Neale Monks explains why the lifespan of a fish is difficult to predict.
Q: I’ve noticed that when you print an article on a fish, you provide a really useful ‘fact file’ containing temperature, tank size, water requirements and so on – but you don’t include an indicative lifespan. Is this because it’s difficult to compare farmed and wild fish?
NICK HARRISON, VIA EMAIL
A: Neale replies: Fish lifespans are very difficult to predict. For a start, many of the species we keep live far longer in captivity than in the wild. Fish like Neons and Betta are more or less annual fish in the wild, not because they are intrinsically short-lived but because their habitats are very prone to drying out at certain times of the year. In fact, many of the egg-scattering species that produce hundreds if not thousands of eggs each time they spawn may experience tremendously high levels of mortality, with perhaps just a handful of those eggs surviving long enough to hatch. Of those that do, it may well be that not a single fry grows into an adult that successfully reproduces. This is why the numbers of eggs they produce, and the frequency with which they spawn, is so high.
Another complicating factor is that we rarely vary the temperature in our fish tank, which means that fish don’t experience the normal seasonality they would in the wild. Why does this matter? Almost all fish are what we might loosely call ‘cold blooded’ which means their body temperature matches that of their surroundings. This, in turn, determines the rate at which metabolic processes such as digestion and growth occur. A general rule in biology is that increasing the temperature by 10˚C doubles the rate at which these processes take place. Or put another way, a fish living at 25˚C will be doing all of the clever biochemistry needed to live and grow at twice the speed of one being kept at 15˚C. Now, imagine a fish species that experiences both summertime highs and wintertime lows. During the winter, everything will be happening much more slowly than in summer, and that includes the rate at which it ages, not just how quickly it swims about or digests food.
Lots of the species we keep in aquaria would naturally have a cooler winter phase when their metabolic rate would slow down, including various Corydoras species, many of the danios, and some of the barbs. Pretty much anything described as subtropical would fall under this heading too. Compared with their potential lifespan if given a cooler wintertime ‘resting’ phase, these species will be appreciably shorter lived if kept in a normal, consistently warm tropical aquarium.
Finally, the process of domestication tends to favour fish that mature quickly, primarily because the sooner a fish reaches a sellable size, the sooner the fish breeder gets a return on his or her investment. All well and good, but it does mean we tend to see fish that are genetically programmed to be smaller and shorter-lived than might be the case in the wild. Add to that the effects of inbreeding, which makes, for example, farmed Betta rather prone to tumours, and the lifespan of domesticated tropical fish varieties is often very difficult to compare with what’s seen with wild-caught members of the same species.
About the best thing we can do is highlight those species that are hard-wired to be annual fish (as with some of the killifish) so that prospective purchasers understand that a breeding colony is the aim with these. Beyond that, there are very few species that cannot be expected to live at least a few years, with fish like tetras and barbs generally living 3-7 years; average-sized cichlids like Angelfish around 10 years; and the larger L-number catfish well over 20 years and potentially 50+ years. But these are all ballpark figures; it’s really difficult to make firm predictions, especially when comparing a domesticated fish with a wild-caught one.