as a process that threatens freshwater systems as runoff can lead to habitat degradation, causing erosion and increased river sediment. Another threat to freshwater systems is pollution, which poses a great threat to existing organisms that inhabit those areas. One of the factors believed to have contributed to the extinction of the New Zealand grayling include the theory of source-sink dynamics. Rivers and streams that supply an optimal habitat for ecosystems are known as sources, whereas those which do not provide the same level of environmental characteristics due to pollution, overfishing, or introduced species, are known as sinks. As the presence of sink rivers increased through pollution, caused by industrial development, increased, the availability of source rivers decreased accordingly. When at sea the New Zealand grayling faced little to no threat, but when migrating back to freshwater rivers or streams faced the threat of entering a sink river, where breeding and inhabiting conditions are unfavorable. As the fish were known to migrate in large shoals, if migration to a sink river occurred, a significant proportion of the population would either find it difficult to reproduce, thereby decreasing the amount of offspring in the next generation, or die off. The increase in sink rivers therefore posed a significant threat to the amphidromous species, as the availability of source habitats decreased and turned to habitats with less than favorable conditions. Another factor assessed to be a cause for the extinction of the species is the harvesting of the fish. Along with the Māori people hunting the New Zealand grayling, the settlement of the Europeans resulted in an increase of fishing. The increase in hunting for the fish, as it was in abundance, led to a decline in the population and eventually extinction, as the reproductive capacity of it was inundated. The final factor believed to have triggered the rapid decline and subsequent extinction of the population of the New Zealand grayling is the effects of introducing trout species from Europe. Brown trout were introduced into the southern oceanic regions during the 1860’s including New Zealand in 1867, and rainbow trout were introduced in 1883. The introduction of both brown and rainbow trout affected endemic species, such as the New Zealand grayling, through direct predation and competition for food and habitat. The non-native oomycete pathogen Saprolegnia parasitica vectored by introduced trout may also have played a strong role in the extinction of the