Cleaning Up Our Seafood

The guys entered the water in their wetsuits and waded out with the long net, slowly encircling an area of the pond and then drawing the circle smaller. The driver of the digger parked on the bank lowered a large scoop net attached to its front arm into the now frothing water. When he raised it again it was full of flapping silver fish called ‘cobia’, each as long as my arm.

One of the guys loosened a drawstring to empty them all onto a metal table on the bank. Those below a certain size were thrown back in the water and those large enough were quickly and methodically dispatched with a metal spike through their brain then tossed into large tubs full of ice on the back of a truck.

The men were a coarse lot, laughing and swearing a lot while they worked, but they seemed like nice people. For a while they gave me the job of repeatedly thumping a metal rod on a wooden jetty that jutted out over the pond, fooling the fish into thinking it was feeding time. The fish created bow waves as they came charging across the huge pond toward us. The guys circled around again with the net.

This time when the fish were dumped onto the table I picked up a smaller one by its tail, trying not to let go as I wrestled it back into the water. I was holding one of the most sustainable fish harvested commercially in Australia, though I was still in the process of learning why, and I found myself wondering what it would taste like.

With our oceans being so overfished, fish farming seemed to make a lot of sense to me. But when cycling past the many floating salmon farms in Tasmania at the start of this journey I heard about the environmental problems with their effluent polluting the waterways. It made me wonder, is there such a thing as truly sustainable fish farming? When I’d asked this to an aquaculture expert, he had pointed to this place in far North Queensland and my curiosity had been burning the whole way up.

From the town of Ayr I’d cycled the 20km out to Pacific Reef Fisheries and left my bike on the verandah of the simple wooden office building. I was soon climbing into a ute with the environment manager, Christian, and we drove into the fishery: a wide expanse of open land filled with a grid pattern of huge square ponds bordered by dirt causeways. Floating devices on each bond were spraying up water to aerate it.

This used to be a small family-run prawn farm with just a couple of ponds, Christian told me, until Pacific Reef Fisheries took it over in 1998. There was a growing market for prawns and the farm was expanded over the next several years, first to 75ha, then to 98ha. The cobia have been a recent addition and make up about 10% of what they do.

Having a larger farm provides economies of scale, he said, but the input costs get large too, particularly water, power and feed. This drives the move towards sustainability, to reduce costs.

It’s the clever way they deal with their wastewater that they’re particularly acclaimed for. He pointed towards the west, explaining that there was an estuary beyond the distant trees there where they drew all their intake water from, and into which their discharge water eventually returns.

‘Our discharge water is cleaner than our intake water’, he told me, giving me a moment to digest this incredible feat. I was eager to find out how they did it, but it seemed that would have to wait.

There are two sides to the daily operation of the fishery, he explained as we pulled up outside a large shed: feeding and water quality. We walked inside and into one of the rooms: a laboratory with a benchtop and basin along one wall, shelves full of vials and a girl and guy sitting looking into microscopes.

Throughout the rest of the day I would often be seeing these two driving amongst the ponds in a small buggy collecting samples from all the water bodies. These were brought back for testing, looking at various water parameters such as dissolved oxygen, mineral content and algae species.

Here began my education in algae, and it was complicated. They aim to get a bloom of micro-algae going in the ponds when they first stock them with prawn post-larvae from the hatchery. The algae provides shade and feeds zooplankton, which the baby prawns feed on (they’ll later be weaned onto artificial feeds). A challenge is that their intake water from the nearby estuary can contain problematic algae species, including one called ‘heterosigma’, which produces toxins harmful to prawns when stressed. A chart on the wall of the laboratory detailed the attributes of this nasty species, with instructions on what to do if it’s detected in a pond: ‘flow the pond and turn all aeration on’, to hopefully flush it out of the system. Fortunately the micro-algae they encourage to grow inhibits this harmful species.

Monitoring the pond is apparently a 24/7 operation, with staff checking the aeration levels throughout the night. At one time a pond could have 8 tonnes of prawns in it, and if things go wrong with the aeration devices it might be only a matter of hours before they all perish.

