Fish: the Species Law and the Temperature Verdict

#Two questions, and almost everyone asks them in the wrong order

The first question a farmer asks about fish is "where do I buy fingerlings". The right first question is "which fish may I lawfully keep here", and the right second is "what does my water do in July".

Get them in the wrong order and you have bought stock you may not legally move or sell, to put into water that will not grow it and may kill it. That is not a rare outcome. In 2023 South Africa's entire freshwater aquaculture sector — 140 farms across nine provinces — reported 451.62 tonnes of fish, while the country imported 7,879.6 tonnes of tilapia (DFFE Aquaculture Yearbook 2024). Imports of one species ran at roughly seventeen times total domestic freshwater production.

Both questions are answerable before you spend a rand. Neither is answerable from a supplier's brochure.

#What the list actually says

The species law sits in the Alien and Invasive Species Lists, 2020, published as GN 1003 in Government Gazette 43726 on 18 September 2020 under the National Environmental Management: Biodiversity Act 10 of 2004. List 7 is the national list of invasive freshwater fish, and item 22 is written as a genus minus two species: "Oreochromis species excluding Oreochromis mossambicus and Oreochromis placidus".

That item carries two categories at once — Category 3 in general, and Category 2 for permitted aquaculture facilities — plus four conditions. Two of them decide questions farmers ask weekly:

• hybrids of tilapia species are treated as the listed tilapia species, so the "red tilapia" fingerlings offered informally at the gate are covered;
• the sale and transport of live tilapia is prohibited except from accredited hatcheries.

Read that against the category matrix in the same gazette — the categories are where people go wrong. For a Category 3 species, possession is exempted — but importing, growing, breeding, conveying, selling, spreading, releasing, transferring between catchments and discharging tank water into a waterway are all prohibited. For a Category 2 species, every one of those requires a permit under section 71(1) of NEMBA.

One more clause converts a design defect into an offence. Notice 2, paragraph 1(e) of the same gazette exempts alien freshwater fish from section 65(1) of NEMBA except for release into rivers, wetlands, natural lakes and estuaries. A pond that overflows into a stream in a February storm is not a leak. It is a release.

#The two traps behind "is it legal?"

Farmers who do check the list still get caught, two predictable ways.

Trap one: "indigenous" is a catchment question, not a national one. A fish can be indigenous to South Africa and still be alien to your river system — extralimital. DFFE's own water use licence application requires you to state whether your species is "exotic, indigenous, or indigenous but extralimital to the area" (DFFE). Mozambique tilapia, redbreast tilapia and sharptooth catfish are all indigenous to parts of this country and alien in others. "It's a South African fish" answers nothing.

Trap two: the NEMBA list is only one of the permits. Provincial nature conservation permits apply to keeping, stocking and transporting fish in every province, listed species or not, and typically run 12 months, renewable, subject to a compliance audit (DFFE). An unlisted species is not an unregulated species.

#The table that ends most of the arguments

South Africa's official reference for aquaculture water quality is the South African Water Quality Guidelines, Volume 6: Agricultural Use — Aquaculture (2nd edition, 1996), from the then Department of Water Affairs and Forestry. Its species temperature table is the most useful single page in this domain.

Two features do the work. Tilapia and catfish both want 28–30 °C to grow properly. And trout run the opposite calendar: rainbow trout's upper lethal limit is 27 °C, so a hot low-flow February kills a South African trout farm, not a cold July.

Then the guideline removes the obvious escape route, flatly: "The costs involved in controlling temperature under pond-culture conditions are prohibitive and not feasible." You cannot heat your way out of your climate in an earth pond. You choose a species that matches it, or you do not farm fish.

#Worked example: Meyerton against the table

Take the course's home farm — Delta Farms at Meyerton, Sedibeng, Gauteng — against that table. These monthly normals were computed independently from the ERA5 reanalysis archive for 1995–2024 at −26.556, 28.016:

Read it against the table using the monthly mean, not the daily maximum — the daily maximum is what everyone reaches for and it flatters a pond badly.

• No month at Meyerton reaches the 28–30 °C tilapia growth optimum. The best monthly mean in the year is about 21 °C in January.
• June and July mean 10.4–10.7 °C — inside the 9–11 °C lower lethal band for Mozambique tilapia. A Highveld winter is not a pause in tilapia growth; in a shallow unheated pond it is plausibly a kill.
• Water crosses 20 °C upward around September–October and back down around April–May: roughly six to seven months in which growth is possible, and none in which it is optimal.
• Sharptooth catfish, at a lower lethal of 8–10 °C, sits just above that winter band where tilapia sits inside it. Two or three degrees is the whole species argument on the Highveld.

Hold one honest limit on that worked example firmly: those are air temperatures, used as a proxy. No measured pond or dam water-temperature series for a Highveld site was available. Real water is damped relative to air — warmer on a winter night, cooler on a summer afternoon, more so with depth. A deep dam will sit above these winter figures; a one-metre earth pond close to them. The direction is sound; the exact months are yours to establish.

#The log that decides the venture

So: name your species, check the current list yourself, get the catchment answer in writing, and read your own water at first light every Monday until spring. Bring both answers to the next lesson, where the money gets counted — and where most honest answers turn out to be no.