Presentation: State-of-Knowledge Initiative for the Special Committee on Sustainable Aquaculture of the British Columbia Legislature

Delivered November, 2006. (Updated with Volume 5, January 2009)

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Near-field Interactions and DEPOMOD

[A full pdf copy for printing may be downloaded from the WAVES database.]

Background

The environmental impacts resulting from the culture of salmon in open net cages in the marine environment occur over a range of space and time scales, and include alterations to ecosystems caused by the enhanced release of nutrients and carbon, discharge of contaminants, theraputants and cross-transmission of pathogens and parasites (Pearson and Black 2001).

One of the most conspicuous and often studied impacts of open net cage fish farms is on the benthic faunal community underneath and in the immediate vicinity (~ 200m) of net cages— the nearfield environment—caused by the enhanced sedimentation of organic-rich solid wastes from the farm (DFO 2004).

Wastes in the nearfield area of deposition, or the ‘footprint of the farm’, result from quickly settling solid wastes primarily composed of faecal material and uneaten food pellets or waste feed.

The very fine and slowly settling portion of the solid waste is transported and dispersed much further away from the farm site—the farfield— and may accumulate in depositional zones. The fate and the proportion of solid wastes transported to the farfield are largely unknown.

The effects of organic enrichment on sediment chemistry and the benthic community have been well documented by many researchers.

Significance

In natural or unperturbed coastal sediments, the oxygenated sediment – water interface is a habitat that contains a variety of aerobic  organisms (i.e. animals that require oxygen).

Enhanced deposition of organic matter will result in an increased oxygen demand by these aerobic organisms and may lead to hypoxic (low oxygen) or anoxic (no oxygen) conditions at the sediment-water interface (DFO 2004).

In the absence of dissolved oxygen, most aerobic organisms occupying the thin sediment-water interface will die, and methane and free sulphides will accumulate in the sediments as organic matter is decomposed by anaerobic organisms.

The degree of impact to the benthic community and habitat depends upon a combination of factors, such as the rate of the organic waste input, bathymetry, circulation and substrate characteristics.
Mathematical or numerical models describing the distribution and impact of solid wastes from marine salmon farms can be useful tools for the management, monitoring and study of the aquaculture industry and its impacts.

Many models of the nearfield distribution of organic wastes and their ecological effects have been developed and continue to be developed. Most recently, the Scottish authorities have developed and commercialized a comprehensive aquaculture waste modelling suite called DEPOMOD (Cromey et al 2002a).

It was designed to be used by industry, consultants and the regulatory agencies as an objective aid or tool to assist in assessing the potential impact of aquaculture operations on the nearfield benthic environment.
DEPOMOD is notable in that several of the modules have been validated to some extent with field observations (Cromey et al 2002a, 2002b, Chamberlain et al 2005).

The overall modelling approach consists of a sequence or progression of sub-models or modules each addressing the important processes and components of the problem. The starting point for all models is the fish production/waste module.

This initial module uses information about the farm configuration (cage dimensions and layout) and fish production (feed input, biomass, size), and then transforms the feed input to a solid waste output from the net pens. The next modelling step in the sequence is the hydrodynamic and waste settling model, or particle tracking model, which distributes the wastes onto the ocean bottom using the sinking rate characteristics for the wastes, together with information about the ocean currents.

The sedimentation rate is a key determinant of benthic impact. Once wastes settle onto the bottom, the currents, if sufficiently strong, may resuspend and redistribute wastes. The final component in this progression of models is the benthic module. In this module, measures of benthic impact such as free sulphide concentration in the sediments or indices of benthic diversity are compared to model-derived sedimentation rates in order to establish semi-empirical relationships.

If significant relationships can be demonstrated, then the model predictions of sedimentation rates may be used to predict the degree and spatial extent of benthic impact at other locations having similar substrates and oceanographic conditions.

Since 2004, DFO has been working with the industry and provincial agencies to test and validate DEPOMOD for use with British Columbia finfish farms.

Using an extensive set of field observations collected by Brooks (2001) at several B.C. finfish farms (The Focus Study), and detailed production and configuration information Chamberlain et al (2005) applied and tested DEPOMOD at one of the farm sites of the Focus Study.

Initial results of the application of DEPOMOD demonstrated significant relationships between predicted sedimentation rates (carbon flux) and several measures of benthic impact, namely sediment sulphide concentration, species diversity, infaunal trophic index (ITI) and faunal abundance. The sediment chemistry and biology showed a clear effect from the deposition of wastes from the finfish farm.

While the initial results of the study were encouraging a clear understanding and examination of the model limitations and uncertainty are fundamental for its evaluation and application. Inherent in the model are the inaccuracies that result from the simplification of the key processes that the model attempts to simulate (e.g. hydrodynamics, particle deposition, etc.).

