Phoxinus phoxinus (European minnow)

varies greatly with factors such as population density and numerous environmental factors (Lien, 1981; Myllylä et al., 1983; Mills and Eloranta, 1985; Mills, 1987; 1988; Museth et al., 2002). 

is still being translocated and introduced into new areas, developing dense populations in most localities. At present there are no serious threats to minnow populations in Norway. However, during the past decades acidification affected this species in some areas of southern Norway, and more than 100 populations were either lost or damaged (Hesthagen et al., 1999). These populations are to a large extent located within the native distribution area for

. However, acidification is no longer a serious threat to minnows in this region. Only a few of the lost populations have been re-established (Hesthagen et al., 2007). Improved water quality in this region facilitates establishment of

in watercourses where it is non-native (cf. Larsen et al., 2007). A similar situation is seen in northwestern Scotland (Adams, 1994).

Originally, minnows were spread because fishermen used it as live bait for catching species like brown trout (Salmo trutta), Arctic charr (Salvelinus alpinus), perch (Perca fluviatilis) and pike (Exos lucius) (Huitfeldt-Kaas, 1918). This practice is considered to be the main reason for most introductions throughout the 1900s. However, minnows have also been accidentally introduced together with stocked hatchery-reared brown trout in a large number of lakes (Borgstrøm, 1973; Lura and Kålås, 1994). Brown trout stocking has been routinely done especially in lakes modified as hydropower reservoirs in order to compensate for reduced natural recruitment (Vøllestad and Hesthagen, 2001).

These reservoirs are often located in the upper sections of watersheds. Whenever minnows were introduced, they were in most cases able to subsequently migrate downstream and become established in more lakes. This frequently occurred during the 1960s and 1970s. Minnows have also been spread through tunnels between watersheds constructed for hydropower development.

In a few cases minnows have been intentionally introduced to provide forage fish for brown trout. In one case minnows have been introduced as a control measure against the so-called tune fly (Simuliidae) (Halleraker and Hesthagen, 1994).

P. phoxinus displays considerable variability in life-history traits, i.e. in age and size at sexual maturity, growth rate and longevity (Mills, 1988). Age at maturity has been recorded over a gradient from 0+ to 6+, in fast and slow growing populations, respectively (Museth et al., 2002). Sexual maturity occurs at a smaller body size and at a lower age in lowland localities compared with those located at a higher altitude and latitude. In most cases, however, size at maturity deviates little from 50 mm. In the river Utsjoki in Finnish Lapland, maturity was strongly size-dependent and delayed until the fish reached 5, 6 or even 7 years of age, with a maximum age of 13 years at a length of only 75 mm (Mills, 1988). In Norway, sexual maturity in minnows has been recorded at between 2 and 15 years. In the alpine lake Øvre Heimdalsvatn, minnows of age 4 and 5 years made up about 67% of the spawning stock (Museth et al., 2002). All mature individuals were larger than 50 mm in length, and only a few specimens were smaller than 55 mm. Whereas no minnows older than 3 years were recorded in River Frome, UK (Mills, 1988), the oldest individual in the alpine lake Øvre Heimdalsvatn was 13 years (Museth et al., 2002). In Norway,

spawns mainly in June and July, depending on altitude and latitude. The fish spawn in shoals over stones and gravel, either in running water or in shallow areas close to the shore line. The adhesive eggs stick to the substratum. In Øvre Heimdalsvatn, spawning activity was observed only 4-8 days after ice break in early June, with the spawning period lasting about 3 weeks (Museth et al., 2002). The adhesive yellow eggs of about 1.0-1.5 mm in diameter hatch after 5-10 days. Individual fecundity is between 200 and 1000 eggs. It may appear that sexually mature minnows change behaviour towards spawning time, becoming more susceptible to fish predation (Museth et al., 2005).

feeds on invertebrates (mainly crustaceans and insect larvae), and some plant material. They may also prey on salmonid alevins (Huusko and Sutela, 1997; J Museth, Agricultural University of Norway, personal communication, 2008).

. In Lake Øvre Heimdalsvatn, the introduction of minnows caused major changes in the benthic community (Brittain et al., 1988; 1995). Zoobenthos diversity declined, with a marked increase in numbers of oligochaetes and small forms, especially chironomids. There was also a marked decline in numbers of Gammarus lacustris, especially the proportion of larger individuals. However, total benthic densities remained similar to pre-introduction. G. lacustris formed a major component of the diet of minnows, while it’s occurrence in brown trout stomachs declined greatly. Lepidurus arcticus also virtually disappeared from the trout diet, probably due to minnow predation. In another Norwegian reservoir, introduced European minnows fed on the planktonic stages of L. arcticus, and after a few years adult specimens became an insignificant part of the diet of brown trout (Borgstrøm et al., 1985). Introduction of European minnows may also cause reduced recruitment in brown trout. In Lake Øvre Heimdalsvatn, the cohort size of age-class 4 was reduced by approximately 50% during a period in sympatry with minnows compared to the situation before the introduction of minnows. There was no significant change in annual individual length increment (Borgstrøm et al., 1996). It is uncertain whether the reduction of trout recruitment was due to direct interactions with minnows in the nursery streams, or an indirect effect caused, for example, by increased brown trout cannibalism. Minnows may prey on salmonid larvae (Huusko and Sutela, 1997).

Raising public awareness appears to be the only measure with some potential to prevent or delay spreading of

to new localities. In some cases eradication from small water bodies may be feasible, in particular if the locality is expected to serve as a centre of further spreading into more localities. Control by habitat modification or bio manipulation may be possible but is rarely investigated.