Rethinking suspended mussel-farming modelling: Combining hydrodynamic and bio-economic models to support integrated aquaculture management

Suspended mussel-farming using cultivation socks, either in long-line or traditional pole cultivation structures, is a widely spread activity around the world. The sustainability and efficiency of the activity depends on a number of interrelated parameters, as is the natural availability of food, the physical and ecological characteristics of the coastal or marine area and the farmers' choices. The natural parameter which influences the activity the most and which can be a major controlling parameter for integrated management is adequate and uninhibited water circulation. The individual characteristics of the farming structures, as well as their spatial placing, can influence circulation, thus affecting food distribution, growth rates, productivity and profits of the mussel-farming activity. To assess this influence in an integrated manner, it is necessary to work in different levels of analysis and transdisciplinary domains, utilising multiple simulation models and taking into consideration the particular characteristics of the activity in different areas. This paper aims to demonstrate such an integrated management approach through: i) the high resolution assessment of the effect of farming structures on circulation, using a novel parameterisation of the cultivation socks as porous media; ii) the calibration of a traditional drag coefficient based on the results of the high resolution experiments, allowing a lower resolution analysis, necessary to implement assessments in cultivation area level and iii) the analysis and evaluation of alternative spatial planning configurations, both regarding hydrodynamics and bio-economic characteristics of the mussel-farming activity. The case study, used to demonstrate the methodology, was Thermaikos Gulf, in Northern Greece, the area producing more than 80% of the national mussel production and which has been facing critical management problems for more than 20 years. The integrated modelling approach resulted in the successful use of porosity for high resolution modelling of suspended mussel-farming in socks and in the multi-level understanding of how the operational characteristics of the activity can influence its sustainability. Additionally, the results shed light in some of the most important problems of the Greek mussel-farming sector. Overall the approach demonstrates that integrated, social-ecological management of productive activities, like aquaculture, require the combination of multiple and transdisciplinary levels of analysis, the development of tailor-made modelling approaches and some outside-the-box thinking, to overcome difficulties related to the availability of information.