| Summary: | Field and laboratory studies were designed to identify nutritional and other factors affecting the feeding and growth of Perna canaliculus within mariculture systems in the Marlborough Sounds.
Field data was collected from mussel farms in Marlborough, and analysed to reveal how environment, community structure, feeding and growth varied between embayments, between farms and within mussel farms.
In the absence of adequate seawater flow through farms, up to 60% reduction of food occurred due to retardation of flow by farm structures and grazing by mussels. Decreased food concentrations limited feeding, condition and growth by mussels living at downcurrent sites within farms. Whereas the presence of adjacent farms did not limit
food or food uptake, the manipulation of stock densities and farm structures may enhance food availability and provide a key to enhanced productivity.
Average feeding rates varied markedly between embayments, and this variation may be related to food availability. Mussels at a seaward site (Richmond Bay) had an even and low condition index whereas those in Crail Bay had a variable and high condition index.
Multiple regression analyses indicated that spatio-temporal changes in mussel length, temperature and food resources could explain the marked differences in feeding rates. Filtration rate was high or maximal below a food concentration of 1.5 ug 1-¹ chlorophyll a, but declined markedly above this threshold when a maximal ration was ingested. Assimilation efficiency declined from 85% during winter to 75% in summer, and did not become limited when maximal volumes of food were eaten by P. canaliculus.
Laboratory studies were designed to test a range of factors (below) that may affect feeding in the field, or perturb feeding in experiments.
Presence of gametes caused an 81% decline in feeding. However, mussels filtered normally after spawning and in the absence of gametes. Filtration ceased for 4 hours after salinity was reduced from 34 to 25ppt, then recovered to rates similar to those recorded at 34ppt. Filtration rate also declined ten-fold as oxygen concentration declined from 5 to 1.5ppm. However, current speeds of 5-28 cm s-¹ did not affect filtration.
After deprivation of food, feeding behaviour depended on food concentration. At 0.3 mgC 1-¹ food, starvation increased filtration slightly. At 1.5 mgC 1-¹ food, filtration rate fluctuated markedly until rates stabilised at 48% of the maximum recorded rate, 18 hours after food deprivation. Feeding and energy budgets were therefore determined during long-term (24 hour) experiments.
Five separate phases of feeding behaviour were identified as food concentration increased from 0.03 - 10.0 mgC 1-¹, a range extending below that recorded in the mussel farms of Marlborough (0.4 - 4.0 mgC 1-¹ food). 1. Filtration rates increased to maxima at 0.3 mgC 1-¹. 2. Filtration then declined to 70% of this maximal rate as food concentration increased to 0.6 mgC 1-¹. 3. Uniform filtration occurred from 0.6 to 1.0
mgC 1-¹. 4. From 1.0-1.4 mgC 1-¹, filtration declined and a maximum ration was ingested. 5. Above 4.1 mgC 1-¹ food, mussels filtered at minimal rate, appeared to ingest a maximum ration, and rejected food in pseudofaeces. Whereas phases 1-5 of feeding occurred in 80mm length class mussels maintained at 180C and in three size classes of mussels maintained at 15°C, only phases 1, 4 and 5 were recorded in 80mm length mussels maintained at 12°C. Gut passage time increased below 15°c. Temperature may therefore affect both feeding behaviour and dietary regulation in P. canaliculus.
Mussels assimilated from 81% to 89% organic matter from I sochlrysis galballa.
Digestion was not affected markedly by changes in ingestion rate, temperature or mussel size. Thus, Growth Potential increased with food concentration to maximal values of 3.7-5.8% body Cd-¹. Energy budgets indicated that growth rate increased with food concentration during phases 1-3 of feeding, but was independent of food during phases 4-5 of feeding. Whereas maximum Growth Potential increased with temperature, it was not dependent on mussel length within the range 30-80mm.
Comparison of field observations with results of laboratory studies indicated that feeding phases 3, 4 and 5 occurred in both situations. Thus, feeding only became severely food limited when food concentration declined markedly. However, only 0.5% of food consumed within farms was produced in situ, and adequate water movement in
embayments and through farms was therefore needed to provide cultivated mussels with food. The structure of the cultivation system may constrain growth by mussels. In particular, the mussels' distribution within farms, and the distribution of farms between
embayments limited production. Further understanding of the biological base of this industry may therefore enhance profit.
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