Feeding behavior and bioenergetic balance of the great scallop (Pecten maximus) and the blue mussel (Mytilus edulis) in a low seston environment and relevance to suspended shellfish aquaculture.

Abstract

The length of the rugged Norwegian coastal line including all islands exceeds the circumference of the earth, and the coastal zone contains many sheltered areas that are potentially suitable for bivalve farming. However, a large fraction of the coastal environment exhibits close to oligotrophic conditions and has significantly lower seston (e.g. food) concentration compared to most bivalve aquaculture sites currently operating worldwide. Aquaculture in low seston environments is a concern, partly because of the food requirement for bivalves to grow (e.g. particulate organic carbon (POC) ~ 200 μg L-¹ or chlorophyll a (Chl a) ~ 1 mg L-¹), but also because bivalves were reported to stop feeding at low seston concentrations (Chl a between 0.5 to 0.9 μg L-¹). Low seston environments have an increased risk of food depletion that can reduce bivalve production due to food limitation, especially when bivalves are kept at high stocking density as in culture. In order to manage bivalve farming in low seston environments there is a need for bivalve production and carrying capacity models. The objective of the thesis was to study the functional feeding responses of two commercially important bivalve species (Pecten maximus and Mytilus edulis) to the dietary conditions in a natural low seston environment. Further objectives were to determine the net zero growth and net zero energy balance of the bivalves to help facilitate growth predictions. On a cultivation unit scale, the aim was to describe physical and biological components of seston depletion processes in long-line suspension culture, to optimize cultivation unit design, carrying capacity assessment and site selection. Studies were carried out to facilitate an improved understanding of the seasonal feeding behavior, physiology and growth of the great scallop (P. maximus) and the blue mussel (M. edulis) under natural low (e.g. suspended particulate matter (SPM) < 1 mg L-¹ or Chl a < 1.5 mg L-¹) and diluted seston concentrations and to provide data for aquaculture and ecosystem models. The results were novel in that they showed that these species do not stop feeding, but rather maintain intermediate to high feeding rates at all low seston concentrations employed. The feeding response to seston quantity was unimodal as previously reported for scallops and mussels, but the response curve was skewed toward lower seston concentrations. Positive tissue growth was detected at seston concentrations of 80 μg POC L-¹ and 0.4 μg Chl a L-¹, which is less than half the values previously reported. The ability of the bivalves to grow under the low seston quantities was primarily caused by continuous feeding at typically high rates and the high net absorption efficiency (range 40 – 80%). Net energy balance predictions for both species overestimated actual growth owing to a wide range of factors including; the inability of bulk seston characteristics to adequately characterize the seasonably variable energy content of the seston and spawning. To increase the accuracy of growth estimates, the biopolymeric constitute of seston should be considered as an alternative to utilizing constant POC/energy conversion factors for ingested food. The spatial distribution of mussel condition index in a long-line (200 x 15 meter) mussel cultivation unit was studied in relation to physical flow reduction and seston depletion by mussel grazing. The food availability, expressed as the product of current speed and Chl a concentration, decreased to less than 20% of ambient within the first 30 meters of the unit. The strong reduction in food availability was the apparent cause of the lengthwise reduction in condition index detected from the edges toward the centre of the unit. The physical and biological data obtained from the long-line mussel unit was used to develop a model of current speed reduction and production capacity according to cultivation unit layout. The model output highlighted the need to optimize the farm length to width ratio to reduce seston depletion and improve flow characteristics. The model has increased the awareness of measuring current speed and seston concentration to identify suitable farming sites in Norway and has motivated farmers to rearrange the farm layout.