Yet the micro-algae in the ponds produces oxygen as it photosynthesises during the day. This has allowed one of the fishery’s major sustainability improvements: switching off the aerators during this time and greatly reducing their power consumption. They’re also moving towards remote sensing of water quality and oxygen levels to optimise the use of the aerators.

Christian and I returned to the ute and drove out amongst the ponds. Up close the water was a murky green colour, which I was told is how it should be due to the micro-algae growing in it. I couldn’t see any prawns, which again Christian said was a good thing. Happy prawns stay hidden in the depths of the pond. Only when they’re stressed do they move to the sides where you can see them. With the cobia it’s the opposite, he said; when they’re stressed you can’t see them.

I’d never been a huge fan of the taste of prawns, which made me the odd one out at Christmas time with the family, yet I was fascinated to hear how these prawns were farmed specifically for Christmas and Easter. From the end of May until mid-July, all the ponds will be empty. They’ll then fill the ponds with water from the nearby estuary and start stocking them with prawn post-larvae from a hatchery down near Guthalungra. By Christmas they’ll be about 150 days old and around 20g each, ready to harvested for the seasonal feasts. The Easter prawns are started in September so that come time for harvesting they’re 180-190 days old, over 30g each and able to be sold at a premium. The ponds are then emptied again and cleaned.

We pulled up between two ponds next to short jetties. Walking out to the end of one, Christian reeled in a rope tied to a wooden post, lifting up a flat basket attached its end. Inside were lots of small brown granules, as well as several baby prawns hopping about trying to escape.

These small nets were the ‘feed trays’ and the brown granules the artificial prawn feed. The prawns were fed manually by workers dropping feed into the ponds each day, but a small sample was placed in these baskets and checked later to see if they were feeding properly.

Here was another one of their sustainability improvements, Christian explained. Their feed used to have a much higher fish content, as well as lots of bycatch (other marine species caught unintentionally by the fishing industry). Now they source their feed from a sustainable feed company that ensures there are no protected species amongst the bycatch and with more soy protein substituted in.

Weaving between the ponds, we reached the fish pond where the cobia were farmed. They had experimented with farming other fish species, Christian said, such as coral trout, barramundi and Queensland groper, and eventually found that cobia suited them best. They weren’t as profitable as the prawns and so only this one pond was allocated to them.

We drove over to one corner where there was a flat-bed truck, a digger, a metal table and a crew of men in wetsuits hauling in cobia for the weekly fish harvest. Christian drove away leaving me with the fish pond manager, Mitchell. A gentle and courteous guy about my age, his manner contrasted with the rougher blokes he was managing. He explained what they were doing and gave me a pair of gloves so that I could join in.

After a couple of hours we had reached their quota: 215 cobia caught in the days haul. The undersized ones thrown back would now grow faster, having less competition for feed and less stress from crowding.

I joined one of the workers, Jason, in the truck driving the tubs full of harvested cobia to the packing shed. Here the tubs were lifted down with a forklift and would apparently be left until the fish had cooled to 5 degrees Celcius. They would then be taken into the sterilised packing room to be boxed up ready for trucking to market.

Returning to the lab, I found Christian in his office next door. ‘How’d you go?’, he asked. I’d wrestled with a few fish and the harvest crew had looked after me well. So far I’d gained a general idea of how the prawns and fish were farmed and I understood two major sustainability improvements, but what about the third and biggest of all? How did they deal with their wastewater?

Driving across the fishery again we pulled up next to two plastic-lined canals, each in the shape of an elliptical racetrack about 50m long. At one point in each canal was a large paddle wheel slowly turning and keeping the dark green water moving. We walked out onto a metal grate that bridged a canal and Christian dipped his hand in and scooped out a handful of green seaweed. It’s a macro-algae called ‘ulva’ or ‘sea lettuce’, he said. It did look leafy and lettuce-like.

This was only the trial system. We drove further away from the main fishery to a neighbouring area of land with a fence around it. A sign asked us to go slowly so as not to kick up dust on the dirt roads. The reason for this soon became clear as we parked the ute and entered the compound through a gate.

