Report: TR 2008/16
Author: John Zeldis (NIWA)

Abstract

Environment Waikato is currently scoping a possible plan change to allow for the diversification of aquaculture within existing aquaculture management areas (AMAs) in the Region. This plan change would allow for the cultivation of species other than mussels, including finfish. The most likely site initially for this would be at the Wilson Bay Marine Farm Zone (WBMFZ) in the Firth of Thames, which is currently consented for mussel aquaculture only (total area about 1200 ha). For eco-physiological and economic reasons, the yellowtail kingfish (Seriola lalandi lalandi) is the most likely candidate for the cultivation of finfish in the Firth of Thames and is the focus for this analysis. However, effects of finfish farming are likely to be similar for most of the species that could be farmed in the Firth, assuming similar farming intensities.

To assist Environment Waikato in considering the proposed plan change, this study compares new N additions from finfish farming with aquatic ecosystem processes of the Firth, riverine and oceanic additions, and losses through hydrographic export, denitrification (the microbially-mediated loss of N to the atmosphere) and mussel harvest. It combines information from a nutrient mass-balance budget for the Firth and estimates of Firth primary production (both obtained using field surveys made in the last decade funded by the Foundation for Research Science & Technology), with estimates of N discharged to the marine environment during fish feeding calculated using feed input, composition and feed conversion ratios (FCRs) provided by NIWA aquaculture specialists. It also compares N discharges from finfish farming with potential N removal caused by existing and future farmed mussel harvests at the WBMFZ. The purpose of the report is to provide perspectives on the relative magnitudes of ecosystem and farm processes under various intensities of finfish farm development, to inform Council decision-making about sustainability of finfish culture in the region. The primary focus of the study is at the Firth-wide scale, but makes inferences about impacts at the local AMA scale.

Key findings are:

1. 1. On average, riverine supply of inorganic and organic N to the Firth is greater than the supply arising from mixing across the boundary between the Firth and the Hauraki Gulf. During periods when ocean downwelling is dominant over the adjacent continental shelf, rivers contribute about 70% of the dissolved inorganic N (DIN) load, and when upwelling is active, 50% of the load arises from rivers.

1. 1. The Firth is a strong net sink for inorganic N, indicating that it denitrifies large amounts of nitrogen gas to the atmosphere on a net basis (about 10,800 t N y-1). DIN inputs to the Firth accounted for only about half of this. Particulate and dissolved organic nitrogen (PON, DON) made up the shortfall, originating from riverine (mainly) and oceanic sources. The mean Firth primary production value was about 28,000 t N y-1 incorporated into organic material.

1. 1. Nitrogen discharged to the marine environment from fish farming is estimated at 60 kg N per tonne of fish production, using a feed conversion ratio of 1.3 (FCR: defined as dry weight of feed added to harvested wet weight of fish) based on kingfish culture results from Australia and New Zealand. For FCR = 1.5, which is within the range of current practice for kingfish culture, about 75 kg N is discharged per tonne of fish produced. About 85% of this will be in dissolved forms (ammonium, urea, nitrate, the sum of which is called dissolved inorganic nitrogen DIN here), and the rest is in particulate form.

1. 1. To place the potential N discharged by fish farming into context, scenarios ranging from 1,000 to 10,000 tonnes of fish production  per year were evaluated at the two FCRs. At a production of 2,000 tonnes and FCR = 1.3, N discharged was estimated to be small relative to other Firth-wide N processes, sources and sinks: 0.4% of the Firth system N primary production, 1.1% of its denitrification rate, 1.1% of inputs of total N (inorganic plus organic) to the Firth from rivers and the ocean and 1.7% of the input of total N from rivers alone. In terms of loads of dissolved inorganic nitrogen (DIN), which is the most bio-available form of N for primary production, discharges from 2,000 tonnes per year fish production were estimated to be 2.7% of DIN inputs from rivers and the ocean, and 3.8% of the loading from rivers alone. These percentages increase by about 25% for FCR = 1.5. For the 10,000 tonnes per year (FCR = 1.3) scenario, N addition from fish farming is estimated at 5.7% of total N inputs to the Firth and 13.4% of DIN inputs, potentially significant relative to Firth-wide loading and other ecosystem processes. 

1. 1. The analyses of this report consider the sizes of fish farm discharges relative to Firth-wide ecological processes, sources and sinks involving N. It is certain that N discharged from fish farms, as proportions of areal primary production, denitrification, and loading from other sources (rivers and oceanic) will be much higher local to the WBMFZ than over the Firth-wide scale. As an example, in principle, the proportions could be 10-fold higher over an area 1/10th the Firth area, (i.e., 1100 km2/10 = 110 km2) surrounding the WBMFZ. The actual degree of this focussing of effects will depend on fish farming intensity and hydrodynamic dispersal of discharged N, and also on any functional effects that discharged N may have on the ecological rates themselves. 

1. 1. If, at the local AMA scale, such discharged N causes significantly increased organic supply (from new phytoplankton and from waste solids directly) sub-oxic conditions could form. This could threaten fish farming, as well as suppress nitrification, a key element of denitrification. On the other hand, if the scale of these loading effects is small relative to hydrodynamic dispersal, such feedback may not occur.
   2. Mussel harvesting removes N from the ecosystem, estimated at about 6 kg N per tonne of green weight mussel harvested. Discharge of N from fish farming is estimated at about 60 kg N per tonne of fish production (FCR = 1.3), such that the harvesting of 10 tonnes of mussels will remove the same amount of nitrogen as added by the growth of 1 tonne of fish. The 2006 Coromandel region annual mussel harvest (21,000 tonnes, 95% of which is in the budgeted Firth area) would remove slightly more N than that discharged by about 2000 tonnes fish production at FCR = 1.3.

1. 1. Because of the focussing of N discharge at the local scale (described above), only mussels growing within the perimeter of effects caused by that focussing will be relevant for remediation. If, for example, N removal by mussels harvested only at the WBMFZ are relevant in this sense (currently 14,000 tonnes) the equivalent N discharge arises from about 1300 tonnes fish production (FCR = 1.3).

1. 1. The uncertainties introduced by the focussing of effects (which are not resolved by the mass-balance approach used here) mean that the local-scale effects of discharged N need to be examined more closely, and are a strong reason to support better-resolved dynamic bio-physical modelling of the local area, including coupling with sedimentary and oxygen dynamics and effects of mussel harvest. Remediation by other forms of co-culture (e.g., algal, deposit feeders) should be also be considered.

1. It is recommended that defensible, locally applicable ‘limits of acceptable change’ are designated for adaptive management of WBMFZ fish farm development. This should be informed by the modelling and by meta-analyses of known fish farm effects from other studies.

Exploring the Carrying Capacity of the Firth of Thames for Finfish Farming: A Nutrient Mass-Balance Approach [PDF, 136 KB]