Week 5: Which fish you may keep, and for how many months

Week 5 — feed-rate ratios, stocking limits and seasonal species rotation, plus the jurisdiction question the thermal model cannot see

22 March 2027

Open slides — week 5

The question#

Aquaponics is sized from the feed, not from the fish and not from the plants. Given your climate and the law where your site is, what can you keep, for how many months, and what does that let you grow above it?

The method: the feed-rate ratio#

FAO Technical Paper 589, Small-scale aquaponic food production, gives the three numbers a small system is sized from:

That last one matters more than it looks. A 1 m³ system loses 10–30 L a day — about 5 % of the household’s daily domestic water. This course uses a placeholder top-up of 20 L/day and sizes it properly in weeks 7 to 9. Aquaponics is not where your water goes.

Worked example: Adelaide#

Adelaide’s monthly mean runs 11.3 to 22.7 °C. No single legal species covers that, so the site needs a rotation — and rotation means a harvest-and-restock cycle, not one continuous stock.

A 4 m² bed at 50 g/m²/day for twelve months:

50 × 4 × 365 = 73 kg feed/year
73 kg ÷ FCR 2.0 = 36.5 kg live fish
× 40 % fillet × 20 % protein = 2.9 kg of protein

Then apply the closure rule. Adelaide’s larvae cover 9 % of the ration, so only 9 % of that fish is grown on site:

2.9 kg × 0.09 = 0.26 kg of protein counted, against 67.5 kg required

The fish tank is a nitrogen delivery system for the grow bed that happens to produce a meal. Size it for the greens.

What the model cannot see: you may not be allowed to keep it#

The screening model ranks species by temperature alone. For Canberra it returns Murray cod and rainbow trout, seven months — and Murray cod is precisely the species an ACT household may not keep without a licence.

The Nature Conservation Act 2014 (ACT) excludes fish from the definition of “animal” unless the fish has special protection status, which covers ACT-listed threatened species and EPBC-listed ones. Silver perch is ACT-vulnerable and EPBC critically endangered; Murray cod is EPBC-listed. Keeping a non-exempt animal with special protection status carries up to 200 penalty units and two years, and the current exempt-animals declaration lists no fish at all. Separately, importing any live fish into the ACT needs a licence or the conservator’s written approval.

Queensland, South Australia and the Northern Territory all define aquaculture around sale, trade, business or research, so a household system for its own table needs no aquaculture licence there. Tilapia is restricted noxious matter everywhere it appears: the Queensland keeping offence is 500 penalty units, about $86,350 at the current unit value — not the $220,000 that circulates online, which is a different Act in a different state at a historical unit value.

And here is the part worth remembering. Re-run the model for Canberra with the two special-protection species removed and the best rotation becomes jade perch and rainbow trout, six months instead of seven. Canberra’s bands do not move at all. Counted fish protein stays at 0.27 kg/year, because it is limited by the larvae supply from week 4, not by the thermal window. The legal constraint costs you nothing you were being scored on — but you still have to design around it, and say so.

Temperature bands, and one conflict#

Five sites#

What this week hands you#

A sized fish and bed pair, the species named with its approval pathway in your own jurisdiction, and the imported-feed line carried forward from week 4.

The operating conditions this week names — temperature band, pH, alkalinity — are also the readings a measurement programme would have to watch. Carry them into Assignment 1 rather than assuming a system that has not been built holds them.

Back to week 5 overview