Weight ratios of the kelps, Alaria esculenta and Saccharina latissima, required to sequester dissolved inorganic nutrients and supply oxygen for Atlantic salmon, Salmo salar, in Integrated Multi-Trophic Aquaculture systems

Abstract Estimates of seaweed nutrient sequestration ability in open-water, Integrated Multi-Trophic Aquaculture (IMTA) required to ‘balance’ nutrients from fish cages have typically assigned a specific nutrient load for a specific fish biomass. The resultant culture area and densities of seaweeds required for full equivalent nutrient sequestration may have meaning only to experienced aquaculturists. Consequently, a novel ratio model is proposed which determines the weight ratio of harvested seaweeds required to sequester an equivalent weight of soluble inorganic nutrients loaded per unit growth of fish. Soluble inorganic nutrient excretion from Atlantic salmon, Salmo salar , was estimated using a semi-stochastic nutritional mass balance approach. Oxygen demand was estimated using respiratory quotients. Nutrient contents of the IMTA kelps, Alaria esculenta and Saccharina latissima , were measured at harvest times, and net oxygen production was estimated using the photosynthetic equation. To quantify uncertainty, input parameters were assigned theoretical distributions (based on empirical or literature data) and the model was run using a stratified sampling approach (Latin Hypercube) over multiple iterations, to generate distributions of weight ratios for the various nutrients. A mathematical simulation of nutrient loading from a salmon farm over a full production cycle was estimated using mean loading values per unit growth, with monthly growth estimated by a thermal growth coefficient (TGC). Results showed that one kilogramme of Atlantic salmon growth (large fish fed a typical commercial feed, ± standard deviation) resulted in the excretion of 29.49 ± 4.20 g nitrogen, 2.26 ± 2.25 g phosphorus and the respiration of 243.38 ± 48.28 g carbon. Dissolved oxygen requirements for 1 kg of growth were 455.29 ± 86.24 g. Salmon smolts placed in cages in spring and harvested 21 months later, load approximately 4 times more nutrients in the second grow-out season. The mean ratios of A. esculenta weight required to sequester nutrients excreted per unit weight of S. salar production were 6.7(± 1.5):1 for nitrogen, 4.8(± 3.0):1 for phosphorus, and 5.8(± 1.4):1 for carbon. Oxygen could be supplied at a weight ratio of 4.1(± 1.0):1. The mean ratios of S. latissima were 12.9(± 2.7):1 for nitrogen, 10.5(± 6.2):1 for phosphorus, and 10.2(± 2.2):1 for carbon. Oxygen could be supplied at a weight ratio of 7.2(± 1.5):1. A. esculenta appears to have almost twice the nutrient sequestration capacity per wet weight than S. latissima . However, culture densities of S. latissima are 1.5 times greater than those for A. esculenta and when spatially weighted this difference is reduced to 1–1.5 times. Numbers of rafts for both kelp species required for full nutrient sequestration from a commercial scale salmon farm exceed the number of rafts which can be practically deployed within a typical site lease area. However, not all inorganic nutrients from cultured fish will be available to IMTA seaweeds, nor should 100% nutrient sequestration need be the only successful endpoint in such systems. These aspects should be considered when assessing the net value of kelps in open-water IMTA systems.