Mortality of first-year cultured northern quahogs, Mercenaria mercenaria, through thermal decline: Impacts of low temperature, the rate of temperature decrease and dietary 20:5n-3 and 22:6n-3.

In a preliminary investigation, first-year cultured northern quahogs, Mercenaria mercenaria (Linnaeus, 1758), were administered a matrix of dietary highly unsaturated n-3 fatty acids, 20:5n-3 and 22:6n-3 to observe their effect on homeoviscous adaptation to low temperature. The quahogs were subjected to high magnitude thermal fluctuations of the natural, declining temperature regime of a temperate estuary during the approach to winter. Inverse correlations emerged between mortality and dietary abundance of 20:5n-3 during the upper range of the temperature decline (18°–12 °C), and 22:6n-3 during the lower range (12°–6 °C), demonstrating their unique roles in supporting homeoviscous adaptation within each respective phase of thermal decline. High survival of a group whose diet was devoid of these n-3 fatty acids was associated with high molar % increases of endogenic non-methylene-interrupted fatty acids, 22:2Δ7,13 and 22:2Δ7,15. Conversely in the current investigation, similarly grouped juvenile quahogs all experienced reductions in incorporated 22:2Δ7,13 and 22:2Δ7,15 when subjected to a mild temperature decrease of 0.2 °C per day, suggesting that high magnitude thermal fluctuation is a signal for the synthesis of non-methylene-interrupted fatty acids. The unfavorable circumstance of this group of quahogs deprived of dietary 20:5n-3 and 22:6n-3 was exacerbated by diminishing incorporation of these compounds and suffered the earliest and highest overall mortality of all dietary treatments. High early-phase mortality of this same group was associated with high levels of previously incorporated 22:6n-3, considered to be homeoviscously unfavorable at the upper thermal range above 12 °C, and this direct relationship fell neatly within the 95% confidence interval of a linear regression model established in the preliminary study. Statement of relevance Results of this supplemental experiment provide novel correlations among thermal decline, diet and mortality of juvenile northern quahogs, distinct from the preliminary investigation cited in this paper. This work distinguishes the effects of absolute temperature decline and rapid decline on mortality of juvenile northern quahogs. This work also demonstrates the requirement of high magnitude thermal decline to signal the endogenic biosynthesis of EPA, DHA and 22:2 NMIs in juvenile northern quahogs in homeoviscous adaptation to falling temperature. Results of this work can be applied to tailor supplemental diets of northern quahog seed to prepare for forecasts of rapid thermal decline. Such timely dietary supplementation may reduce loss of bivalve seed which might improve the industries annual production and profitability. [ABSTRACT FROM AUTHOR]