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

Delivered November, 2006.

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Far-field and Ecosystem Interactions

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Background

Waste material from finfish aquaculture originate as uneaten feed, fish feces, and net-pen wastes, which are then dispersed by tidal and wind/wave transport.           

Potential environmental impacts may be categorized into three types of broad-scale changes distant from farm sites: sedimentation, eutrophication (excessive nutrient concentrations) and effects on the food web. (DFO, 2003).

Sedimentation and Resuspension

Observations made within the vicinity of finfish net-pens (nearfield) are typically associated with the deposition of solid-waste particles characterized by large and or dense particles.

The dispersal of solid waste under these circumstances is quantified using empirical hydrodynamic models which produce nearfield footprints of the deposition shadow below a farm system (Cromey et al. 2002; Stucchi et al. 2005). The widespread movement and distribution of fine particulates in the farfield environment is driven by both transport and dispersion processes.

Transport provides large-scale movement of water parcels linked to prevailing currents, while dispersion involves the blending and dilution of water parcels within close proximity of each other (DFO, 2004). Particle behaviour is also an important consideration for farfield transport models as passive transport or deviations from current directions will influence the distance traveled by waste material.

Coastal circulation and mixing models that provide temporal and spatial predictions of currents have been developed to aid in the prediction of farfield transport (Hargrave, 2002, Foreman et al. 2006).

Benthic resuspension and widespread lateral transport of waste material depends on tidal flow, wind and wave energy, patterns of turbulence, waste-particle aggregation, and seafloor morphology. Benthic transport models require a range of components including hydrodynamic transport and mixing processes, descriptions of deposition and resuspension criteria, multiple size classes of a variety of particle types, and variations in seafloor properties (DFO, 2004).

The critical thresholds for the settling and resuspension of both feed pellets and faecal matter are considered important inputs for benthic transport models.

Erosion and transport characteristics of a wide size-range of feed pellets have been examined in water-flow flumes and experiments regarding fecal transport are currently being designed (Sutherland et al. 2006). Flume-based research will also consider solubility or degradation factors of a variety of feed and faecal material based on different diet compositions and fish conversion rates under various tidal and storm simulations.
The use of laboratory and on-site submersible flume experiments can overcome the challenges faced with collecting spontaneous erosion measurements and fast-moving transport processes from near-seabed environments.

In summary, circulation and benthic transport models can aid in site selection and serve as both research and monitoring tools to increase our knowledge regarding the processes that lead to nearfield and farfield effects.
Tracking particles of aquaculture waste material in both the near- and farfield environment requires the identification of chemical tracers.

Specific indicators, such as the stable carbon isotope signature in feed pellets, have been picked up in a nearfield water column during a routine feeding event (Sutherland et al. 2001).

In addition, Sara et al. (2004) have shown that both farm-derived carbon and nitrogen sources can be isotopically detected through water column and sediment sampling within the vicinity of a net-pen system, suggesting their usefulness in a farfield setting if large differences between pen-derived and natural stable isotope signatures exist.

Zinc, an essential element in fish diets added to prevent cataract formation in juvenile fish (Richardson et al. 1986), is considered a chemical tracer of feed pellets. However, interactions between hydrography, particle aggregation properties, seafloor bathymetry, and post-depositional processes will influence the different patterns of dispersion of different pen-derived chemical markers.

As a result it is important to standardize background concentrations of various elements using sediment trace-metal ratios (zinc : lithium) before identifying zones of potential impact (Yeats et al. 2005). In addition, the ratio of copper and lithium in sediments have been shown to indicate a nearfield influence from copper-based antifoulings used on net-pen systems.

Eutrophication/Organic Enrichment

The term eutrophication is generally used to describe a multi-faceted alteration to the physical, chemical and biological environment associated with nutrient loadings and typically results in increases in biological oxygen demand (BOD) or low-oxygen conditions.

Eutrophication of coastal estuarine waters from finfish aquaculture occurs through direct inputs of waste material (feed and faecal matter) or through increased loadings of inorganic nutrients (e.g. nitrogen, phophorus).

Although the prediction and observation of far-field effects of inorganic nutrient inputs within coastal marine environments is very challenging (e.g. Broughton Archipelago), mass-balance nutrient budgets on an inlet-wide spatial scale have been carried out in the Southwest New Brunswick region (Strain and Hargrave, 2005).
Benthic organic enrichment has been examined along nearfield – farfield gradients extending along the seafloor as far as 1 to 5 km from netpen systems in the Pacific Region.

Correlations between sediment texture, inorganic nutrients (carbon, nitrogen, phosphorus), trace-element ratios (zinc : lithium), low-oxygen conditions (sulfide indicators), and faunal communities were observed. On one occasion the distribution of these benthic variables was linked to a depositional footprint of a farm site delineated from an acoustic survey of the seafloor.

Such technology produces pictures of the seafloor which outline changes in seabed topography and sediment characteristics. Acoustically-derived seafloor maps will help identify farfield depositional zones and steer sampling programs designed for farfield initiatives.

Data collected to date suggest that high-resolution pictures of the seafloor ground-truthed with measurements of sediment properties may provide insight regarding the relative importance of ongoing and widespread benthic transport processes versus episodic and focused transport which likely occurs during storm events.

Ecosystem interactions

The concept of ecosystem level interaction involves a large food web structure made up of plankton, fish, benthic and macroalgal communities existing in a broad-scale environment including subtidal and intertidal regimes.

The response of benthic organisms to organic enrichment and low-oxygen conditions has been observed mainly in the nearfield environment. Long-term monitoring research programs are required along a nearfield - farfield gradient to detect potential changes in species community composition at farfield distances. In conjunction with chemical tracers and organic enrichment indicators, chronological changes in community structure will be an important variable in the assessment of nearfield – farfield effects resulting from finfish aquaculture.
A DFO working group has been established to focus on ecosystem-based research and develop environmental research priorities related to aquaculture science.

References

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