1
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. THE LAST WORD Big fish in a… Is it true that goldfish (or other captive fish) grow in proportion to the size of the tank they live in? I have a small goldfish in a tank at home but have seen much larger goldfish in ponds and believed they were a different species. But a friend insists they are the same, with the size difference being caused only by the volume of the body of water in which they live. Is my friend right? And if so why? n It depends. Some Koi, a related but different species, are easily confused with large goldfish. Real goldfish have been bred and cross- bred for more than 1000 years, and are now regarded as a single species. But there are various breeds that differ in size, even allowing for pond volume. Water supply is seldom a constraint on marine fish, but ponds and streams are another matter. Freshwater fish are generally adapted to grow according to available resources. I successfully raised baby guppies in a wine glass and they grew apparently healthy and happy, but were less than half the normal size and produced very small litters. Much the same applies to other freshwater fish and even tadpoles, and multiple factors seem to control their growth. Temperature, depth and mass of water seem to be important, as do famine and crowding. Apparently, a common control factor is the secretion of certain protein molecules into the water by the fish. These stunt the growth of young fish, and affect the smallest more rigorously than the larger ones. So commercial fish breeders need to be lavish with the turnover of fresh water if they are not to be stuck with unsellable runts. Jon Richfield Somerset West, South Africa n What determines the ultimate size of goldfish will include genetics and the availability of resources such as food. But there are social factors at work too, and these are of particular interest to ecologists, fisheries biologists and fish farmers. Goldfish are schooling fish, meaning that they naturally form loosely coordinated groups that spend variable amounts of time together. Shoaling fish such as herring, by contrast, form tightly bound groups that swim and forage together all the time. Within the groups, bigger goldfish are dominant, and they occupy the best microhabitats and take advantage of the best sources of food. Such fish are able to grow faster and avoid predators for longer, and this in turn increases their dominance. While all fish grow throughout their lives, they normally grow fastest when young, and the rate tends towards zero as they age, so if a goldfish does not become dominant in its first year, it is unlikely ever to reach its genetically possible maximum size, no matter how long it lives or how well it feeds in later years. This explains the common situation in which a pond is occupied by one or two big goldfish and lots of smaller ones. Even if the large fish are removed, the remaining ones will never get much larger. However, the size of aquarium and pond fish is usually more to do with environmental factors than social ones. Indeed, the mbuna cichlids of Lake Malawi, which are some of the most intensely social fish, commonly get bigger in aquaria than in the wild because they are given a much richer source of nutrients (in the form of fish food) than they would get in the wild (algae and, to a lesser degree, microinvertebrates). There’s no reason goldfish should become stunted in ponds if water quality is good and food supply rich and varied. But because garden ponds are often filtered (to remove fish waste) and almost always receive few water changes, which would dilute the end products of biological decay and filtration, water quality in ponds may not be as good as the pond keeper thinks. This, more than anything, explains why goldfish in ponds are smaller than expected. That we so often see stunted goldfish in ponds, aquaria and bowls says less about their ability to “grow to the size of the container” and more about their ability to hold onto life where other fish would simply die. Neale Monks Peterborough, Cambridgeshire, UK The author writes about fish health for Practical Fishkeeping magazine and is the editor of Brackish-Water Fishes: An Aquarist’s Guide to Identification, Care and Husbandry (TFH Publications, 2006) – Ed This week’s questions DO THE SPLITS Sometimes I buy raw carrots that have split lengthways. They have a deep crack along one side, but still seem edible. What causes this? Does the same thing happen in similarly shaped vegetables such as parsnips? Pavel Mojmir Bratislava, Slovakia GET UP, SIT DOWN Working as a paramedic I often attend people who have fallen and, though they are uninjured, are unable to get themselves up. If they have been lying on the floor for an hour or longer they almost all exhibit the same strange symptom. When helped to stand, they lean backwards and if not supported would fall over. Sitting these people upright for a few minutes before standing removes the problem. What causes this and why do they always lean backwards? Nich Woolf By email, no address supplied “Bigger fish occupy the best microhabitats and take advantage of the best sources of food” Last words past and present, plus questions, at last-word.com The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphin Will we ever speak dolphin?
