Evidence for ‘escape through spawning’ in large gilthead sea bream Sparus aurata reared in commercial sea-cages

S Somarakis,C Saapoglou,Cs Tsigenopoulos,M Pavlidis,T Dempster

doi:10.3354/aei00057

Abstract

During the last decade, farming of species that may reproduce within marine fish farms has become more common. However, knowledge of the extent and ecological effects of reproduction of farmed fish within commercial farms is sparse. We examined whether large gilthead sea bream Sparus aurata reared in farms in Greece produced fertilized eggs that survived and dispersed into surrounding waters. Extensive histological investigation of gonads sampled from 5 different farms revealed that female sea bream cultivated beyond the size of sex reversal matured, ovulated and released eggs during the normal spawning period of the species. Annual fecundities (108 000 ± 60 081 eggs kg−1 of female in farms) were low relative to commercial broodstock and decreased considerably with sex ratio (i.e. with fish size in the cage). An egg-collector experiment and site-intensive ichthyoplankton study with genetic identification of sparid eggs demonstrated that sea bream eggs were spawned in farms and released to the environment. If 5−10% of the 130 000 tons yr−1 production is fish of larger size, we estimate that 3.5 × 1011 to 7.0 × 1011 eggs yr−1 are released into the Mediterranean Sea. Whilst we documented low daily survival rates of fertilized eggs in the vicinity of sea-cages, our findings imply that the escape of eggs from sea bream farms may have ecological consequences, which likely depend on the sex ratio and intensity of farming within specific regions. To avoid potentially negative ecological and genetic con sequences of escape through spawning, farms with large fish should not be placed close to the nursery grounds of wild sea bream, such as in coastal lagoons.

Highlights

The escape of fish from sea-cage aquaculture may lead to undesirable genetic effects in native populations through interbreeding, and ecological effects through predation, competition and the transfer of diseases to wild fish (Fleming et al 2000, CIESM 2007, Thorstad et al 2008, Jensen et al 2010, Arechavala-Lopez et al 2012)
We examined whether large gilthead sea bream Sparus aurata reared in farms in Greece produced fertilized eggs that survived and dispersed into surrounding waters
Whilst we documented low daily survival rates of fertilized eggs in the vicinity of sea-cages, our findings imply that the escape of eggs from sea bream farms may have ecological consequences, which likely depend on the sex ratio and intensity of farming within specific regions

Summary

Introduction

The escape of fish from sea-cage aquaculture may lead to undesirable genetic effects in native populations through interbreeding, and ecological effects through predation, competition and the transfer of diseases to wild fish (Fleming et al 2000, CIESM 2007, Thorstad et al 2008, Jensen et al 2010, Arechavala-Lopez et al 2012). A second form of escape has come into focus, involving the escape of fertilized eggs spawned by farmed individuals from sea-cages Escaped cod eggs disperse into the environment (Jørstad et al 2008, Uglem et al 2012), survive to become young-of-the-year, recruit to spawning populations and successfully spawn in the wild (van der Meeren et al 2012). The broader ecological consequences of this new form of escape are largely unknown

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