Carrying capacity of native low-trophic resources for fish feed ingredients - the potential of tunicate and mussel farming

The exploitation of organisms foraging low in the food web 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. The primary objective of this project was to determine the trophic resources and carrying capacity for these native suspension feeders C. intestinalis and M. edulis. The sub-goals supporting this objective were; 1) determining the growth rate and feeding physiology of C. intestinalis in Norwegian coastal waters, needed to parameterize growth modelling (DEB), 2) determining efficiency of turning suspended particulate food sources into tissue/biomass, and 3) to model aquaculture-environment interactions and estimate carrying capacity. Growth experiments in the upwelling area of the Lysefjord show a fifteen-fold increase in wet weight of C. intestinalis from about 1 g wet weight in late July until maximum weight in late October. This demonstrates substantial difference in growth pattern and life history strategy between M. edulis and C. intestinalis which need to be considered for any assessment of carrying capacity and trophic recourse exploitation. A comparative experimental study of M. edulis and C. intestinalis revealed on potential methodological influences on the determination of how they retain different sizes of food particles, or particle retention efficiency (RE). Results showed that the retention efficiency of mussels does not always conform to the traditionally assumed model of suspension-feeding bivalves, where RE decline below a maximum retention of particles larger than 3-7 ?m. In our experiments, the mussels had a maximum RE for particles >8?11 ?m, while RE for smaller particles declined gradually to 30?40% retention of 2 ?m particles. C. intestinalis were observed to have a markedly different RE than mussels and retained >70% of particles larger than 1 ?m. The new understanding on retention efficiency can be critical to estimates of production carrying capacity of mussels and tunicates. Results on temperature effects on physiology of C. intestinalis were attained from experiments also designed to study possibly future climate change (warming, salinity and acidification). Under low food availability the effects of elevated temperature (and pCO2) were energetically mitigated by increased clearance rate and absorption efficiency and reduced metabolic rate. Comparisons of physiological responses between M. edulis and C. intestinalis in feed individuals showed M. edulis to be more tolerant to both salinity and pCO2 perturbations. In response to elevated pCO2 both species show decreases in growth potential due to reductions in clearance rate. At elevated pCO2, salinity had no further effect on growth potential of M. edulis, whereas C. intestinalis showed 100% mortality at the lowest salinity (15 psu). To investigate if these results hold true in natural environments mesocosm experiments were carried out in China. Results suggest that under natural conditions Crassostrea gigas (Pacific oyster, an invasive in Norwegian waters) is more sensitive to elevated pCO2 and may have less ability to compensate via feeding plasticity when compared to M. edulis. Results from physiological experiments were used to parameterize The Dynamic Energy Budget (DEB) model satisfactorily reproduces growth of C. intestinalis , estimating parameters from project results. DEB models for C. intestinalis and M. edulis are used in a multiple box ecosystem model to assess carrying capacity, and additionally to explore different stocking densities, upwelling alternatives, and the creation of new cultivation areas. Results from the seston gradient in Lysefjord suggested that individuals of C. intestinalis that are naturally settled and acclimatized to low seston concentrations show a larger capacity for physiological compensation than animals naturally acclimatized to higher seston concentrations closer to the upwelling. The difference in growth pattern and life history strategy between M. edulis and C. intestinalis shown in the Lysefjord environment reveals the intricacy in comparing trophic efficiency for these species in biomass production. The apparent seasonal segregation in growth and ecological efficiency of these species is demonstrated using growth modelling. Results on feeding physiology provide new knowledge critical for assumptions used to estimate clearance rate in bivalves will improve assessment of their ecosystem interactions, including assessment of carrying capacity. Project results thereby impact on the academic understanding of suspension feeding, assessment of ecological interactions, trophic efficiency and ultimately industry development and society.

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 have 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. Farming large-scale biomasses of hi ghly efficient suspension feeders in coastal ecosystems will compete with the wild stocks for limited resources such as space and food. Competition for these resources has implications through all trophic levels for the sustainability of marine ecosystems , including maintaining biodiversity and commercial fisheries and aquaculture productivity. In order to model ecological carrying capacity and to evaluate production capacity and sustainability, it is imperative to know the functional feeding physiology o f the organisms under native environmental conditions. In this project we propose to model production and carrying capacity of the tunicate (Ciona intestinalis) and the mussel (Mytilus edulis), and to explore scenario building and optimization processes a t aquaculture sites under natural and forced upwelling events. The model input data will be based on eco-physiological measurements obtained under native environmental conditions. The project will also compare the organisms? efficiency of turning suspende d particulate food sources into biomass.