO explains the occurrence of fish communities [42] so it is a basic parameter to asses water quality. DO values in all the sampling points were higher than the one observed in river Tea [42].…”

“…DO explains the occurrence of fish communities [42] so it is a basic parameter to asses water quality. DO values in all the sampling points were higher than the one observed in river Tea [42]. In comparison with Reference Condition the values registered in San Xusto were also higher (Table 3).…”