Evaluation of the extent and ecological consequences of escape of fertilised eggs from sea-cages
WP Leader: Dr Stelios Somarakis, Hellenic Centre of Marine Research, Greece
Determine the extent and timing of spawning within sea-cage fish farms of Atlantic cod and sea bream (i.e. escape of fertilised eggs) at an industry-wide scale
Assess the quality of released eggs
Assess the survival and distribution of escaped eggs through empirical studies and modelling
Suggest and evaluate the need for implementation of mitigative strategies for reducing or preventing the escape of eggs
Over the last decade, the culture of species that may reproduce within sea-cages has become more common. For example, in Greece, changes in the rearing processes have resulted in the presence of large gilthead sea bream individuals (larger than 500 g) in cages during the normal reproductive period of their wild counterparts (November-March). There is evidence that sex inversion and the production of both male and female gametes occur within cages under the present industrial rearing pattern. In the culture of Atlantic cod, some fish mature during the first year of culture, while the majority of farmed cod are believed to mature during the second culture year. This means that almost the entire culture stock in any particular farm has the potential to spawn in the sea-cages before they are slaughtered. There might be considerable potential for larvae from escaped eggs to experience favourable conditions for survival and recruitment to coastal stocks. This may cause significant ecological and genetic effects in wild populations in the future.
The lack of knowledge on the extent and consequences of egg escape in a commercial culture restricts our ability to predict the need to implement mitigative actions aimed at reducing escape of fertilised eggs from sea-cages.
This WP consists of both an extensive field program and a modelling component.
In the field program:
Farmed fish will be randomly collected at selected cod (Gadus morhua), sea bream (Sparus aurata) and meagre (Argyrosomus regius) farms in Norway, Greece and Spain, respectively. These fish will be used for analyses of reproductive status, gonad size, individual egg number and spawning period.
Cod eggs from wild and cultured fish will be compared with respect to a range of egg quality parameters including egg dry weight, diameter and fatty acid content.
Fertilized sea bream and meagre eggs from selected farms will be collected and transferred to the laboratory where their development, survival and hatching rates will be analysed.
The fine-scale distribution and abundance of sparid and sciaenid embryos around sea cages will be studied in Greece and Spain respectively using vertical plankton tows.
Two types of models will be developed in this WP:
a simple descriptive model to predict possible survival rates of cod eggs and larvae originating from farmed fish spawning in sea-cages.
a coupled 3D hydrodynamic and biological model, will predict the dispersal pattern of escaped eggs from cod farms and nearby natural spawning areas
Results and Discussion
During the first year of the program, only Tasks related to the collection of farmed fish at selected cod, sea bream and meagre farms in Norway, Greece and Spain, respectively, have advanced but work is still under progress.
Preliminary results of histological analysis of gonads from large sea-breams kept in cages in Greece indicate that fish started to mature normally in autumn 2009.
Section of a sea-bream ovary from a Greek farm in the Ionian Sea (eastern Mediterranean).