Production of juvenile salmonids in small Norwegian streams is affected by agricultural land use

Abstract

Summary 1. We estimated the biomass and production of juvenile anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) (parr) in 12 streams in the Skagerrak area of Norway to identify controlling environmental factors, such as land‐use and water chemistry. 2. Production estimates correlated positively with fish density in early summer, but not with the size of the catchment. The summer biomass of age‐0 brown trout and Atlantic salmon was smaller than that of age‐1 and constituted 27.4 and 25.7%, respectively, of the total biomass of the two groups. 3. Mean production of brown trout from July to September varied between streams, but in most cases it was below 2 g 100 m−2 day−1. Yearly cohort production from age‐0 in July to age‐1 in July was 10 g m−2 or less, with mean annual production of 1.32 g 100 m−2 day−1, equivalent to 4.8 g m−2 year−1. The corresponding annual cohort production of Atlantic salmon was 0.38 g 100 m−2 day−1 or 1.4 g m−2 year−1. Annual production to biomass ratio (P/B) for brown trout of the same cohort in the various streams was between 1.47 and 4.37; the overall mean (±SD) for all streams was 2.25 ± 0.94. Mean turnover rate of Atlantic salmon was 2.73 ± 0.24. 4. Production of 0+ brown trout during the summer correlated significantly with the percentage of agricultural land and forest/bogs in the catchment, with maxima at 20 and 75%, respectively. Age‐0 brown trout production also correlated with concentration of nitrogen and calcium in the water, with maxima at 2.4 and 14 mg L−1, respectively. 5. The results support the hypothesis that brown trout parr production reflects the quality of their habitat, as indicated by the dome‐shaped relationship between percentage of agricultural land and the concentration of nitrogen and calcium in the water.

Transmission dynamics of the monogeneanGyrodactylus salarisunder seminatural conditions

“…Johnson et al (2011) showed that the risk of infection in guppies (Poecilia reticulata) was 12 related to fish density. The density of fish in the present experiment was 2-3 times higher than observed in other water bodies in Fennoscandia, although there are considerable differences between sites and years (Johansen, Elliott & Klemetsen 2005, Jonsson et al 2011. Atlantic salmon are territorial, and generally avoid direct contact (Aas et al 2011), and it is possible that the high density together with the frequent electrofishing may have caused a higher degree of interaction between individual fish than would naturally occur, leading to a higher probability of infection.…”

Water level influences migratory patterns of anadromous brown trout in small streams

“…In small streams, males attain maturity younger and smaller than females (Jonsson et al, 2001). In small streams, males attain maturity younger and smaller than females (Jonsson et al, 2001).…”

“…Earlier, electrofishing showed that the postspawners left the streams when the spawning was finished, and recovered tagged trout indicate that many return to sea (Knutsen, Knutsen, Olsen, & Jonsson, 2004;Olsen, Knutsen, Simonsen, Jonsson, & Knutsen, 2006). Both streams support partly migratory populations, that is a part of the stocks remain freshwater resident, whereas the anadromous fish feed at sea from smolts to adulthood (Jonsson et al, 2001). Less than 100 anadromous trout returned to the stream each year.…”

“…Degerman, Leonardsson, & Lundqvist, 2012;Jensen, 1968;Jonsson, 1985). Streams smaller than 1 m 3 /s, however, are typical spawning habitats of anadromous trout (Elliott, 1994;Jonsson et al, 2001), but these streams are usually abandoned by the adult fish outside the spawning season (Jensen & Rikardsen, 2008;Jonsson et al, 2001). Streams smaller than 1 m 3 /s, however, are typical spawning habitats of anadromous trout (Elliott, 1994;Jonsson et al, 2001), but these streams are usually abandoned by the adult fish outside the spawning season (Jensen & Rikardsen, 2008;Jonsson et al, 2001).…”

A conservation plan for Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta) in a region with intensive industrial use of aquatic habitats, the Hardangerfjord, western Norway

“…Hatchery-produced fish are sometimes released in order to compensate for reduction in freshwater habitat due to hydropower production or other human activity (Jonsson et al 2011). However, using hatchery fish removes part of the process of natural selection (Einum & Fleming 2001;Araki & Schmid 2010;Lorenzen et al 2012).…”