Everywhere they are appearing! Species that don’t belong, but they enjoy the experience anyway. Fish are among the most invasive of animals, with fishermen themselves also responsible for introducing attractive species to catch. The American cutthroat trout Oncorhynchus clarkia lives in coldwater tributaries in the west of North America, with many endangered sub-species. It has a relative we may know better that migrates much more and has many sub-species too, this is the famed rainbow trout, Oncorhyncus mykiss (RBT), that has been classed as an invasive alien in up to 45 countries, after its introduction. For more fish (the bitterling) and a host of others from pythons to Japanese knotweed this is our catalogue of IAS.

The related Pacific salmonids here have interfered terribly with the natural ecosystem. Loss of the cutthroat and the invasiveness of the rainbow have substantial ecological and even evolutionary consequences for biodiversity and individual plants and animals in their habitats. The rainbow for example is now hybridised in so many ways that a pure subspecies is hard to locate. This paper seeks to characterise how dispersal and interaction influence these two species. Ryan P. Kovach and his 5 colleagues, from the University of Montana and Montana Fish, Wildlife and Parks write in the Proceedings of the Royal Society B on Dispersal and selection mediate hybridization between a native and invasive species.

External fertilisation contributes greatly to hybridisation in many fish, but the behavioural similarities and genetic similarity of these Pacific salmon also create hybrid fish. In nature, the mechanisms of this process need studying; hence the current paper. There should be selection for the cutthroat subspecies known as Westslope cutthroat trout (WCT) as the juveniles show better survival than the hybrids. However, either sustained dispersal or the reduced effects of outbreeding maintain the hybrid population.

Over 24 years, the ancestry of invasive rainbow trout (RBT) in 2 native WCT populations was related to their appearance (phenotype) and the strength of selection measured. Very mixed-ancestry adults from other locations were also recorded in an effort to check introgression into the population. The locations were at Langford and Cyclone creeks. While Langford is smaller, stable and cold, Cyclone creek is almost 30% wider, and 3.5^oC. warmer in the summer.

A small number of the fish concerned are resident but many migrate and return for breeding 2-4 years later. Stocking of RBT stopped in 1969, leaving highly backcrossed hybrids and some pure RBT. In the case of WCT, there was almost no hybridisation in the 1980s, but the invasion of RBT and hybrids is very evident afterwards. By 2003-2007, RBT alleles were found in 58% of adults in Cyclone and 87% in Langford.

The juvenile fish give the clue about what is happening in the ecosystem. They had less RBT input to their genotype than the adults. This means selection against the RBT admixture in both streams in every year. Immigration is highly indicated in the samples. Other RBT sources invade repeatedly, it seems with 12 pure RBT captured as 5.2% off the total entering Langford creek between 2003 and 20007. No pure juvenile RBT were ever captured leaving.

So the strong selection for the native WCT would seem to suit them to these 2 different creeks. Immigrants with a high proportion of RBT genes promotes the ill-adapted hybridisation, proving that genomic extinction can happen without the animal being fit. At the end of the study period, only 17% of Langford’s fish were pure WCT despite an accurate measurement of the selection coefficient acting against RBT introgression at an incredible 0.88.

The direction of selection then cannot be estimated simply in any species. The role of dispersal in driving hybridisation is difficult to measure but the dynamics here prove how important it is. Worldwide biodiversity needs to note this important study and deal effectively with the consequences. The fittest do not always survive, Mr. Darwin.