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Nathan Hill reveals a fish that's a symbol of sustainability, bringing hope and financial support to the very people who need it.

One definition of the word cardinal is: 'paramount, or of foremost importance'. For the indigenous peoples of the Rio Negro it’s patently clear just how important the Cardinal tetra really is.

This fish provides them with income without overexploiting natural reserves, which helps to retain the unspoilt nature of their homeland and cultural heritage.

The Cardinal tetra (Paracheirodon axelrodi) could be the most economically important ornamental fish on this planet. From South America, in a distribution that ebbs from Brazil across to Venezuela and Colombia, it is the staple fish that every hobbyist has seen and most will have kept.

For its novel appeal — and few freshwater fish have such a striking iridescence — there is a serious side to the Cardinal and that’s the topic of sustainability. As well as being a global aquarium favourite, the handling of the Cardinal represents an attainable ethical ideal; a perfect working example of how responsible fish collection can be performed. It’s not lightly that the supporters behind the Cardinal tetra trade bandy their optimistic slogan: 'Buy a fish, save a tree'.

Buying Cardinal tetras preserves foliage and peoples in those regions of South America where the fish proliferate.

Project Piaba, established in 1989 under the coordination of Ning Labbish Chao, has sought to refine, investigate and promote tetra harvesting for optimal benefit to the communities there.

Rather than shameless profiteering with exports, Chao has put into place a project that now sees financial benefits to a wider medley of individuals.

Later in the project’s life, financial injection from philanthropist Dr. Herbert Axelrod gave rise to the Dr. H R Axelrod Ornamental Fish Laboratory, which was further upgraded to the Centre for Aquatic Conservation.

Since then Piaba has provided education, scholarships for students, and promoted awareness of its cause.

Figures vary from source to source, but bolder claims cite numbers of up to 30 million fish exported annually from the Rio Negro and conservative estimates of up to 15,000,000 fish reflect the scale of the operations.

Paracheirodon axelrodi accounts for around 80% of these total exports. This in turn provides an income for roughly 60% of the riverine inhabitants of the Negro. Many families rely on it for their daily sustenance, but we aquarists so easily take it for granted.

There have been concerns over the impact of removing so many fish from the region, but on the grander scale of things the figures are small.

It is understood that billons of Cardinals perish each and every year due to the onset and subsequent decline of the rainy seasons. The Cardinal’s wildlife cycle could be considered closer to annual fish like killifish.

Any wild Cardinal reaching more than two years of age is considered a true veteran.

The benefits of distributing income from these fish are clear. It lessens the divide between rich and poor in a region where inequality could be rife. Moreover, it means indigenous peoples need not resort to alternative methods of generating income, such as ‘hack and slash’ tactics to destroy forestry to make way for short-term crop gains.

South America has been under the scrutiny of the affluent world for some time in its approach to razing huge areas to make fast gains on maize, cattle harvests or even contraband like cocaine. Projects like Piaba that serve to undermine the 'here today, gone tomorrow' strategies of aggressive farming, are pivotal in preserving large expanses of wilderness.

Without even realising it, money from Cardinals that you have purchased has likely trickled into the preservation of riverside regions or the rainforest.

Changing colours

Paracheirodon axelrodi is the original double-take fish. Even non-fishkeepers are hypnotised by its sharp, crisp markings and the way that the fish almost glows as though it has created itsown light.

The colours are attributed to iridiophores, and these, along with the cytoplasm and guanine layers in the fish’s skin, have long been subject to research.

Cardinals have a degree of control over their own colours, increasing the cytoplasm layer in their skin in the daytime, altering the refractive properties and, subsequently the wavelengths of light reflected. This is worth bearing in mind when buying some, because a retailer hates to be asked for 'six of the greener ones'. Given mere minutes, any markings may become very different.

In their natural range of the tannin-stained, rich-red rivers of the Rio Negro and Rio Orinoco, those colours serve as communication and predator deflectors. They are pretty, but a predator can find it hard to determine exactly where the fish is. Is the body actually in the water, or can it see the reflection of the fish bouncing from the underside of the water’s surface?

Pending region of capture, markings can fundamentally differ. Those from the Rio Orinoco have a blue stripe that reaches just short of the soft, fleshy adipose fin, while those from the Rio Negro have blue that extends beyond.

Orinoco fish also have wider red sections in the caudal peduncle. There are golden and silver-striped forms, rarely seen in the trade, as well as farmed colour morphs, mostly from the Czech Republic.

Come the night the colours change, with the red fading while the blue band alters to reflect more blue and UV light, contrasting their greener, daytime forms.

It’s understood that night time induces a shift in the spectral detection range of their corneas too, their eyes shifting wavelength so they can still see each other.Feeding and keeping

Cardinals are moderately easy to sustain, as testified by the endless thousands of community tanks that house them.

To see them at their best, recreate the wild environment. A tank of 80cm/31” or more works well for small shoals, though many keepers have housed them in tanks at 45cm/18” or less. A larger tank is preferable for relaxed fish and stronger shoaling inclinations.

Filtration need only generate light flow as these fish aren’t familiar with swirling torrents. I advise an internal canister or a small external filter connected to a spray bar to distribute weaker currents.

