First records seem to be from 1910 at Lake Banyoles (which we visited when you were over) for ‘improvement of wild stocks’. Here’s an extract from a 2001 paper which explains a little more:
“Often the introduction of one species produces the need to introduce further species. Following introduction of major predators into fish communities which are not adapted to heavy predation, the decline in native species is such that it has often been assumed to be necessary to introduce a forage species more closely adapted for life with the predators. In fact, the introduction and spread of major predators, such as the pike Esox lucius, largemouth bass Micropterus salmoides, wels catfish Silurus glanis, Europea perch Perca fluviatilis, zander Stizostedion lucioperca, created the need to introduce and spread forage species, such as gudgeon Gobio gobio, roach Rutilus rutilus, rudd Scardinius erythrophthalmus, pumpkinseed Lepomis gibbosus, bleak Alburnus alburnus, and white bream which are more closely adapted to survive alongside the predator.
A range of reasons are cited for introducing fish species for the ‘improvement’ of wild stocks. The major motivation is to introduce some element that is perceived as lacking from the fauna of a water body. This is usually termed to fill a ‘vacant niche’ or some variant of this. Although not strictly in line with the niche concept, which sees the niche as a property of the organism, the idea of a vacant niche is used to describe the perception that there are resources within a water body which are not being used efficiently for lack of a suitable species. It usually applies in Spain to new habitats such as reservoirs or regulated rivers, where the native fauna lacks elements competent to establish themselves in the new water body.”
