Reidar Borgstrøm

Mercury concentrations are low in commercial fish species of Lake Ziway, Ethiopia, but stable isotope data indicated biomagnification

Stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C), complemented by stomach content data, were used to assess the food web structure and trophic transfer of mercury (Hg) in four commercial fish species of Lake Ziway, Ethiopia: Nile tilapia (Oreochromis niloticus), catfish (Clarias gariepinus), Tilapia zillii, and golden carp (Carassius auratus). Total mercury (THg in mg kg(-1), ww) concentrations were low, with mean values of 0.033, 0.034, 0.025, and 0.011, in C. gariepinus, T. zillii, C. auratus, and O. niloticus, respectively. The relationships between mercury concentrations against total length (TL) and total weight (TW) were positive and significant in T. zillii, C. auratus, and C. gariepinus (P<0.01), but not in O. niloticus, which even showed a decreasing tendency with increase in TL and TW. Regression of log THg vs. δ(15)N among all fish species showed a significant correlation, indicating that mercury is biomagnifying along the food web of Lake Ziway. Isotope ratios indicated that C. gariepinus occupied the highest trophic level of the food web of Lake Ziway; but contained similar THg concentrations as T. zillii, which is located at a lower trophic level, probably due to a faster growth rate of C. gariepinus, and thus an example of biodilution.

A comparison of AFLPs and microsatellites to identify the population structure of brown trout (Salmo trutta L.) populations from Hardangervidda, Norway

A total of 495 fish from 11 Hardangervidda lakes were genotyped in order to compare amplified fragment length polymorphisms (AFLP) and microsatellites in terms of their capacity to infer population genetic structure. The 11 microsatellites used in this study gave a greater polymorphism information content and greater gene diversity, with an average of 14.8 alleles per locus, than the six AFLP primer combinations used. However, the AFLPs resulted in 178 polymorphic loci and a 3.1 times larger marker index (effective multiplex ratio multiplied with the gene diversity). Comparable population structuring, for example in terms of distinguishing fish from the different river systems, was obtained with both marker systems. An AFLP and microsatellite multilocus Bayesian assignment test with the structure program divided the fish into six groups largely concurrent with main branches on a population neighbour‐joining tree. Yet, the admixture status of individuals is mostly contradictory in the AFLP and the microsatellite analyses. The results are discussed concerning migration between lake populations.

Selective exploitation of large pike Esox lucius—Effects on mercury concentrations in fish populations

The present study outlines two main trends of mercury transfer patterns through the fish community: 1) the Hg concentrations increase with increase in the trophic level, with top predators having the highest concentrations, and 2) a fast growth rate may dilute the concentrations of Hg in fish muscle tissue (growth biodilution). In 2004, an extensive reduction in number of large pike (Esox lucius L.), was initiated by selective gillnet fishing in Lake Arungen, Norway, in order to increase the pike recruitment due to an expected reduction in cannibalism. In this connection, total mercury (THg) concentrations in the fish community were studied both before (2003) and after (2005) the removal of large pike. The delta(15)N signatures and stomach content analyses indicated that pike and perch (Perca fluviatilis L.) occupied the highest trophic position, while roach (Rutilus rutilus (L.)) was at the lower level, and rudd (Scardinius erythrophthalmus L.) at the lowest. The piscivores, pike and perch, had the highest concentrations of THg. The biomagnification rate of THg through the food web in the fish community was 0.163 (per thousand delta(15)N), with the highest uptake rate (0.232) in perch. A significant decrease in THg concentrations was found in all fish species in 2005 compared to 2003. Removal of the top predators in an Hg contaminated lake might thus be an important management tool for reducing Hg levels in fish, thereby reducing health risk to humans.

Diet overlap between introduced European minnow (Phoxinus phoxinus) and young brown trout (Salmo trutta) in the lake, Øvre Heimdalsvatn: a result of abundant resources or forced niche overlap?

