Environmental Impact of Exposed Salmon Farms: Transect Reveals Matching Gradients in Seafloor Ecosystem Structural and Functional Proxies

Abstract

Exposure of fish pens to strong currents in the open ocean facilitates the dispersion of farm-derived organic-rich solid wastes (faeces, uneaten feed) and thus lessens the effect of waste deposition on the seafloor ecosystem. Quantifying this effect requires tools that can detect deviations in seafloor ecosystem functioning, including trends that, if left unattended, accumulate to cause adverse effects. Here, we demonstrate a spatial gradient in the seafloor O₂ demand, maintained by local deposition of farm-derived organic waste, that could be used as such a tool. To detect this gradient, we measured the in situ sediment-seawater O₂ flux with 2 seafloor landers, a benthic chamber lander and an aquatic eddy covariance lander, deployed at 19 sites along a 3000 m transect heading towards a salmon farm in the D’Entrecasteaux Channel, Tasmania, Australia. We found that the seafloor O₂ demand gradually increased within approximately 500 m of the farm from a background level of 220-560 to 1922 µmol m⁻² h⁻¹. The observed gradient was consistent with a gradient in the biological structure of the seafloor, demonstrated by traditional analyses of benthic macrofaunal assemblages. We discuss how regular monitoring of the seafloor O₂ demand with autonomous benthic landers can support proactive ecosystem-based management of open-ocean fish farms.