Sex Determination

Effective management of our precious reef fish in the Great Barrier Reef requires detailed information on the biology of the fish. Reproduction is a highly important aspect of fish biology that is of special interest to fisheries managers, since successful reproduction ensures recruits for the fishery in the years to come. Thus, managers must consider anything that affects reproduction when regulating fisheries.

Questions of particular importance include:

• How many males compared to females are in the population (sex ratios)?
• Do particular species of fish change sex, and if so at what size/age does the change occur?

Before you can have sex however, you need sexes, that is, males and females in the population.

WHY DO WE SEX FISH?

Determining if a fish is male or female provides fisheries researchers with a lot more information about the reproductive characteristics of fish populations than just the sex of the fish. For example, it can also give us insight into other interesting characteristics about fish reproduction, such as whether the fish change sex during their lifetime and when fish reproduce in each year.

Identifying individuals that have both sperm and eggs within their gonads simultaneously indicates that species is hermaphroditic (reproduces as both male and female) and may even change sex.

Over the last 20 years it has been discovered that many species of coral reef fish do change sex. For example, coral trout and red throat emperor actually begin life as females but have the ability to change sex at some point in their life cycle to become males.

Other information such as the size and age of individuals that are changing sex allows us to determine at what point in the life cycle the fish change sex. Fisheries managers use this information to decide such things as what should be the minimum or maximum sizes for keeping fish to eat such that at least some fish are assured of spawning before ending up on the table.

Fish gonads Photo by CRC Reef.

HOW DO WE TELL THE SEX OF THE FISH?

All fish have a pair of reproductive organs called gonads, which produce roe or eggs (females) or smelt or sperm (males). The gonads are located at the top of the gut cavity, just below the fish’s backbone.

In some cases it is possible to determine the sex of a fish just from looking at the colour and shape of the gonad, but often a more detailed examination is required to tell the sex and developmental stage of the fish. This process is known as histology.

The process is quite technical, but it basically involves a few straightforward steps.

Embedding a gonad Photo: CRC Reef

First, the gonads are removed from the fish and placed in chemicals to preserve the tissues and prevent decay.

Second, slices or sections are cut from the gonad and placed in small blocks of wax to enable ease of handling during processing. These sections are around 5mm thick.

Sectioning a gonad Photo: CRC Reef

Third, a very thin section is cut from the piece of gonad in the wax block, using a special machine called a microtome – which has a very sharp blade! These sections usually are around 5microns thick (= 0.005mm, or 5-1000ths of a mm) and are mounted on glass slides about 7.5cm long by 2cm wide.

Part of sectioning process Photo: CRC Reef

Fourth, the section is stained with three different chemical stains to improve the appearance of eggs and sperm when viewed under a microscope. The stains turn different parts of the gonad tissues into different colours and so allow easier identification of the main features of the gonad.

Finally, the thin, stained sections are viewed under a microscope to determine if the fish is a male or a female, or in the process of changing sex.

Section of a ovary (female) Photo: CRC Reef	Section of a testis (male) Photo: CRC Reef