A study of the diet of native brown trout (Salmo trutta) parr and introduced European minnow (Phoxinus phoxinus) in the subalpine lake, Øvre Heimdalsvatn, showed that the two species had considerable dietary overlap, both in the littoral zone and in the outlet of the lake. Chironomidae constituted a substantial proportion of the diet of the two species in both habitats. The results indicated that both zooplankton (Cladocera) and large macroinvertebrates (EPT-species) made up a higher proportion of the minnow diet in the early phase of the minnow establishment (1975–1977) than later, and that the significance of small macroinvertebrates (Chironomidae) as prey has increased during the same period. Dietary analysis of the sympatric brown trout and minnow population in Øvre Heimdalsvatn does not provide a definitive conclusion about the degree of competition between the two species. However, together with the findings in other studies in Øvre Heimdalsvatn that have documented reduced recruitment and individual growth of brown trout, decreased individual growth of minnows and a marked decline in the density of large crustaceans (Lepidurus arcticus and Gammarus lacustris) in the shallow littoral of the lake during the last decades indicates that competitive interactions between the two species are likely. This is probably an example of the competition between two fish species with a high degree of dietary overlap when living in sympatry, most likely caused by absence of alternative prey and alternative habitats due to the high predation risk for both brown trout parr and minnows in deeper parts and in the open waters of the lake.

Habitat use and diet of sympatric Arctic charr (Salvelinus alpinus) and whitefish (Coregonus lavaretus) in five lakes in southern Norway: not only interspecific population dominance?

Stable coexistence of Arctic charr and whitefish does occur in a number of native lake fish communities in Scandinavia. Even so, whitefish introductions into Arctic charr lakes have resulted in serious decline and possibly local extinction of Arctic charr. In this article, we analyze the habitat use and diet of the two species in five Norwegian lakes differing in basin shape and environmental conditions. In two of the lakes, both species are native, and appear to live in a relatively stable coexistence. Here, whitefish mainly occupy the littoral and upper pelagic zone, while Arctic charr live in the deeper habitats. Diets are generally quite different in terms of the zooplankton species eaten. In the three other lakes, either whitefish or both species have been introduced. In the shallowest lake, habitat segregation is similar to that seen in the pristine lakes, although Arctic charr appears to be on the brink of extinction. In the remaining two lakes, however, Arctic charr dominates, and occurs in higher numbers than whitefish in all the habitats. Our observations indicate that coexistence of the two species in oligotrophic and relatively pristine lakes requires an extensive profundal zone to serve as a refugium for Arctic charr. If the littoral zone is rendered inaccessible or unprofitable for whitefish due to dominance of a third competitor or predator, or as a result of lake regulation, then Arctic charr may be the dominant species.

Evidence of salmon lice-induced mortality of anadromous brown trout (Salmo trutta) in the Hardangerfjord, Norway

Abstract The Hardangerfjord, western Norway, is an area with a high concentration of salmon farms, high levels of infection of salmon lice in anadromous brown trout, and declining trout populations. This study assessed the marine survival rate of anadromous trout from the River Guddalselva, in the central part of the fjord, and tested the hypothesis that trout populations in this area are depressed by salmon lice infection. From 2001 to 2011, all descending smolts and trout returning from the fjord were captured in the traps at the field station of the Institute of Marine Research. In 2004 and 2005, parts of the smolt cohorts were treated with the Substance EX to prevent sea lice infection. From 2007 to 2010, all smolts (n=3557) were also tagged with individual tags. The results show a survival rate in the sea of only 0.58–3.41% for tagged smolts, which is extremely low. The highest survival rates appeared in the years with the lowest recordings of salmon lice in spring. The survival rate of Substance EX-treated smolts and controls was 3.41% and 1.76%, respectively. These findings suggest that salmon lice infection is an important contributor to the high mortality of anadromous trout populations in the Hardangerfjord.

Chemical and ecological effects of contaminated tunnel wash water runoff to a small Norwegian stream

