We look after our fish very well, don’t we. Whether it is a fish only aquarium or reef aquarium, a lot of thought goes into choice, taking into account compatibility, size and character. All in all, the fish should be healthy and happy.
The marine aquarium is a stable environment (or should be) with all the parameters at their proper levels. The fish receive a varied and appropriate diet. There is little stress I would imagine, apart from the odd squabble that usually doesn’t amount to much. Predation doesn’t exist.
On the wild reef the fish are more stressed as they hunt for food, attempt to avoid becoming food, maybe defend a territory, ensure they have a safe haven for the night and additionally have the need to breed.
Disease is a threat in both environments, but more so in the aquarium. The aquarium gallonage is really miniscule compared to the enormous gallonage of the reef, so the appearance of disease is much more serious. Just consider the entry of oodinium. The fish in the wild may meet it once or twice or not at all. If it does appear it is unlikely to cause a problem. So disease is an area where captive fish could be more stressed than their wild counterparts.
So the lifespan of fish that are commonly kept in captivity is easy to record. All that is required is for the aquarist to note the date when the fish arrived at his/her aquarium. My fish have dates recorded – for example, the flame angel (Centropyge loricula) is 5¾ years old. This excludes the period in transit and with the retailer of course.
Knowing the real age of a fish is close to impossible. How long has the fish been in the wild? Who knows, maybe there is a size related calculation that could assist? But size relates to diet and overall health to a considerable degree.
Perhaps a comparison could be made between wild and captive fish. The fish selected would have to be of the same type and of the same size. So taking the flame angel, it is easy to note the date the fish entered an aquarium, but the fish in the wild would have to be tagged or similar, and how would track of it be kept? No, it isn’t going to happen.
So the only lifespan that could be measured would be of fish in an aquarium. That would not reflect the potential real lifespan, of course, as there are many variables and a lot of the variables have changed.
An inquisitive aquarist keeping fish in captivity will just have to refer to a record and state to a fellow aquarist with pride ‘I’ve had this fish for x years.’