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Information on the spatial and temporal characteristics of ambient dissolved oxygen in the southwestern New Brunswick area of the Bay of Fundy is presented to help develop an understanding of dynamics of oxygen and salmon cage culture in the region. Some modelling efforts focussed on dissolved oxygen issues associated with fish farming in the area are also presented. A description and application of a simple oxygen depletion index is proposed to help identify the influence of salmon cage culture on the regional and farm-scale dissolved oxygen concentrations. The chapter concludes with a brief summary and discussion of observations and model development required to enhance the understanding of oxygen dynamics in the farms and bays of southwestern New Brunswick. The knowledge can be used by industry in their farm management practices and by environmental regulators in their efforts to define and sustain water quality standards.

Salmon aquaculture discharges organic wastes into the marine environment. Salmon metabolism and the waste discharges add nutrients and organic matter to and remove oxygen from both the water column and sediments. The salmon industry in Southwestern New Brunswick (SWNB) is used to illustrate how waste discharges can be estimated in the absence of detailed information on farm operations. A fish growth model and mass balance calculations are used to estimate carbon, nitrogen and phosphorus wastes at the farm scale. Feed nutrition data and environmental measurements are used to partition wastes into fractions, and to estimate the oxygen demand. The predicted demand at the time of maximum discharge is 200 times greater than the oxygen uptake measured in surface sediments at cages, suggesting that most farm wastes are dispersed over wide areas and do not accumulate directly under cages. The number of farmed fish in an inlet is then used to predict total discharges to that inlet. In SWNB, salmon aquaculture is the largest anthropogenic source of organic input to the coastal zone. The significance of the wastes on inlet scales (2–25 km) is evaluated by comparing element fluxes through salmon farms with fluxes due to natural processes: primary production, nutrient regeneration, and community respiration. In intensively farmed bays, fluxes due to salmon farms reach values of 20, 330 and 160% of those due to natural processes for oxygen, nitrogen and carbon, respectively: significant changes to the ecosystem have occurred in these bays. Spatial scales are critical in describing such impacts: effects will be greater close to the farms, and smaller when averaged over larger areas.

Seasonal measurements of plankton (phytoplankton and bacteria) biomass and abundance, primary production, and nutrient demand were conducted in the coastal waters of southwestern New Brunswick (SWNB) in 2000–2002 to investigate the far-field effects of finfish (salmon) aquaculture on the pelagic ecosystem. Plankton biomass and production varied seasonally with peak concentrations and activity in summer–fall and lows in winter. Nutrient demand followed a similar pattern with nitrogen (nitrate and ammonium) turnover times ranging from greater than a week in winter to less than a few days in summer. Ammonium concentrations were elevated at the aquaculture sites relative to control sites, however, effects on other nutrients, phytoplankton biomass, bacterial abundance, and primary production were not discernible despite the significant flux of nutrients into the system from finfish farming. Several lines of evidence point to the conclusion that primary production in SWNB is under light rather than nutrient control and that phytoplankton there have limited capacity to process additional nutrients produced as aquaculture in the region expands. The ratio of bacterial abundance to phytoplankton biomass (B/P ratio) is proposed as an easily measured water-quality indicator for assessing the trophic balance (autotrophy vs. heterotrophy) of the pelagic ecosystem in coastal waters.

Seasonal transitions from oxygen production to oxygen demand at coastal aquaculture sites in southwestern New Brunswick (SWNB) can be defined in terms of the production-respiration (P/R) ratio. During the summer, when P/R is greater than 1, an autotrophic ecosystem is in place, and dissolved oxygen (DO) in surface waters remains above thresholds for optimal fish growth. During the fall and winter, when P/R is less than 1, a heterotrophic system is in effect, and DO can decrease to below threshold. Photochemical decomposition may act as a seasonal link, contributing to the onset of net oxygen demand by facilitating the breakdown of terrestrial and marine organic carbon in the fall. The overall carbon load is not a useful index of the bioreactive material that creates oxygen demand. Instead, bacterial number and chlorophyll concentration (expressed in terms of a bacteria:chlorophyll ratio) may be better indicators of the seasonal regulation of oxygen dynamics by the ecosystem. Low DO is the cumulative effect of sustained oxygen demand; a seasonal change in P/R from greater to less than 1 is an early warning of this demand. The application of both indices (P/R and DO) in tandem can be used to develop ecosystem-sensitive plans for the management of water quality at aquaculture sites.

Benthic organic matter (OM) enrichment is a frequent environmental effect of coastal aquaculture. There is a need for simple, general methods of predicting and monitoring such effects. One approach uses a geochemical relationship between total sulfides and redox potential to define organic enrichment. This empirical relationship developed for shallow, macrotidal aquaculture sites in Southwest New Brunswick (SWNB), Canada, is used to determine organic matter (OM) loading and to monitor shifts in benthic quality. A similar relationship is seen for salmonid farm sites in coastal Newfoundland but not for sediments under mussel farms or at nearby reference sites. Overall, redox potentials and total sulfides are not correlated and 70% of observations fall below the relationship documented for SWNB. Sediments in Newfoundland coastal waters are often rich in OM, [1.5 to >30%, median 8% loss on ignition (LOI)]. In addition, they are seasonally (<0°C) or always cold (−1.8 to <5°C). Much of the coast is exposed to high-energy conditions often with an effective fetch exceeding 700 km. Inner basins of some bays and fjords are protected from the waves associated with such exposure and may naturally experience seasonal anoxia that will significantly influence sediment-water exchange processes. All these factors will moderate sedimentation, remobilization and eventual remineralization of organic matter. The application of simple geochemical indices of organic enrichment as monitoring and assessment tools must be tempered with understanding of the environmental processes that regulate these factors under diverse geomorphological conditions.

