Feeding sea-caged southern bluefin tuna: the search for a manufactured feed

Why the industry needed to move away from trash fish#

The farming of southern bluefin tuna (Thunnus maccoyii) in sea cages began in 1990 in response to falling quotas for ocean-caught tuna. By enhancing quality through supplementary feeding in cages, Port Lincoln's fishing community found a profitable outlet in the Japanese sashimi market. By 1994/95 the industry was producing about 2 000 tonnes worth $60 million, and the paper reports potential for the farmed tonnage to climb toward 3 000 tonnes carrying a value near $90 million by the following season.

That growth rested almost entirely on wild-caught forage fish fed whole or chopped, chiefly pilchards together with jack mackerel, blue mackerel and herring, with about half of the 15-20 000 tonnes used annually sourced from overseas. The authors set out several concerns with this approach: limited natural stocks of trash fish in Australia, quarantine risk from imported shipments, variable volume and quality of pilchard supplies, feed costs that were hard to mechanise, and the possibility that trash fish diets were capping growth while releasing waste nutrients into Boston Bay.

The paper frames manufactured feed development as a way to address all of these issues at once: better matching nutrition to requirements, improving storage stability and product quality, lowering feed conversion ratios and associated transport costs, and cutting environmental and disease risks linked to fresh fish waste. The authors estimate the economic benefit of a suitable manufactured feed at as much as $9.5 million per annum to the Tuna Boat Owners of Australia and $5 million per annum to feed manufacturers.

This pattern of relying on unprocessed marine resources while searching for a formulated alternative echoes work summarised in the RAAN Conference Proceedings collection, where nutrition researchers across several livestock sectors wrestled with similar questions about matching feed supply to animal requirements under commercial constraints.

How the sea-cage system works#

Wild tuna are caught between December and February using purse seine nets towed around a school, then swum into a towing cage and brought back to Boston Bay at 1 to 2 knots, a journey sometimes covering several hundred kilometres. The paper notes that this non-handling approach, with fish swimming and feeding normally throughout, reduces stress and injury compared with poling.

Commercial cages are made of high density black polyethylene, averaging 40 metres across, with an inner net (mesh 60-90 mm) holding the fish and an outer predator net (mesh 150-200 mm) weighted to the sea floor. Some fouling of the nets is considered useful so the tuna can see and avoid them. Fattening was originally planned to run six months, but the paper reports that marketable fish could be produced within three months depending on stocking size, opening the possibility of two farming cycles a year.

Digestive biology behind the feed problem#

Understanding gut structure and function was treated as a starting point for feed design. Drawing on earlier anatomical work by Gibbs and Collette, the paper describes a blind-sac stomach with a very large capacity, able to hold around 10% of the fish's body weight in pilchards in one feeding, plus a short lower intestine with little segmental difference along its length.

Enzyme work at the University of Adelaide found proteolytic enzymes similar to those in terrestrial animals in the pyloric caeca, including notable dipeptidyl peptidase IV-like activity, with diet type having little effect on which enzymes were present. This pointed away from enzyme supply as the bottleneck and toward factors such as protein type, mechanical processing, residence time and gut microenvironment as more likely limits on digestion.

Microbiological surveys found large bacterial populations, up to 10 to the seventh power per millilitre in some samples, with proteolytic organisms dominant in the pyloric caeca and a mix of lipolytic and proteolytic types in the intestine, attached differently to feed particles versus the gut wall depending on location.

Four growth experiments, 1994 to 1997#

Four large-scale feeding trials were run with sea-caged fish through the Cooperative Research Centre for Aquaculture, one per year given harvest timing. The first, in 1994, compared pilchards with two manufactured forms, a soft pellet encased in a sausage-style skin and a drier extruded pellet, across three cages. Tuna rejected the extruded option but, after a 42-day weaning period, accepted the softer form; feed conversion ratios of 7:1 were recorded for pellet-fed fish against 13:1 for pilchard-fed fish, with comparable growth and meat quality.

