Assessment of options for managing seabed recovery in marine finfish aquaculture

Intensive fish farming can result in significant benthic impacts, with excess feed and waste products altering the nutrient balance of the system, changing sediment-water nutrient fluxes and influencing ecological dynamics. The degree of impact depends upon the scale and efficiency of the farming operations and the nature of the receiving environment. Estuarine and coastal areas are dynamic, and frequently subject to natural sources of organic enrichment; this adaptability may increase their resilience to fish-farm impacts. However, farm-based enrichment sources are typically at different spatial and temporal scales and potentially to a much greater degree than would occur naturally. Understanding the way in which the environment responds to farming operations, in particular the systems’ ability to recover such that impacts do not irreversibly alter the ecosystem function, allows for farm-based management of impacts which can optimise farming productivity.

This study presents a meta-analysis of aquaculture data from a range of locations and input scenarios, and compares performance under 2 key management models: i) “natural” sediment remediation and ii) active intervention, to assess recovery outcomes. We also consider the possible effects of changing biogeochemistry on sediment contaminants that may commonly be found in association with sources of organic enrichment (i.e. heavy metals).

Results suggest that recovery rate is contingent upon the biological function of the sediments. When ecosystem processes are affected to such an extent that anoxic conditions prevail, nitrogen re-cycling is less efficient and the rate of recovery (as indicated by oxygen penetration) is reduced. Active remediation approaches that enhance oxygen penetration into sediments by physical disturbance may be useful in stabilising and standardising sediment condition. Biological enhancement, with the addition of bioturbators, was not a viable option where sediment conditions were markedly deteriorated/anoxic, although certain functional types did appear to improve oxygenation/nitrogen recycling under mildly enriched conditions.