Cleaning and washing of road tunnels are routinely performed and large volumes of contaminated wash water are often discharged into nearby recipients. In the present study, traffic related contaminants were quantified in tunnel wash water (the Nordby tunnel, Norway) discharged from a sedimentation pond to a nearby small stream, Arungselva. In situ size and charge fractionation techniques were applied to quantify traffic related metal species, while PAHs were quantified in total samples. All metals and several PAHs appeared at elevated concentrations in the discharged wash water compared with concentrations measured in Arungselva upstream the pond outlet, and to concentrations measured in the pond outlet before the tunnel wash event. In addition, several contaminants (e.g. Cu, Pb, Zn, fluoranthene, pyrene) exceeded their corresponding EQS. PAH and metals like Al, Cd, Cr, Cu, Fe and Pb were associated with particles and colloids, while As, Ca, K, Mg, Mo, Ni, Sb and Zn were more associated with low molecular mass species (<10kDa). Calculated enrichment factors revealed that many of the metals were derived from anthropogenic sources, originating most likely from wear of tires (Zn), brakes (Cu and Sb), and from road salt (Na and Cl). The enrichment factors for Al, Ba, Ca, Cr, Fe, K, Mg and Ni were low, suggesting a crustal origin, e.g. asphalt wear. Based on calculated PAH ratios, PAH seemed to originate from a mixture of sources such as wear from tires, asphalt and combustion. Finally, historical fish length measurement data indicates that the fish population in the receiving stream Arungselva may have been adversely influenced by the chemical perturbations in runoffs originating from the nearby roads and tunnels during the years, as the growth in summer old sea trout (Salmo trutta L.) in downstream sections of the stream is significantly reduced compared to the upstream sections.

Impacts of a small dam on macroinvertebrates: A case study in the Tinau River, Nepal

Macroinvertebrate composition, eco-morphological habitat descriptions and substrate composition were assessed above and below a small dam in the Tinau River, Nepal to explore the impact of the dam on biotic conditions. Four sites, one about 1.5 km above, one just above the dam site, one just below, and one about 2.5 km below the dam, were selected for qualitative and quantitative samplings of macroinvertebrates. The Nepalese Biotic Score method was used for the biological water quality assessment. The most abundant groups of macroinvertebrates among the total collection were of the family Chironomidae (53.5 percent) followed by Ephemeroptera (36 percent) and Trichoptera (5 percent). Only one of the dominant families, Baetidae, showed significant variability with substrate composition. The dam building had significant impacts on the macroinvertebrate composition just above the dam site, probably as a result of deposition of inorganic material within the small reservoir and changes in water speed. Damming of the Tinau River thus seems only to have a relatively minor impact on the river biota downstream of the dam site. The water quality of the river was assigned as Class II in all the stations, indicating its suitability for drinking after treatment.

Stable isotope-based determinations of the average temperatures experienced by young-of-the-year Svalbard Arctic charr (Salvelinus alpinus (L.))

Mean ambient water temperatures experienced by individual young-of-the-year (YOY) Arctic charr, Salvelinus alpinus (L.), from a Svalbard lake were estimated using measurements of oxygen stable isotopes (δ18O) derived from fish otoliths. Otolith-derived water temperatures differed significantly from temperatures recorded at the outlet river of the Dieset watercourse but were consistent with temperatures previously recorded in shallow littoral areas of other Svalbard lakes where YOY charr are commonly found. This indicates that fixed-point monitoring does not necessarily represent the temperatures and thermal habitats used by individual fish. Otolith-derived water temperatures were also positively related to fish length-at-capture and otolith size, although much of the variation remained unexplained. Differences among individuals could be related to variability in food availability and food intake as well as variation in the initial fish size at hatching. Implications for subsequent investigations into how YOY charr respond and adapt to future climate change are discussed.

Differential changes in growth patterns of anadromous brown trout and Atlantic salmon from the River Etneelva over a 25-year period

Abstract Freshwater and marine growth of anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) from the River Etneelva was analysed in relation to river, fjord and ocean temperatures during the periods 1976–1982 and 2000–2007. Anadromous brown trout grew more slowly through their first and second summers in the sea during the last observation period compared to the first period, and there were more growth checks in the scales sampled from the last period. The reduced growth in length corresponds to a weight reduction of about 20–40%. In fresh water, there was no change in growth rate of the trout parr, while in Atlantic salmon, growth rates of both parr and post-smolts increased from the first to the second period, possibly due to the corresponding increase in temperatures in the river and the marine environments. Anadromous brown trout stay in the fjord during the whole marine feeding period, and therefore the reduced growth in the sea must be caused by factors operating in the fjord environment. Although changes in food availability may also play a part in the observed changes, the negative trend for anadromous brown trout is most likely related to the high infection levels of salmon lice (Lepeophtheirus salmonis) observed in the central and outer parts of the Hardangerfjord.