Assimilation capacity of organic matter from salmon aquaculture (ACOM): Improving model predictions of benthic impacts

Description

The spatial scale, magnitude, and persistence of effects to the benthos, or seabed, from biochemical oxygen demanding (BOD) matter are influenced by a range of factors that control the deposition, recycling, and transport of aquaculture wastes, or effluent. Previous research had shown that there is a relationship between aquaculture waste from fish net-pens and reduced oxygen levels in the sediments. Because different species require different amounts of oxygen to survive, being able to assess oxygen levels within the sediment can provide important clues to potential impacts of aquaculture wastes on the benthic habitat. However, this cause-effect relationship neglects to consider critical post-deposition transport and recycling processes that influence benthic impacts. An important, but poorly understood factor is the inherent "assimilative capacity" of different benthic habitats to mineralize (recycle) this material without altering the natural oxic state of the sediments. This multidisciplinary research project aimed to increase scientific expertise and knowledge of the major processes that determine seabed assimilative capacity, including waste deposition and dispersion rates, the oxygen supply to the benthos, and the decay rate of organic matter under a range of environmental settings and seabed types (mud, sand, mixed substrate). The project worked to develop a sub-module that will work with the FVCOM water circulation model to allow researchers to incorporate both location-specific data on water currents as well as sampling data in order to gain a better understanding of how the waste is being deposited or dispersed within a given area. This sub-module allows for greater accuracy in predicting the spatial scale and the magnitude of benthic impacts resulting from BOD matter effluents from open-water finfish farms.

Findings

Median grain size, a basic seabed characteristic, affects the capacity of salmon farm sites to naturally assimilate excess organic matter and limit the zone of impact. Waste recycling or decomposition at salmon farms was indicated by remote sensor measurements of sediment oxygen consumption. Waste recycling was rapid under the typically hypoxic conditions that occur in cohesive silt (mud) substrates but was only detected under oxic conditions in permeable sand substrates. With increased organic matter enrichment, the development of hypoxia in sand substrates limited the capacity of the microbial community to assimilate organic wastes. Erodibility values for the waste material suggested that resuspension and transportation from both sediment types occurs. Results on the degradation and movement of farm wastes inform the continued improvement of model predictions of aquaculture environment interactions.

Publications

Law, B.A., Hill, P.S., Milligan, T.G., Zions, V.S., 2016. Erodibility of aquaculture waste from different bottom substrates. Aquaculture Environment Interactions, 8, 575-584. DOI:10.3354/aei00199.

Program Name

Program for Aquaculture Regulatory Research (PARR)

Year(s)

2014 to 2017

Principal Investigator(s)

Peter Cranford
Research Scientist, Bedford Institute of Oceanography
1 Challenger Drive, Dartmouth, Nova Scotia
Email: Peter.Cranford@dfo-mpo.gc.ca

Team Member(s)

Brent Law

Fred Page

Terri Sutherland

Shawn Robinson

Herb Vandermeulen

Susan Haigh

Collaborative Partner(s)

Raymond Bannister, Institute of Marine Research, Norway