Specifically, DNA and microsatellite or minisatellite probes will be isolated for the detection of fragment size polymorphisms which will be tested for their Mendelian inheritance and for their ability to provide means of identification of individual fish and for parent offspring matching in breeding experiments. In the use of catfish, the probes will also be used to confirm the gynogenetic status of fish produced in laboratory experiments. For the sea bream in particular, the probes will be used in a breeding experiment designed to identify parents that produce fast growing progeny. About 30 adults (of which about half will be males and half females) will be mass spawned and the resulting progeny will be grown together in one pool. Blood will be removed from each parent and DNA extracted. The DNA will be stored for subsequent scoring of genotypes when the probes become available. Random samples of about 150 progeny will be drawn from the common pool at 2 periods, at the age of 7 and 10 months. These offspring will be weighed and their DNA extracted for genotyping. The use of DNA probes will make it possible to identify the parents of each offspring. This information will be used to search for statistical associations between growth rates (weight, the age of 7 or 10 months) among full sibs, half sibs and unrelated progeny. Parents whose progeny grew faster will be identified and can be used subsequently as exclusive parents for the productio of a new set of progeny. The development of genetic probes will be done by all contractors. Contractor 1 will undertake the responsibility for the probes of the sea bream and also for the first phase of development of probes for the catfish. Contractor 2 will be responsible for the first phase of development of probes for sea bass. The application of the probes in the sea bream breeding experiment will be done by contractor 1 in the Institute of Marine Biology of Crete which maintains a large programme of research on sea bream aquaculture. Contractor 3 has an ongoing research programme on catfish, part of which is to produce and evaluate the importance of gynogenetic stocks for aquaculture. The probes will be used to screen for gynogenetic fish.