Gammarus lacustris (G. O. Sars, 1863)

Effect of Eurasian minnow introduction on the two crustacean species Gammarus lacustris and Lepidurus arcticus

Gammarus lacustris and L. arcticus , are known to be staple food organisms for brown trout in high mountain lakes, and they may be heavily preyed upon by fish ( Qvenild 2022). At high population densities of brown trout, they may be grazed near to extinction ( Aass 1969; Qvenild & Hesthagen 2020). With introduction of Eurasian minnow, the increased predation pressure has been linked to decline in brown trout prey availability ( Borgstrøm et al. 1985; Museth et al. 2007). In Lake Øvre Heimdalsvatn, G. lacustris and L. arcticus were originally a major part of the brown trout diet (Lien 1979). Some decades after the introduction of Eurasian minnow, G. lacustris seem still to be an important food item ( Borgstrøm et al. 2010). However, predation from brown trout and Eurasian minnow has obviously had a negative effect on the occurrence of L. arcticus . An even more devastating effect of Eurasian minnow introduction was observed in the high-altitude mountain lake, Lake Sylvetjønne in Jotunheimen ( Hesthagen 2005). Originally populations of L. arcticus and G. lacustris provided extraordinary catches of large brown trout. After an accidently introduction of Eurasian minnow in the 1980s, this outstanding fishery has been strongly reduced.

Before the invasion of Eurasian minnow, L. arcticus and G. lacustris were important prey items in Lake Skjerja ( Smukkestad 1980; Eriksen & Garnås 1985, 1988; Eriksen 1991,1992). In 1995 they were virtually grazed to extinction ( Eriksen 1995). A high effort with baited traps started in 1994, and it is assumed that the sustained effort with traps has reduced the predation pressure from the minnow. In 2002, the two crustacean species were again present ( Rognerud et al. 2003). Stomach content is regularly checked by the local fish guard who confirms that L. arcticus and G. lacustris still seem to be important in the lake (Ivar Sygnabere, pers. comm.). Good individual condition of brown trout (Fultons condition factor = 1.16), and the red flesh colour, give the impression of fish of high quality with good access to important crustacean food items. In contrast, L. arcticus and G. lacustris no longer seem to be important in the brown trout diet in the nearby Lake Skaupsjøen where no systematic reduction of Eurasian minnow is performed ( Borgstrøm 2009). A large fraction of big-sized brown trout in Lake Skjerja may also be crucial in regulating the abundance of Eurasian minnow as experienced in Lake Øvre Heimdalsvatn ( Museth et al. 2003).

Climate change implications

A near relationship between air temperature and water temperature is generally accepted ( Kvambekk & Melvold 2010). The pattern with warm and cold summers is reflected in a variety of other lakes, both on Hardangervidda and elsewhere ( Qvenild 2022). On Hardangervidda, an increase in air temperature has been observed since the 1980s (seklima.no). Hence, this trend is also assumed to be reflected in warmer aquatic habitats as seen in the long-term series in Lake Øvre Heimdalsvatn (Sildre.no). This perpetuating trend of warmer lakes is anticipated to continue and will most likely affect the future status of the lakes.

The brown trout has proved to be sensitive to the change in climatic conditions on Hardangervidda, with populations ranging from near collapse in the first decades of the 20 th century, to an almost instantaneous change to very rich fisheries in the 1930s ( Qvenild 2022). Climate induced strong year classes dominated the fisheries in the 1930s. A similar strong year-class appeared in 1997. In the last decade, the warm summers 2014 and 2018 seem to have produced strong year-classes. Such climatic induced variations in recruitment and growth pattern may be stronger with a warmer, wetter and more unpredictable climate. Such a climate may also induce increased productivity in lakes and rivers and with an increased nutrient input from terrestrial habitats.

Due to the removal of Eurasian minnow, G. lacustris and L. arcticus are still available food organisms in Lake Skjerja. Even though these two crustaceans can survive water temperatures above 20°C for shorter periods of time, it should not exceed 16 and 18°C for longer periods, respectively ( Lakka 2020; Wilhelm & Schindler 2000). During the period 2017–2023, the water temperatures have sporadically exceeded 18°C in Lake Skjerja, but only for short periods. In the warm summer 2018, the maximum temperature was 18.5°C. Hence, harmful high temperatures do not seem to be any threat for the existence of the two species at this point.

A study of the distribution of L. arcticus in the Fennoscandia Mountain range, indicated that an average water temperature (1 July – 15 September) of 14°C is close to the upper thermal threshold for the species ( Qvenild et al. 2021). In lakes with higher temperatures, L. arcticus seem to suffer, likely because of life cycle mismatches. In some years (2018 and 2021), the mean water temperature approached this limit in Lake Skjerja. However, the lake still seems to have a viable population of L. arcticus . Lake Skjerja is a very shallow lake and thus, it lacks proximity to a colder refugia in the hypolimnion. However, extreme year to year variations may be problematic for a species like L. arcticus , which has proved to be very sensitive to various environmental disturbances ( Lakka 2020; Qvenild 2022).

A warming of lakes may be tolerated by G. lacustris through its phenotypic plasticity in the reproduction traits (Qvenild et al. 2020). The water temperature in Lake Øvre Heimdalsvatn has also increased in recent years, and the summer average temperature has exceeded 14°C in some years. Despite the invasive Eurasian minnow, G. lacustris is still an important food item for brown trout ( Brittain et al. 2019). In contrast, L. arcticus seem to suffer, likely due to the combined effect of a high predation pressure from Eurasian minnow reinforced by the warmer water, which likely is beneficial for the minnows. Lepidurus arcticus , both being a prey and a predator, is assumed to have structural effects of the freshwater ecosystem ( Jeppesen et al. 2001). Hence, a collapse of the L. arcticus may have severe effects on the food web dynamics in the lake.

As indicated by catches of Eurasian minnow with baited traps in Lake Skjerja, a higher water temperature may benefit this species by increased recruitment and individual growth. This can cause increased predation pressure on the two crustacean species, in addition to a stronger competition for these food items with the brown trout. Increased periphytic production and its effect on the grazer dominated ecosystem, may be in favour of Eurasian minnow. Hence, warmer freshwater ecosystems can be an additional challenge in managing sympatric brown trout and Eurasian minnow populations in the future.