Towards sustainable aquaculture

Cultured kingfish cruise a tank at Bream Bay Aquaculture Park. (Photo: Alan Blacklock)

The microbial processes at work in the sediment beneath a fish cage during decomposition of organic material, such as fish faeces. (Graphic: Hilke Giles)

Four cores in the lab experiment. (Photo: Kay Vopel)

A close-up of the layer of fish faeces on one of the sediment cores. (Photo: Kay Vopel)

Hilke Giles measures oxygen in the cores during the laboratory experiment. (Photo: Kay Vopel)

In the field and in the laboratory, NIWA scientists are exploring ways to help New Zealand's aquaculture industry grow without damaging the environment. Hilke Giles, Kay Vopel, and Steve Pether have investigated how open-water finfish farming affects the chemistry of marine sediments.

Aquaculture is a rapidly growing industry in New Zealand with plans to raise annual production from today’s $300 million to $1 billion by 2025. New Zealand aquaculture is dominated by products with high quality but moderate value, such as GreenshellTM mussels, Chinook salmon, and Pacific oysters. Some of the industry’s planned growth can be met through improvements of these species; however, the greater opportunity lies in new higher-value species, such as kingfish (Seriola lalandi lalandi).

NIWA has made great progress in breeding kingfish. A remaining obstacle to large-scale commercial cultivation is the potential effect on the marine environment. One way of learning about these impacts is by simulating them in the laboratory.

Environmental effects of fish farming

The main concern about developing a new marine fish farm is usually how it may affect water quality. The key to understanding these effects lies with the processes that take place in the sediment.

Fish are typically farmed in coastal areas in cages near the water surface. Fish faeces and uneaten feed sink to the seafloor, where they enrich the sediment with organic material. Fish farms also have other effects on the ecosystem, but this enrichment is the most significant effect.

In the sediment, organic material is broken down by a complex network of microbial reduction–oxidation (or redox) reactions. This process provides food for microbes and therefore increases their activity. Microbes that live in the upper few millimetres of sediment use oxygen as a source of energy to break down organic material and thus create an oxygen demand in the sediment.

Below this upper sediment layer, organic matter is broken down by microbes that use other sources of energy, mainly nitrate, iron and manganese compounds or sulphate. This process is called anaerobic decomposition and produces reduced end-products. These end-products are mobile in the sediment. They diffuse within the water between the sediment grains (porewater) and are redistributed if the sediment is mixed, for example by burrowing animals or by resuspension ofthe sediment by strong currents. If these reduced end-products enter the upper sediment layer they are oxidised by oxygen and this also creates an oxygen demand in the sediment.

The large amount of organic matter deposited below fish cages changes the rates of almost all redox reactions. It also changes the flux of reaction products between the sediment and the overlying seawater. These fluxes can have important consequences for the seawater quality. For example, fish farming can result in the release of nutrients from the sediment that support the growth of algae in the water. It can also result in the release of toxic hydrogen sulphide. The effects of fish farming on ecosystem functioning thus depend on redox reactions in the sediment.

Simulating kingfish farming in the laboratory

To find out what happens when fish faeces are deposited on sediment, we set up an experiment in the laboratory. We submerged cores of coastal sediment in seawater tanks. To some cores we added faeces from kingfish living in tanks at NIWA’s Bream Bay Aquaculture Park, while some sediment cores were left untreated.

In the coastal environments typically used for aquaculture, tidal currents resuspend the sediment surface, lifting the top sediment layer so it mixes with seawater before settling down again when the currents weaken. We also wanted to investigate if this process modifies the effect of fish farming on the sediment. To simulate resuspension, we vigorously stirred the surface sediment of some sediment cores before adding faeces.

Our results

As we expected, adding fish faeces to the sediment increased the sediment oxygen demand significantly. We found that:

  • Oxygen in the sediment decreased more rapidly with depth in the cores where fish faeces were added, due to the high oxygen demand by microbes.
  • Oxygen demand was less when sediment was resuspended before fish faeces were added.
 

Implications for fish farming in New Zealand

Our experiment suggests that fish farms in areas where sediments are periodically resuspended by tides and currents may have less effect on the sedimentary environment than fish farms in calm areas. We’ll learn more about these effects with more detailed analyses of our results.

We’re on the brink of understanding the connections between fish farming, environmental parameters, and processes in the sediment and seawater, and soon we’ll be able to give better advice regarding the ecosystem effects of fish farming.

Further reading

Bruce, M. (2006). New aquaculture species: adding real value to New Zealand seafood. Water & Atmosphere 14(3): 10–11.

Bruce, M.; Forsyth, A.] (2007). NIWA: helping aquaculture reach the billion dollar aim. Seafood New Zealand 15(6): 11–15.

Nodder, S.; Zeldis, J.; Pilditch, C.; Law, C.; Currie, K.; Giles, H. (2005). Mud, glorious mud. Water & Atmosphere 13(3): 20–21.

Vopel, K.; Giles, H.; Zeldis, J. (2007). Colourful sediment. Water & Atmosphere 15(2): 22–23.

Teachers’ resource for NCEA Achievement Standards or Unit Standards:
Aquaculture Level 3 US16674, US19772, Level 4 US19773
Biology Level 2 US6309
Geography Level 1 AS90204
Education for SustainabilityLevel 2 AS90811
Science Level 1 AS90188, Level 2 US6352, Level 3 US21613, US6355

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