Fish feed waste enhancement of the particulate food supply and performance of mussels Mytilus edulis
suspended near salmon cages at an integrated multi-trophic aquaculture (IMTA) sitewas assessed using a
multi-indicator approach. Dietary indicators included bulk measurements of seston quantity and
nutritional quality, proximate analysis (PA), fatty acid (FA) and stable isotope (SI) composition. Mussel
tissue indicators consisted of PA and FA composition. Mussel performance was assessed from
physiological integrations (scope for growth, SFG), growth efficiency (K2) and condition index (CI). All
measurements were made over 2 days at a commercial IMTA farm and a monoculture mussel farm in the
Bay of Fundy (Canada). Significant differences detected in seston quantity and quality were within the
range of natural spatial variability. The SFG of IMTA mussels was lower (28.71 J h1) than monoculture
mussels (38.71 J h1) and reflected site differences in natural food availability and composition that
affected absorption rate. PA of mussel organs didn't reflect a significant fish feed contribution to the
mussel diet. However, dietary enhancement and assimilation of fish feed waste was demonstrated by
significantly higher levels of feed FA biomarkers 20:1v9, 18:2v6, 18:1v9 and low v3/v6 ratio in seston,
mussel tissues and feces at the IMTA site than at the mussel farm. SI (d13C and d15N) in seston and mussel
feces significantly differed among sites and IMTA mussels had significantly higher CI (21%) than
monoculture individuals (16%). Itwas concluded that bulk indicators of the diet, short-term physiological
integrations, and PA of mussel tissues have a limited capacity to detect dietary enhancement at IMTA
sites. FA and SI tracers of fish feed waste were shown to be more sensitive for detecting the low-levels of
diet enhancement within the large range of natural seston variation.