Growth of the northern quahog, Mercenaria mercenaria, in an experimental-scale upweller

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Upwellers have proven to be extremely effective as bivalve nursery units and their use is steadily increasing in North America. The re-analysis of previous work by others suggests an asymptotic relationship between growth (% volume increase per day) and chlorophyll-a effective flow rate (the amount food flowing past a unit biomass of northern quahogs, μg per minute per liter of northern quahog volume). An experiment field study was conducted to define the relationship between food flow and bivalve stocking density. Furthermore, this study was designed to investigate other significant environmental parameters influencing bivalve growth in an experimental-scale upweller system. Northern quahog, Mercenaria mercenaria (Linné), seed were grown from ∼2 (longest axis) to ∼13 mm in an experimental-scale floating upweller from June 21 to August 19, 1999 (four separate experimental periods) in Point Judith Pond, Wakefield, Rhode Island. Flow rates and stocking densities were varied in order to produce a chlorophyll-a effective flow rate range of 360 to 1,500 μg · min-1 · l-1, and growth and environmental parameters were measured semiweekly. During the first two-week experiment (June 21 to July 7) an asymptotic relationship was observed between growth (% increase/day) and chlorophyll-a effective flow rate. A significant difference in growth was found between the treatments. The difference in the functional relationship between experiments 1 and 3 was possibly related to lower DO values, which reduced differential growth in experiment 3. In experiment 1, the low-biomass treatments grew faster than the high-biomass treatments. A significant difference in growth between treatments was also observed in experiment 3, although the asymptotic relationship was less pronounced. In experiment 3, the high-biomass replicates grew faster than the low-biomass replicates. Experiments 1 and 3 both experienced similar environmental conditions; however, experiment 1 encountered higher morning dissolved oxygen (DO) levels. In addition, the within experiment variability in experiment 3 was much less than the variability in experiment 1; therefore, accentuating growth differences in experiment 3. In both experiments 1 and 3 maximum growth occurred near treatment 2 in a range of chlorophyll-a effective flow rates of 550 to 650 μg · min-1 · l-1. In experiments, 2 and 4 there were no significant differences in growth between treatments. Growth appeared to be limited by low oxygen. In order to eliminate the effect of food limitation on growth, the upper third of the replicates (the fastest growing animals) were used to calculate the relative growth rate (RGR) during the two-month experiment. Growth was linearly correlated with morning-dissolved oxygen (R2 = 0.42) and with chlorophyll-a (R2 = 0.35). The critical DO threshold for growth in upwellers appears to be 5 ppm, below which growth is adversely affected. During this study, morning DO levels were less than 50% saturated, indicating the potential for DO levels to be increased. Future research should investigate methods for elevating DO levels in upwellers.

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Journal of Shellfish Research





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