For sea bream and sea bass, knowledge regarding how escapes might affect ecosystems is limited. Intentional releases of cultured sea bream for stock enhancement have been reported from the southern Atlantic coast of Spain, and in the Bay of Cadiz (Sanchez-Lamadrid 2002, 2004). Released fish moved less than 10 km from the release point. Good growth rates and condition indices indicate that the released fish adapted to life in the wild and suggest that populations of wild fish could also be altered by released fish. For example, there is correlative evidence of a substantial increase in wild populations of sea bream after fish farming began in the Messolonghi lagoon, Greece (Dimitriou et al. 2007). Dempster et al. (2002) found very few sea bream near sea-cages in which sea bream were being reared, which suggesting either low levels of escape or that escapees move rapidly away from the farms to other habitats. Based on the ecology of sea bream and the location of most fish farms in areas close to wild sea bream habitats, it is probable that escapees would mix with their wild con-specifics. Consequently, the potential exists for escapees to interact negatively with wild populations, through interbreeding, competition and transfer of diseases and pathogens.
The only published long-term data available on escapes of sea bass indicate that when sea bass cultured from western Mediterranean populations escaped in the eastern Mediterranean, they established and maintained distinct populations of the western Mediterranean phenotype without introgressing with the local population (Bahri-Sfar et al. 2005). Escapes of sea bass in some locations may be particularly problematic, principally where local populations are small or in areas outside the natural distribution of sea bass. For example, sea bass do not naturally occur in wild habitats around the Canary Islands and their recent appearance there in coastal waters is due to escapes from sea-cages (Toledo Guedes et al. 2009). Escape of sea bass from fish farms in such areas is thus an introduction of a non-native species.
Possible impacts of ‘escape through spawning’ of sea bream
In the Mediterranean region, information about spawning by fish kept in sea-cages is sparse. In Greece, the largest EU producer of sea bream, a spectacular increase in both the number of fish farms and their production capacity took place over the past decade, accompanied by a substantial decrease in the price of sea bream. This industrial development led to structural and functional changes in the rearing process. Farming durations increased from just 12 to 18 months before 1995 (Petridis & Rogdakis 1996) to durations of up to 40 months after 1999 (Dimitriou et al. 2007). Gilthead sea bream is a protandrous hermaphrodite species and the increased farming duration has resulted in the production of fish of a size compatible with that necessary for fish to reach the stage of sex inversion and female sexual maturation, normally observed at the age of 2-3 years in the wild (Zahra et al. 1978). The aforementioned changes in rearing processes have resulted in the presence of large gilthead sea bream individuals (larger than 500g) in cages during the normal reproductive period of their wild counterparts (November-March: Bauchot & Hureau 1986). There is evidence that sex inversion and the production of both male and female gametes occur within cages under the present industrial rearing pattern (Dimitriou et al. 2007). A doubling of the population of wild sea bream within the Messolonghi lagoon in Greece, based on standardised commercial fishing trap catch returns, correlates with the advent of farming sea-bream to large sizes in the region. Spawning within sea-cages is suspected to have led to greater recruitment to wild sea bream stocks (Dimitriou et al. 2007). Ecological and economic consequences of this population shift have ensued as while more wild sea-bream are now available to the fishery, they are of much smaller mean size resulting in an overall lower economic return to local fishers.