Lighting need not be intense, as the reflective nature of the fish will bounce off whatever is available. Low output T5 or even T8 tubes work wonders without bleaching the fish into the background.

Consider the kinds of branches and litter that would fall into slow moving Amazonian rivers or flooded forest canopies where the fish spend time during each flooded season.

Fine sands, wood tangles, mats of leaf litter, seed pods, some floating greenery and large doses of blackwater extract will create a home from home. If using external canisters, peat can be used in one of the chambers to release tannic and humic acid.

If that’s too environmentally unsound, or if you lack a canister, use almond leaves or alder cones to create those dark conditions that are so loved by Cardinals.

Cardinals are found alongside huge ranges of other fish in the wild, including hatchets, pencilfish, corydoras and loricariid catfish such as otocinclus, and a long list of other tetras, cichlids and more. Researching a Cardinal’s tank mates is part of the fun!

Keep temperatures higher, ideally at 28°C/82°F, although if this becomes prohibitive for the other fish they will tolerate much lower and may even start to breed when things cool to 24°C/75°F.

Feed a mix of dried and frozen foods, and live Daphnia when available. Wild morsels of choice include mesofauna — tiny bugs that occur naturally on leaf litter — and tiny insects and crustaceans. Colour-enhancing flakes work wonders at bringing out the red in the flanks.

The fish will also take a degree of algae food but generally swing to the carnivorous end of the omnivore spectrum.

Wild disease

Be vigilant with new purchases. As most are wild harvested, they can bring in unwanted pathogens.

Monitored exports have revealed fish that carry Gyrodactylus, Ichthyopthirius, Piscinoodinium, Procamallanus worms and the dreaded Neon tetra disease, Pleistophora. Be aware of the signs of any of these illnesses, and watch new arrivals like a hawk for the first fortnight: assuming you haven’t quarantined them first.

Tough cookies

Despite being delicate in the aquarium, Cardinals withstand some conditions that no other fish can — particularly a level of acidity that would kill others.

In the wild, they’ll usually be found in waters with a pH value somewhere between 5.5 and 6.0. In extremes they have been recorded at 3.5pH, with zero-associated hardness — the pH of a typical orange. In tests it’s found that lethality starts to take its toll at 3.35pH.

This extreme tolerance is only possible in water-borne humic acid, which helps to reduce sodium loss in the fish. In typical acidosis it’s often the osmoregulatory effect of losing sodium that kills.

Many people misunderstand this acidic tolerance and have applied Cardinals to incredibly low pH tanks in absence of humic acids where they have perished. Although such low pH levels are tolerated, for most Cardinal keepers they are unnecessary and the fish will be happy at a pH value of 6.0 for its entire life.

The man behind the name (picture above by George Farmer)

It’s impossible to talk of the Cardinal without referring to Dr. Herbert R Axelrod — the person they’re named after and who has popularised the species.

A man simmering with character, Herbert has an ebullient history during which he advised Sir Winston Churchill on goldfish, collected alongside the Japanese emperor Hirohito, caught wild Amazonian beasts for Walt Disney, liaised with King Leopold III of Belgium and was invited by the then president of Brazil, Humberto Castelo Branco, to help draw up conservation plans for the Amazon.

As well as gaining degrees in physics, biology, mathematics and chemistry, Herbert also found the time to learn at least six languages, so it’s difficult to imagine how he found the opportunity to explore the world at the same time as establishing his huge publishing empire —which survives to this day.

Explore Herbert did, and he had a hand in introducing many new species of fish that carry his name. The echoes of 'axelrodi' and 'herbertaxelrodi' in the monikers of many varieties really bear testimony to Herbert’s aquatic doggedness.

Herbert came across the Cardinal in 1954, thinking the fish to be a new giant variant of the Neon tetra.

Returning with some to America, he noted that they behaved and spawned differently to classic Neons, and he sent some to Dr. Leonard P Schultz at the Smithsonian institute.

Leonard buzzed at the new fish. He pointed out that it was not just a new species but at the time belonged to a fresh genus separate from the Neon due to its different dentition.

Eventually in 1956 the fish was described and named Cheirodon axelrodi — later revised to Paracheirodon alongside Neons — in Herbert’s honour.

There was initially some professional friction because taxonomists Weitzman and Myers had also described this fish and they had called it Hyphessobrycon cardinalis.

Over the next 18 months debate raged until the International Commission of Zoological Nomenclature swung towards axelrodi.

On a few levels the 'discovery' of the Cardinal is far from undisputed. First of all, we know that South American natives were aware of these fish for millennia, so any claim to be the 'first' is hopelessly optimistic.

It transpires that the real Western 'discoverer' was most likely the limnological researcher Harold Sioli, who stumbled on them two years prior to Herbert in 1952. However, thinking they were a variety of Neon he made nothing of his find.

There are also reports of the fish being transported around South America by Captain Malm, a Brazilian pilot, who distributed but never described them or took the 'discovery' further.

Irrespective of who cast eyes on the Cardinal first, what remains undisputed is that Herbert was a pioneer who put them on the fishkeeping map.

Despite any disputes over discovery, it’s a happy outcome that the fish now supports communities, protects habitat and cultures, and pleases aquarists across the globe.

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