Hybridization and genetic population structure of Alosa population in the United Kingdom.

Human-mediated habitat fragmentation has been proposed as the main factor driving hybridization between the sympatric migratory European shads Alosa alosa and Alosa fallax, which has co-occurred with substantial population declines in A. alosa. In river systems across Great Britain, shad are negatively affected by navigation weirs constructed in the last 150 years that impede their spawning migrations. Consequently, the aim here was to assess the impact of human disturbances on the genetic introgression and population structure of shad in Great Britain through genotyping 119 Alosa spp. using 24 microsatellite loci.

Loss of genetic integrity and biological invasions result from stocking and introductions of Barbus barbus: insights from rivers in England.

Anthropogenic activities, including the intentional releases of fish for enhancing populations (stocking), are recognized as adversely impacting the adaptive potential of wild populations. Here, the genetic characteristics of European barbel Barbus barbus were investigated using 18 populations in England, where it is indigenous to eastern-flowing rivers and where stocking has been used to enhance these populations. Invasive populations are also present in western-flowing rivers following introductions of translocated fish. Two genetic clusters were evident in the indigenous range, centered on catchments in northeast and southeast England. However, stocking activities, including the release of hatchery-reared fish, have significantly reduced the genetic differentiation across the majority of this range. In addition, in smaller indigenous rivers, populations appeared to mainly comprise fish of hatchery origin. In the nonindigenous range, genetic data largely aligned to historical stocking records, corroborating information that one particular river (Kennet) in southeast England was the original source of most invasive B. barbus in England. It is recommended that these genetic outputs inform management measures to either restore or maintain the original genetic diversity of the indigenous rivers, as this should help ensure populations can maintain their ability to adapt to changing environmental conditions. Where stocking is considered necessary, it is recommended that only broodstock from within the catchment is used.