Date of Award

2000

Degree Type

Thesis

Degree Name

Master of Science in Fisheries, Animal and Veterinary Science

Department

Fisheries, Animal and Veterinary Science

First Advisor

Joseph DeAlteris

Abstract

The growth of the northern quahog, Mercenaria mercenaria, has been mathematically modeled over both the nursery stage and the complete life so as to develop expectations of growth as a function of environmental suitability. Then, the growth of northern quahogs in an experimental nursery upweller was evaluated as a function of system operating parameters (flow rate and stocking density) and other environmental parameters to determine the limiting factors in this critical phase of shellfish aquaculture.

The von Bertalanffy growth equation was used to predict increases in shell length (millimeters), weight (grams), and the relative growth rate (% increase per day) at various instantaneous growth coefficients (K). The relative growth rate (RGR) was also determined over a number of time intervals, including 1, 4, 7, 14, and 28 days. The age at which the maximum shell length and weight was reached varied with K. A higher K (0.30) resulted in rapid growth and an earlier asymptote, while a lower K (0.20 and 0.10) resulted in slower growth and a later asymptote in the animal's maximum shell length and weight. The RGR averaged over an annual time interval (annual RGR), as predicted by von Bertalanffy, decreased rapidly as the northern quahog aged, approaching 0.5 % increase/day after age 2. Annual RGR at different K values was similar, indicating that RGR was insensitive to changes in K. During the first growing season (210 days in the northeast), the increase in shell length predicted by von Bertalanffy was linear with a slope determined by K, that is, a greater slope results in a higher K. A similar relationship was apparent with weight. The RGR, however, varied greatly during the first growing season. Specifically, the RGR was 11% increase/day at 90 days after spawning and 2% increase/day at 210 days after spawning. The RGR at different K values was also insensitive to changes in K. There were no detectible differences between RGR determinations at a K of 0.10, 0.20, and 0.30 with varying time intervals (T); however, the value of RGR at a given point in time varied substantially with the time interval used to calculate RGR. The larger the growth interval, the larger the 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 shell lengths and RGRs were compared to observed shell lengths and RGRs from a field experiment growing northern quahogs in an experimental-scale upweller (nursery stage). The 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 was higher than expected and followed the general trend of decreasing RGR over time.

Northern quahog seed were grown from ~2 (longest axis) to ~13 mm in an experimental-scale floating upweller from June 21 to August 19, 1999 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 per minute per liter of northern quahog volume (μg ·min-1·r1), 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 initial 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 initial 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·r1. In experiments 2 and 4, there were no significant differences in growth between treatments.

Growth appeared to be limited by environmental conditions. In order to eliminate the effect of food limitation on growth, the upper third of the replicates (fastest growing animals) were used to calculate the 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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