The study of fish migration is characterized by both strengths and weaknesses. The weaknesses are largely the products of limited study and resources. Few of the many species of fish have been studied. The studies that have been carried out have concentrated on freshwater and inshore aspects of migration. The major offshore oceanic migrations are still virtually untouched in terms of experimental research into controlling mechanisms. Within the areas which have been intensively studied there has been some tendency for research to advance unevenly. Over 2000 papers have been published on fish temperature preference, while other areas, such as genetic variation in migratory behaviour, have received little study until very recently.