In front of us were several massive versions of the elliptical racetrack canals, lined up in a row and each extending to a distant set of buildings. This was Pacific Bio, a subsidiary of Pacific Reef Fisheries, where they grow algae in the wastewater from the fishery and use it to produce a number of useful byproducts.

About a dozen years ago Pacific Reef Fisheries partnered with James Cook University, who set up the smaller trial algae ponds to test the concept. The trial ran for eight years, after which they expanded to this much larger commercial facility.

A week later I would visit James Cook University, cycling into the campus in Townsville and up to the raised wooden buildings of the Marine Sciences department, to meet with the enthusiastic and bright-eyed Professor Rocky De Nys, who explained how the algae system worked.

‘All our work started when there was a carbon price in Australia’, he said, taking me into their laboratory full of big tubs of algae-filled water. They found that growing seaweed was a good way to capture carbon, but then the carbon price ended. Fortunately they found it was also a good way to capture nitrogen, which was valued by the aquaculture industry because their discharge into the ocean is so heavily regulated.

With municipal wastewater, the best practice levels of nitrogen are 5mg/L. When it comes to aquaculture, they’re required to get down to 3mg/L, while best practice is 1mg/L. Algae is the only way to remove these last small amounts of nitrogen.

To begin with, they took a sample of Townsville wastewater containing many different species of algae and cultivated it, letting it all grow and grow. Soon there was only one species left: the ulva, which had outcompeted the rest. It’s continually growing, enabling them to harvest about a quarter per day as it quickly replenishes itself.

Professor De Nys showed me the screen they ran the water over to harvest the algae and the modified washing machines that worked like centrifuges to remove the water. At Pacific Bio they use a giant screw press, he told me, but here our university budgets are tight. The seaweed was then dried and pelletised.

He led me outside where there were more large tubs full of murky green water — ‘You’d think it would grow best in the lab, but of course everything grows best outside where there’s more sun.’ — and showed me a box of dried seaweed pellets.

At Pacific Bio they separate out a product they call ‘PlantJuice’, he said, which contains nitrogen, phosphorus and plant hormones that encourage root development. They then sell this in large vats to the nearby cane farms where it’s sprayed over the crops along with fertiliser.

It’s an incredible system when you consider all the pieces. The aquaculture discharge water gets cleaned, reducing nitrogen being discharged onto the nearby Great Barrier Reef. The cane farmers want nitrogen for their crops and they get this from the algae pellets and PlantJuice, meaning less fertiliser is used. The carbon captured by the algae is sequestered into the crop soils, improving soil carbon, and the plant hormones from the algae boost crop growth. It’s a beautiful circular system and it’s profitable, Professor De Nys said. ‘Cane farmers can’t get enough of it.’

Back at Pacific Reef Fisheries, Christian explained the journey that the fishery wastewater goes on: first into long settlement ponds, then through a sand filter before being pumped into the ‘high rate algae ponds’ or HRAP. After the algae cleans the water it’s discharged into the mangrove system, a network of artificially created mangrove channels surrounding the fishery. The mangroves uptake anything the algae doesn’t, while also providing habitat for wildlife.

Their discharge water comes out so clean that they’re planning to make the system closed-loop, returning it to be re-circulated as the intake water. This would also make the fishery more bio-secure, keeping out nasty species of algae.

Pacific Reef Fisheries were the first in Australia to be certified under the Aquaculture Stewardship Council, which distinguishes farms that meet the highest standard of sustainability. They are also taking this model further, with a new prawn fishery and algae farm being constructed down near Guthalungra.

I took one last look at the giant algae canals, then climbed back into the ute with Christian. He needed to go into Ayr to pick up some tools and gave me and my bike a lift in with him.

As we drove I considered all I’d seen and learnt. From the blooms of micro-algae that feed the baby prawns and help oxygenate the water, to the macro-algae and mangroves that help clean the wastewater, I found it amazing how natural systems were being harnessed to yield better outcomes for the fishery and the environment. It seemed there was a sustainable future for our seafood after all.

Huge thanks to Christian, Mitchell, Wayne, Brad and the fish harvest crew from Pacific Reef Fisheries and to Professor Rocky De Nys from Janes Cook University, for taking the time to talk with me and show me around.