The importance of size and growth rate in determining mercury concentrations in European minnow (Phoxinus phoxinus) and brown trout (Salmo trutta) in the subalpine lake, Øvre Heimdalsvatn

Abstract

The alien fish species, European minnow (Phoxinus phoxinus), was recorded in the mountain lake, Øvre Heimdalsvatn, southern Norway, for the first time in 1969. After some years minnows were observed in the diet of brown trout (Salmo trutta). In 2006, brown trout had significantly higher d 15 N values than European minnow, indicating that brown trout holds the top trophic position in the lake food web. The d 15 N signatures of brown trout were significantly correlated to fish size, age and mercury concentration, while d 15 N signatures in minnows did not show any change with fish size, age and mercury concentrations, indicating that European minnow remain at the same trophic level throughout the life history. The mercury concentrations (mg kg -1 w.w.) in brown trout and minnows were in the range 0.014-0.093 and 0.014-0.16, respectively. For both species, positive relationships between total mercury concentrations and fish length, weight and age were found. Although European minnow was found as prey in brown trout stomachs, minnow had significantly higher mercury concentrations than brown trout. It is most likely that the higher mercury concentrations in the European minnow are due to the high metabolic rate, with more energy spent on maintenance than on growth as a consequence of its small size and very low annual growth rate compared to brown trout. Changes in population densities of both brown trout and European minnow may influence their habitat use, feeding preferences and metabolism, and may thus potentially lead to changes in the mercury loads of both species.

Plerocercoids of the cestode Diphyllobothrium ditremum in brown trout Salmo trutta: substantial increase in infection after establishment of European minnow Phoxinus phoxinus

“…show overdispersed distribution in fish (Halvorsen & Andersen, 1984;Kristoffersen, 1993;Knudsen & Klemetsen, 1994;Knudsen et al, 1996), as found in the present study as well, and heterogeneity among hosts in exposure to infective parasite stages is suggested to be of importance (Valtonen & Julkunen, 1995). The 13 C values of S. trutta that ranged from −30·2 to −23·0 ‰ indicate carbon sources from littoral, planktonic and benthic habitats of the lake, in accordance with the results of Jenssen et al (2010). Although the 13 C signatures in some individuals may indicate a high intake of planktonic food, there was a very low occurrence of copepods in stomach contents of S. trutta during the summer and early autumn in 2013 (present study).…”

“…The low δ 15 N signatures in S. trutta from Øvre Heimdalsvatn, both in 2013 and in the study by Jenssen et al . (), indicate invertebrate feeding (Cabana & Rasmussen, , ), with no distinctly piscivorous S. trutta among the analysed individuals. The δ 15 N signals may also partly be due to the feeding on S. trutta eggs during the spawning period, as observed in autumn 2013.…”

“…The δ 13 C values of S. trutta that ranged from −30·2 to −23·0 ‰ indicate carbon sources from littoral, planktonic and benthic habitats of the lake, in accordance with the results of Jenssen et al . (). Although the δ 13 C signatures in some individuals may indicate a high intake of planktonic food, there was a very low occurrence of copepods in stomach contents of S. trutta during the summer and early autumn in 2013 (present study).…”

“…or by feeding on infected conspecifics, two main hypotheses may be formulated: plerocercoid burden in S. trutta is caused mainly from copepod feeding, or there is a positive relationship between piscivory and prevalence (fraction of fish with the parasite) and intensity (number of parasite individuals in each fish) of Diphyllobothrium plerocercoids in S. trutta . The concentration of total mercury (ΣHg) and the ratio between 15 N and 14 N ( δ 15 N), both serving as indicators of piscivory, were found to be low in S. trutta muscle tissue from Øvre Heimdalsvatn, suggesting a more invertebrate‐dominated diet (Jenssen et al, ). There might be inter‐individual variation, however, and it is expected to find a positive correlation between plerocercoid burden and both ΣHg‐concentration and δ 15 N signal.…”

Long-term trends and variation in 137Cs activity concentrations in brown trout (Salmo trutta) from Øvre Heimdalsvatn, a Norwegian subalpine lake

“…Surette et al, 2006;Syväranta et al, 2006;Desta et al, 2007). However, in Øvre Heimdalsvatn there were no signs of any changes in mercury concentration in the brown trout as growth stagnated (Solhaug Jensen et al, 2010).…”