Effect of intertidal air exposure and handling husbandries on shell development, meat condition and survival of farmed Pacific oysters (Magallana gigas, Thunberg 1793)

Abstract

Oyster farming practices are advancing towards optimising production amid shifting market demands and changing climates. This requires a refined understanding of the complexities in handling strategies involving air exposure via suspended line or off-bottom culture and handling techniques on production and market qualities of the oyster. This study investigated the interactive effects of air exposure (0, 4, 11.5 h d⁻¹) and handling/grading frequencies (0, 4, and 7 x) over a 350-d culture period on the shell quality, growth, survival, meat condition and biofouling in Pacific oysters. Shell quality metrics included weight, thickness, chalkiness, compressive strength, density, breakage, broadness and cup depth indices, roundness, and exterior colour hue. Subtidal oysters (0 h air exposure), characterised by stable, warmer temperatures, higher salinity and chlorophyll abundance, exhibited accelerated growth. This group attained larger sizes with thicker shells of lighter hue, albeit with reduced shell density and increased chalkiness, predisposing them to greater fragility. Conversely, prolonged air exposure (11.5 h d⁻¹), characterised by fluctuating, averagely cooler temperatures, reduced salinity and higher turbidity water, resulted in the smallest oysters with the densest, broadest but shallower-cupped and darkest hued shell exteriors. A moderate (4 h d⁻¹) air exposure yielded an intermediate shell with superior compressive strength and thickness. While the interactive effect of air exposure and handling on shell quality was nuanced, a hierarchical influence on survival was observed. Air exposure demonstrated a positive influence on survival and condition index while negatively influencing fouling. The potential physiological mechanisms underlying variations in shell quality are discussed. There is the potential to optimise shell quality in oyster production through a judicious combination of moderate to high air exposure regimes with carefully defined handling protocols. A chequerboard of options is provided.