One unifying principle proposed for the environmental influence of aquaculture is that when flushing is poor (>2 d.), the maximal biomass produced in an area will be constrained by accumulation of waste products in the water column. In the Bay d'Espoir estuarine fjord on the south coast of Newfoundland, under-ice salmonid culture in cages in protected bays with low flushing rates (5 to 20 d.) is a challenging component of the annual production cycle. However, in two years of environmental monitoring of such protected bays, no significant change to water quality was observed. A measurable influence on the benthos was more frequently detected, but localized. Thus the inconsistency of Bay d'Espoir; it has a low flushing rate, yet there was no observable change to the water column. Possible reasons for this are discussed, and include: the sheer amount of water (i.e., potential for within-basin mixing/dilution and biodegradation) in this estuarine fjord; increased surface transport of nutrients; the benefit of fallowing; and, diminished relative loadings to the water column and benthos in winter conditions for an industry in its early stages of development. Further refinement of assimilative capacity estimates for this and other similar suboptimal areas will have to resolve this apparent contradiction prior to espousing “unifying principles”.

The main components of lagrangian (particle tracking) models used in the assessment of aquaculture impact have been reviewed. Trends in input data requirements have also been examined, with some emphasis on general modelling issues such as standardisation of data and scenario complexity. Specific data aspects were tested in a flux and benthic effects (impact) model and their importance assessed. Dispersion coefficients resolved from drifting buoy data were found to influence model predictions widely depending on the criteria used in the analysis. Different lengths of hydrographic data sub-sampled from a 206 day record measured at a Scottish fish farm also resulted in different model predictions. This highlighted the importance of taking representative hydrographic measurements for regulatory modelling of maximum farm biomass. Both cage movement and the timing of feeding and defecation events were also tested using observed data and found to be of less importance.

An analytical near-field depositional model for solids wastes (organic matter from waste feed and faeces) from open net pen culture of finfish is presented. The model is based on the premise that the statistics of the depth-averaged currents, which are assumed to be normally distributed, determine the distribution of wastes on the ocean bottom. Using a farm configuration consisting of a single net pen, the model is used in a diagnostic mode to quantitatively examine the effects on the waste depositional field or footprint of the farm that result from changing the depth under the net pens and changing the statistics (standard deviations) of the depth-averaged velocity. The model is also used to examine the changes to the farm footprint that result from orientating a two by four linear grouping of net pens perpendicular to and parallel to the principle current direction. The model was tested on an operating Atlantic salmon farm by comparing the predicted organic matter fluxes from the farm with the vertical fluxes of organic matter measured by sediment traps. Based on a rather limited data set, predicted organic matter fluxes were found to be about four to five times higher than observed sedimentation rates. Further, the model predictions were sensitive to the value used for feed waste. The limitations and uncertainties in the model assumptions, parameterizations and in the methodologies used to validate the model are discussed and recommendations for future research are provided.

Organic enrichment of sediments underlying fish farms in temperate and tropical coastal zones is reviewed to identify similarities and important biogeochemical differences. Improvements in technology have allowed farms to move from depositional sites to more erosional offshore locations. However, low cost farms are still being located in sheltered areas, in particular in the tropics. Important differences in the response of sediment geochemical variables to organic enrichment are associated with finfish aquaculture located under highly diverse hydrographic and sedimentological conditions in different coastal areas. In temperate latitudes where farms are often located over soft bottom, organic enrichment increases sediment microbial activity and may alter benthic community structure. Enhanced anaerobic activity may lead to accumulation of sulfides with adverse effects on aerobic bacteria, plants and fauna due to progressive oxygen depletion. In warm temperate waters, such as the Mediterranean and tropical latitudes, many farms are located in more advective areas with coarse-grained carbonate-rich sediments. Effects of organic enrichment in these areas are less well described, but studies have also shown sulfide accumulation in sediments indicative of deteriorated benthic habitats.

The results of metal analyses carried out on surficial sediment samples collected in the coastal waters of southwest New Brunswick and the Broughton Archipelago in British Columbia have been used to investigate the use of heavy metals as tracers of salmon farm wastes. New Brunswick in particular has seen rapid expansion of open cage salmon aquaculture in recent years. While techniques have been developed to identify benthic impacts directly beneath the cages, no method has been developed to determine the fate of dispersed wastes. We show that Zn and Cu, two elements associated with aquaculture operations, can be used to identify farm wastes in sediments at some distance from the cage sites. Geochemical normalization for grain size is needed in order to see the small tracer signals. Excess Zn and Cu levels are found in the sediment at varying distances from the salmon cages in depositional areas in southwest New Brunswick and in the Broughton Archipelago. Evidence that links these observations to salmon aquaculture development is described.