Laura Hill, of Port Chalmers School, asks :-

How can you tell a male fish from a female fish?

Trudy Walters, a marine zoologist at the University of Otago's Portobello Marine Laboratory, reponded.

That depends on the species of fish that you are trying to sex. Spotties, for example, are quite easy - small fish with a single black spot on their side are females. Larger fish whose spot has changed into a longer shape and possibly split into two or three, are male. Spotties all start life as females, and change into males when they get to a certain size!

Sharks are also simple to sex - males have structures called claspers on their pelvic fins, while females do not. Male seahorses have a pouch with an opening which looks like a 'tummy button' for carrying eggs and young.

Sandager's wrasse, like many other species of wrasse, have different colours to distinguish sex - females are pinkish brown, with two brown blotches on the side, while males develop a series of colourful bands, patches, stripes and patterns on their body and fins.

Goldfish are hard to tell whether they are male or female for most of the year. However, in spring and summer the male goldfish get white spots on their gill plates and the females don't. For half of the year you can sex them and for half you cannot.

Hence with some species of fish it is easy to sex them by looking at them but not all fish are so easy to sex. Most can only be sexed by looking at their reproductive organs when they are dead. The reproductive organs are always found in pairs, just under the backbone. Male fish have testes, which are white or yellowish. Females have ovaries, which are usually pinkish coloured.