In addition, there is uncertainty regarding a number of key parameters settings and the consequent effect these have on the model outputs. Finally, errors in the input data (current measurements, feeding rates, etc.) will also lead to inaccuracies in the model outputs.

The study of Chamberlain et al (2005) explored the effects of uncertainty in several parameter settings used in the model and identified limitations of the model and made recommendations for further research and testing of the model. One of the key parameter settings in the model is the proportion of feed that is uneaten or wasted.

There is some debate about the setting of this value as the industry believes the feed wastage rate to be <5% while Strain and Hargrave (2005) report that the rate is closer to 15%.

At present, DFO in collaboration with B.C. Ministry of Agriculture and Lands (MAL) and the BC Salmon Farmers Association is conducting a study to determine feed wastage rate in order to better constrain the large range of uncertainty in model outputs produced by this parameter setting.

Another aspect that warrants further research is the resuspension processes within the model.

Testing of DEPOMOD continues at several B.C. finfish farm sites having a range of environmental conditions. Data collected are part of the B.C. government’s compliance monitoring program and observation collected by the industry as required under the province’s Finfish Aquaculture Waste Control Regulation.

DEPOMOD is currently used by DFO, B.C. MAL and by the industry. A Methods and Setting document has been written (Stucchi and Chamberlain 2005) and adopted for the application of DEPOMOD for B.C. finfish farms.

DEPOMOD simulations produced by the industry or MAL and quality controlled by DFO scientists are used for farm siting and farm configuration.

In DFO, DEPOMOD is used as tool within a risk based management framework for the finfish industry in B.C. (DFO 1998). DFO uses the DEPOMOD predicted footprint in conjunction with the results from the physical and biological assessments to assist the industry and/or DFO to site farms away from sensitive habitats.

In those cases where the predicted waste deposition rate exceeds a regulatory threshold (5 g C/m2/day) a Fisheries Act Sec 35(2) Authorization to harmfully alter, disrupt or destroy fish habitat (HADD) is issued. Compensatory habitat is a requirement of the Authorization and is based on the predicted footprint.

This is to ensure that DFO's Policy for the Management of Fish Habitat guiding principle of ‘No Net Loss’ of productive capacity of habitats is achieved. (DFO 1986).

References

Brooks, K.M. 2001. An evaluation of the relationship between salmon farm biomass, organic inputs to the
    sediments, physicochemical changes associated with those inputs and the infaunal response – with
    emphasis on total sediment sulfides, total volatile solids, and oxidation-reduction potential as surrogate
    endpoints for biological monitoring. 172 pp.

Chamberlain, J., Stucchi, D., Lu, L. and C. Levings. 2005. The suitability of DEPOMOD for use in the
    management of finfish aquaculture sites, with particular reference to Pacific Region. DFO Canadian Science
    Advisory Secretariat Research Document 2005/035. 53p.
    http://www.dfo-mpo.gc.ca/csas/Csas/Publications/ResDocs-DocRech/2005/2005_035_e.htm

Cromey, C. J., T. D. Nickell and K. D. Black. 2002a. DEPOMOD- Modelling the deposition and biological effects
    of waste solids from marine cage farms. Aquaculture. 214, 211-239.

Cromey, C. J., T. D. Nickell, K. D. Black, P. G. Provost and C. R. Griffiths. 2002b. Validation of Fish Farm Waste
    Resuspension Model by Use of a Particulate Tracer Discharge from a Point Source in a Coastal Environment.
    Estuaries Vol. 25, No. 5, 916-929.

Fisheries and Oceans Canada, 1986. Policy for the Management of Fish Habitat. Ottawa: Fish Habitat
    Management Branch.

Fisheries and Oceans Canada, 1998. Decision Framework for the Determination and Authorization of Harmful
    Alteration, Disruption or Destruction of Fish Habitat. Ottawa: Habitat Management Branch.

Fisheries and Oceans Canada, 2004. A Scientific review of the potential environmental effects of aquaculture
    in aquatic ecosystems. Volum III. Can. Tech. Rep. Fish. Aquat. Sci. 2450: :ix + 117 p.

Pearson T.H. and Black K.D. 2001. The environmental impacts of marine fish cage culture. In: Black KD (ed)
    Environmental impacts of aquaculture. CRD press, Boca Raton, Florida, p 213

Strain, P.M. and B.T. Hargrave. 2005. Salmon aquaculture, nutrient fluxes and ecosystem processes in
    southwestern New Brunswick. Chapter 2 In: The Handbook of Environmental Chemistry. Environmental
    Effects of Marine Finfish Aquaculture. Volume 5: Water Pollution. Springer, Berlin Heidelberg New York

Stucchi, D.J., and J. Chamberlain. 2005. DEPOMOD Canada Methods and Settings V2.0. Fisheries and Oceans
    Canada, Pacific Region. (unpublished document)
    http://www-heb.pac.dfo-mpo.gc.ca/publications/pdf/finfish_mfeap.pdf