Modeling growth of the northern quahog, Mercenaria mercenaria

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Growth of the northern quahog, Mercenaria mercenaria (Linné), was deterministically modeled over the life span of the bivalve as well as during its first growing season (nursery stage) in Rhode Island waters. Specifically, the von Bertalanffy growth equation was used to predict increases in shell length (millimeters), weight (grams), and the relative growth rate (% increase in weight or volume per day) at various instantaneous annual growth coefficients (K). The relative growth rate (RGR) was also determined by averaging over different time intervals including 1, 4, 7, 14, and 28 days and annually. The age at which the maximum shell length and weight was reached varied with K. A higher K (0.30) resulted in more rapid initial growth and an earlier asymptote, while a lower K (0.20 and 0.10) resulted in slower initial growth and a later asymptote. RGR averaged over an annual time interval (annual RGR) decreased rapidly as the northern quahog aged, approaching 0.5% increase/day after age 2. The annual RGR's at different K values were similar, indicating that RGR was relatively insensitive to changes in K. During the first growing season (210 days in the northeast), the increase in shell length predicted by the yon Bertalanffy equation was linear with a slope determined by K, that is, a higher K resulted in a greater slope. RGR, however, varied greatly during the first growing season, decreasing from 11% increase/day at 90 days after spawning to 2% increase/day at 210 days after spawning. The RGRs at different K values were also insensitive to changes in K. There were no detectable differences between RGR determinations at a K of 0.10, 0.20, and 0.30 with varying time averaging intervals (7); however, the value of RGR at a specific time varied with the time interval used to calculate RGR. The a and b coefficients estimated for the weight-length relationship from the adult and nursery stage northern quahogs differed from each other, and published measures from Narragansett Bay northern quahogs. This suggests that researchers should use data collected from northern quahogs in a size range similar to that being modeled when estimating biomass from length and abundance data. Predicted lengths and RGRs were compared to observed lengths and RGRs from a field experiment growing northern quahogs in an experimental-scale upweller (nursery stage). Our northern quahogs grew at a K of 0.25 indicating favorable conditions for growth. Early in the experiment (between 70 and 100 days after spawning), the experimental RGR differed markedly from the predicted measure; however, after 100 days post spawning the experimental RGR followed the general trend of decreasing RGR over time, but was higher than predicted.

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Journal of Shellfish Research





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