Modelling the Growth Of Atlantic Rangia, Rangia Cuneata, in Response to Freshwater Inflow, Trinity River Delta, Galveston Bay



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Rangia cuneata, commonly known as Atlantic Rangia, is an oligohaline clam found in abundance along the northern Gulf of Mexico. Rangia cuneata was selected as one of a suite of indicator species for establishing freshwater inflow regimes in Texas estuaries, including Galveston Bay. Within Galveston Bay, the highest abundances of R. cuneata have been in the Trinity River Delta. In the delta, meat index (MI), a health indicator, was found to increase during periods of prolonged freshwater inflow. To build on these results, the primary objective of this study was to model growth of R. cuneata in response to freshwater inflow. Ten sites in the Trinity River Delta with historic accounts of R. cuneata were equipped with continuous salinity and temperature monitoring devices from February 2018 to August 2019. Rangia cuneata were sampled quarterly using two collection methods (hand sampling and clam rake) for abundance, size, and MI. Overall, 559 R. cuneata were collected throughout the study. The average MI for the study period was 22 ± 0.32%. Meat index exhibited negative correlation with salinity and sediment percent fines. Rangia cuneata shells were examined to evaluate the influence of environmental conditions on shell growth and determine the shells age. Shell index (shell weight/ total shell dimensions (length + width + height)) is a new growth index suggested as a long-term health indicator. Rangia cuneata were aged based on winter marks in outer shell layer. Measured age along with length data were fitted in von Bertalanffy growth model. The output function of the model was rearranged and used to measure age of all collected clams (n = 599) throughout the study based on their length data. The Trinity River Delta had a growing population with majority of individuals at 3-4 years old and the oldest individuals were 6.5-7 years old. Shell growth represented by both annual growth bands width and annual increase in height was higher in years with higher average freshwater inflow. Rangia cuneata has been shown to be a responsive indicator species to multiple metrics that are directly related to freshwater inflows within the Trinity River Delta. While meat index is a short-term metric to measure R. cuneata health, shell growth (measured by shell index) is a better metric to understand long-term growth and health. Continued growth and health monitoring using multiple techniques are suggested to better understand R. cuneata response during periods of low flow.



Rangia cuneata, Atalntic Rangia, Trinity River Delta, Galveston Bay, Meat index, Freshwater inflow, Shell index, von Bertalanffy Growth Model