Allozyme diversity in brown trout (Salmo trutta) from Central Spain: Genetic consequences of restocking A. MACHORDOM*, J. L. GARCI  A-MARI  N , N. SANZ , A. ALMODO  VAR* AND C. PLA *Seccio Ân de Ecologõ Âa, IMIA, Direccio Ân General de Agricultura y Alimentacio Ân, Comunidad de Madrid, Finca El Encõ Ân, Apdo 127, E-28800-Madrid, Spain y Laboratori d'Ictiologia Gene Âtica, Facultat de Cie Áncies, Universitat de Girona, Campus de Montilivi s/n. E-17071-Girona, Spain SUMMARY 1. The brown trout (Salmo trutta) represents one of the main freshwater resources in Spain, but habitat alterations and overharvesting have contributed to the decline or disappearance of numerous natural populations. In addition, reinforcement programs of wild populations based on releases of hatchery reared fish of exogenous origin compromise the conservation of remnant native trout resources. 2. We present allozymic data from Central Spain trout populations including stocked and unstocked populations. Although the levels of genetic variation observed were low and affected by hatchery releases ( p = 18.23%, Ho= 3.39%), they were within the range observed in other European areas. 3. The effective introduction of hatchery reared fish is genetically homogenising the populations in the studied area and disturbing the ancestral pattern of genetic variation that distinguishes the Tajo and Duero basins. Within the eight natural populations analysed, seven had alleles assigned to the foreign trout. The introgression in these populations, following the LDH-5*90 allele frequency, ranged between 2% and 29.4%, but those values are not in concordance with the respective stocking effort undertaken in each population. Moreover, the release of hatchery-reared fish does not solve the problems related to the reduced size of wild populations and their recruitment instability. Keywords: brown trout, Salmo trutta, genetic variability, restocking, introgression Introduction The brown trout (Salmo trutta L.) represents one of the main freshwater fish resources in Europe, both for its commercial interest and, especially, for its great sport fishing value. Nevertheless damage to natural habitats and probably overfishing have caused the decline or disappearance of numerous natural populations in Spain (Garcõ Âa de Jalo  n & Schmidt, 1995). The Spanish public administration responsible for the management and conservation of this natural resource has been attempting to maintain salmonid quality and quantity in continental waters for the past 100 years via stocking with hatchery-reared fish of foreign origin. Only recently has attention been focused on the genetic population structure of the native Spanish trout and the genetic consequences of foreign intro- ductions. Genetic variation is an important feature of populations, both for short-term fitness and long-term survival, as it allows adaptation to changing environ- mental conditions to occur (Ryman, 1981; Allendorf, Ryman & Utter, 1987; Utter, 1991; Dowling & Childs, 1992; Presa et al., 1994). Genetic variation is similarly important in farmed populations, allowing selective breeding and preventing loss of fitness due to inbreeding depression (Bartley et al., 1992). Previous studies indicated that native Spanish populations belong to ancient lineages of the species, and that the Iberian gene pools were characterised by: (i) strong individuality based on distinct allele frequencies in both adjacent and distant rivers (ii) Freshwater Biology (1999) 41, 707±717 ã 1999 Blackwell Science Ltd. 707 Correspondence and present address: A. Machordom, Museo Nacional de Ciencias Naturales. Jose  Gutie Ârrez Abascal, 2. 28006 Madrid. Spain. E-mail: mcna133@mncn.csic.es y y y