The second trial, in 1995, weaned fish onto a formulated sausage-skin diet within 14 days across six cages, but that diet had been diluted with water and flour during manufacture, and pellet-fed fish showed lower weight gains, higher feed conversion ratios and more variable performance than pilchard-fed fish over 105 days. High early mortality and poaching losses complicated interpretation.

The third trial, in 1996, tested a spray-coated pellet with higher replication, but a storm stirred sediment in the bay, causing gill irritation and asphyxiation that killed 75% of the research farm's fish and around $50 million worth of fish industry-wide, so growth data were never collected.

The fourth trial, in 1997, compared pilchards against a CRC Aquaculture Mash, a Northern Bluefin Tuna mash, an extruded pellet and two proprietary diets. The CRC mash achieved the best manufactured-feed result so far, with mean growth rates around 45% of those on pilchards but notably less variation between fish.

Why particle size and feed form mattered so much#

A recurring theme across the trials was that diet form, not just composition, governed performance. Morphology and digestibility work suggested that sausage-skin and spray-coated pellets broke down quickly in the tuna's stomach because of small ingredient particle size, passing rapidly into the pyloric caeca and intestine where the short transit time limited nutrient absorption.

Pilchards and well-bound diets, by contrast, broke down slowly, delivering a steadier trickle of nutrients through the intestine. The paper notes that even chopping pilchards and placing them inside a sausage-type casing reduced growth performance relative to feeding them whole, despite identical nutritional content, reinforcing that physical form rather than composition alone was limiting uptake.

Digestibility trials with a Peruvian fish meal diet in sausage skins measured dry matter, nitrogen and energy digestibility along the stomach, pyloric caeca, proximal and distal intestine, though negative nitrogen digestibility values were recorded in the stomach and pyloric caeca, attributed to endogenous nitrogen contributions. Transit time for the manufactured diet was estimated at 5.3 hours, while pilchards took longer, though an exact figure could not be determined because sampling did not run long enough to dilute the marker fully.

A comparable problem of measuring digestion accurately in a confined animal, though for a very different reason, appears in an earlier study on digestibility estimates in caged sheep, where cages were used to control intake precisely rather than to farm the animals.

What the amino acid comparisons showed#

With no established nutrient requirements for the species, the authors built an estimated ideal amino acid profile from red and white muscle of wild tuna, then compared it with the amino acid balance used in SBT diet formulations before 1997. That comparison, reported in Table 1 of the paper, identified surpluses of several essential amino acids including methionine and the branched-chain and aromatic types, alongside a shortfall of histidine in the earlier formulations.

The authors caution that this profile matters most once dietary protein levels are reduced and fresh fish or fish meal are progressively replaced by alternative protein sources; at high protein levels, amino acid balance was considered to have little practical relevance because much of the surplus would simply be deaminated and wasted. We think this qualifier is important: the amino acid comparison is a reasonable starting estimate rather than a validated requirement, and the paper itself treats it that way.

Research directions proposed for sea-caged southern bluefin tuna nutrition#

Having reached a manufactured feed supporting roughly 70% of the growth and condition achieved on trash fish, the authors proposed using the CRC Aquaculture Mash as a research tool from July 1997. Planned work included testing tuna responses to changes in dietary protein and moisture content, evaluating natural colour enhancers such as astaxanthans, assessing feeding frequency effects on growth, developing extruded feed forms at larger scale, and exploring ways to cut back on fresh fish, fish meal and fish oil inclusion levels in formulations.

The paper is candid about the practical constraints shaping this work: the high value of experimental fish, limited ability to replicate treatments given accommodation costs, difficulty maintaining stable experimental conditions in the ocean, and the near impossibility of routine faeces or digesta collection. These limitations are worth keeping in mind when weighing how far any single trial's results can be generalised to commercial-scale farming. The catalogue also holds a separate record on terrestrial nutrition principles applied to abalone farming, another aquaculture sector where feeding standards had to be built without an existing base of requirement data.

More broadly, the Recent Advances in Animal Nutrition proceedings from this period captured a wide spread of feeding research across livestock industries, and this tuna paper sits as one of the more unusual entries, addressing a marine species farmed in open water rather than a paddock or shed.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Feedipedia animal feed database: an open-access database of animal feeds

Questions#

— The Livestock Library team