Life cycle and feeding habits of the threespined stickleback Gasterosteus aculeatus (Linnaeus, 1758): an alien species in the southeast Caspian Sea

The feeding habits and life cycle of a population of the alien threespined stickleback Gasterosteus aculeatus, were studied in the southeast Caspian Sea from December 2000 to November 2001 to evaluate its potential impact on the indigenous commercial species. The abundance (A%) and occurrence (O%) of prey items in G. aculeatus stomach content were calculated for Gammarids, Nereis sp., Chironomidae, fishes, Oligochaeta, fish eggs, and Hirudinea. The potential effects of the alien threespined sticklebacks on juvenile Persian and Russian sturgeons were discussed. The gonadosomatic index of female sticklebacks showed that breeding season began at March and lasted five months in the southeast Caspian Sea. The condition factor, somatic condition factor, and hepatosomatic index increased at the beginning of the spawning season and decreased with its progress. A significant negative correlation between gonadosomatic index and somatic condition factor in females indicates priority of reproduction over somatic growth; this can help the alien species to rapidly increase population size, thus enhancing its negative impacts on indigenous fauna. The similar or even higher mean weight and length of the alien sticklebacks in the study area in comparison with other native populations of sticklebacks might imply suitable conditions of the Caspian Sea for the establishment of alien populations of this species.