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Wednesday, November 2, 2016

Making salmon farming more sustainable

Irrigation farmers have found aquaculture is both profitable and a well-suited addition to crop production.
Murray Cod - Fishes of Australia
Murray Cod - Fishes of Australia

Water to be used for irrigation is first used in fish-growing ponds. Dissolved oxygen and nutrient levels are monitored and at regular intervals the water enriched with fish waste is pumped onto cropland where it provides not only water but fertiliser too. Fresh irrigation water is used to top up the fish-growing ponds, replacing the water that is pumped periodically from them.

In Tasmania, intensive aquaculture has been relying on the ecosystem to process fish waste. In upper layers of water in which sunlight penetrates, the addition of fertiliser promotes the growth of phytoplankton. This has proven to be sufficient so that dissolved oxygen levels in the water remain suitable for fish farming.

A problem that has been found is that at some locations or during extended periods of calm seas, fish waste can accumulate in deep water layers where bacterial action dramatically reduces dissolved oxygen levels. Later just one severe storm can then create an upwelling from this deep water layer with catastrophic effects - suffocating the stock in fish growing pens.

Solving this problem may be profitable for the industry. It is also a better option than leaving this potential for occasional but large financial loss to chance.

The fish waste descending from the fish growing pens should be removed at the same rate it is created. A floating containment and low-cost pumping mechanism (see below) could be designed to do this. This will get rid of the possibility that an oxygen-depleted water layer can develop in a basin below and near the growing pens.

The removed fish waste should support a commercially viable companion industry. For instance, the fertilser may be useful in another aquaculture industry - one producing fish feed via the growth of phytoplankton and algae for marine species that can be harvested for production of fish feed.

Update 25 June 2017 - more sustainable salmon farming

"A trial by Australia's biggest salmon company to collect fish waste under its pens in Macquarie Harbour on Tasmania's west coast looks on track to meet with official approval."

Update 4 July 2017 - Tassal's salmon farming waste disposal system

Tassal's salmon farming waste disposal system
It is hoped the collection of fish faeces will stop degradation of the seabed.

Fishy Farms - Landline - ABC

PIP COURTNEY, PRESENTER: Tasmania's salmon and trout industry has a farmgate value of $500 million and employs 1,500 people. The industry is expected to double by 2030, but not everyone is happy. Conservationists and some rural residents are questioning aquaculture's rapid expansion and its effect on the state's waterways. Fiona Breen with this report.

FIONA BREEN, REPORTER: During a big storm here on Macquarie Harbour huge amounts of seawater flushed into the harbour, pushing toxic water near the sea floor up to the surface. 270,000 of Petuna's fish suffocated.
The company had started expanding three years ago, but stopped after in-house monitoring of the waterway at the edge of Tasmania's famous World Heritage-listed forests revealed serious problems.
Organic loads in the water were up and dissolved oxygen was down. The storm earlier this year killed hundreds of tonnes of salmon and trout here on Macquarie Harbour.

An Open Ocean Trial of Controlled Upwelling Using Wave Pump Technology

Although wave pumps have only been successfully deployed as a small-scale means to generate power (Isaacs et al. 1976), a variety of other applications have recently been proposed, including increasing primary productivity and fish production (Kenyon 2007; Kirke 2003), fueling aquaculture (Liu and Jin 1995)...

Each of these proposals requires pumps that would remain operational in the open ocean long enough to generate and sustain phytoplankton blooms. The minimum operational time scale needed will depend on the pump efficiency, the number of pumps deployed, and the physical and chemical characteristics of the target region. Given our present understanding of phytoplankton bloom dynamics, this time scale is likely be on the order of weeks to months. In addition, these proposals assume a predictable biological response of the upper ocean to deep water additions.

Angelicque White, Karin Björkman, Eric Grabowski, Ricardo Letelier, Steve Poulos, Blake Watkins, and David Karl, 2010: An Open Ocean Trial of Controlled Upwelling Using Wave Pump Technology. Journal of Atmospheric and Oceanic Technology, vol. 27, issue 2, p. 385-396, doi: 10.1175/2009JTECHO679.1.