A paper has recently been published by Gordon Copp, Lorenzo Vilizzi, and Rodolphe Gozlan in the journal Aquatic Conservation: Marine and Freshwater Ecosystems which eludes to the fact that the higher the population density in England, the more likely the natural ecosystems can be tainted with pet fish.
This is not a new piece of information. It has been known for quite some time that the ecosystems which are closer to roads, fish markets and pet shops are more likely to be subjected to pet fish than those which are not close to these avenues.
The object of the study was to perform a statistical analysis to take a look at the spacial relationships between pet fish contamination and the demographics which led to these fish being released into the wild, and also to test whether or not these demographic factors are a reliable way of estimating how many alien species are introduced into these ecosystems.
The case study was carried out using an intermediate scale for all of England, dividing the country into 1500 squares of 10 square kilometers each.
The study consisted of the following data sets to be used in their analysis: non-native fresh water fish occurrences in the wild; the numbers of non-native fish imported, and demographic information such as: numbers of humans, pet shops, garden centers and fish farms per unit area.
The study found that the incidences of pet fish contamination directly co-related to the density of the human population.
If you would like to learn more check out the paper: Copp, GH, L Vilizzi and RE Gozlan (2010) The demography of introduction pathways, propagule pressure and occurrences of non-native freshwater fish in England. Aquatic Conservation: Marine and Freshwater Ecosystems 20, pp. 595–601.