Effect of Pacific Oyster (Crassostrea gigas) Stocking Level on Water Nutrient Concentrations and Productive Output in a Saline Recirculating Multitrophic Aquaponic System with Gilthead Seabream and Salicornia

The balance between fed and extractive compartments is critical in recirculating multitrophic aquaponic systems (RMAS). This study evaluated the effects of Pacific oyster (Crassostrea gigas) stocking level on water quality, gilthead seabream (Sparus aurata) performance, oyster growth, glasswort (Salicornia europaea) production, and whole-system fresh-product output. Nine independent RMAS received no oysters (O-00), 30 oysters (O-30), or 60 oysters (O-60). Ninety juvenile seabream (79.7 ± 13.8 g) were reared for 134 days with salicornia, while oysters received no supplemental feed. Oyster inclusion was associated with lower nitrate and phosphate concentrations, whereas other water-quality variables showed no treatment-related differences. Fish growth, feed conversion ratio, survival, slaughter traits, fillet pH, and scale cortisol did not differ among treatments. Oyster survival was 100%, with modest positive biomass gain that did not increase proportionally at the higher stocking level. Salicornia yield was numerically highest in O-30, and feed-normalized apparent total and extractive fresh-product outputs were greater in O-30 than in O-60. Pacific oysters can therefore be integrated into saline RMAS without detectable adverse effects on fish performance, but increasing oyster abundance does not necessarily enhance system performance. Increasing oyster abundance did not provide proportional improvements in system performance under the conditions tested.