Some marine organisms foraging low in the food web have a large production potential and can comprise a vast biomass. The exploitation of such organisms through production and harvesting has gained interest in recent years, in part to meet the increasing demand for a new, healthy and sustainable feed sources for finfish aquaculture. The suspension feeding tunicate Ciona intestinalis and the mussel Mytilus edulis are proposed as candidates for large biomass production. In order to model production and ecological carrying capacity for farming these species, we have completed a number of physiological experiments in C. intestinalis including the determination of metabolic costs and the ability of the organism to acquire and assimilate energy for growth from the natural seston throughout the year. The acquisition of the data has involved the development of new methods suitable for the use of natural seston in specific environments, such as the large scale controlled upwelling in the Lysefjord providing us with a natural gradient in seston quality. The experimental data were used to parameterise Scope for Growth (SfG) and Dynamic Energy Budget (DEB) models for C. intestinalis. As previously done for M. edulis these models will be applied to assess production and carrying capacity at aquaculture sites.
C. intestinalis demonstrate highest growth rates when primary production is low. This may, in part, be possible due to a greater retention efficiency of smaller particles by C. intestinalis compered to M. edulis. Laboratory experiments also suggest that C. intestinalis can energetically compensate (i.e. maintain SfG) via increased clearance rate and absorbance efficiency when exposed to low natural seston concentrations. Results from the seston gradient in Lysefjord also possibly suggest that individuals that are settled and naturally acclimatised to low seston concentrations are more adapt at this response than animals naturally acclimatised to higher seston concentrations closer to the upwelling, although seasonal responses in feeding plasticity are less clear.
Laboratory incubations used to parameterise the models suggest that this feeding plasticity in response to food limitation may also help, at least in part, to maintain SfG when C. intestinalis are exposed to combined environmental stressors also associated with climate change (e.g. warming, salinity and acidification). Interestingly, this maintenance of SfG does not necessarily result in the maintenance of growth when under stress and C. intestinalis may divert energy to storage (increasing the carbon content of tissues) while decreasing in size, possibly to increase metabolic efficiency (i.e. the Lilliput effect). This work demonstrates energy available for production is more dependent on feeding plasticity, i.e. the ability to regulate clearance rate and absorption efficiency, in response to environmental stress than on more commonly studied changes in metabolic costs. Such understanding is necessary if we are to safeguard future low trophic production and select appropriate sites for aquiculture.