Fact Sheet: Baldchin Groper

Baldchin groper

Choerodon rubescens

Baldchin groper, affectionately known as ‘baldies’, are greatly prized for their high-quality white flesh. They are powerful swimmers and quite capable of breaking a fishing line as they dive for cover among rocks and coral.

Exclusive to WA waters

Baldchin are endemic to WA, where they are found in waters between Coral Bay and Cape Naturaliste. They are most abundant at the Houtman Abrolhos Islands, 60 kilometres west of Geraldton.

range of baldchin groper

A long, slow-growing life

Baldchin can live for 28 years. However, like many bottom dwelling fish, they are slow-growing. For example, it takes them around 5 to 7 years to reach 40 centimetres in length. Because of their slow growth rate, baldchin groper populations are likely to take a long time to recover from sustained overfishing.

What’s in a name?

Baldchin groper are members of the Labridae or wrasse family. With protruding tusk-like teeth in both jaws, they are one of the largest species of tuskfish, reaching about 75 centimetres in length and 7 kilograms in weight.

Baldchin can be identified by the white patch on their pectoral fin and their white chin, giving this species its common name. Their body colour is quite variable, ranging from yellowish-brown for juveniles to pinkish-grey and even greenish-blue for large males.

Reef-lovers

Baldchin live on the continental shelf, mostly on reefs less than 100 metres deep. They are carnivores and feed mostly on echinoderms, molluscs and crustaceans. Juvenile baldchin are often found in shallow weedy areas near reef.

baldchin groper are sedentary, living on the bottom near reef structures

Gender-benders

It may sound strange to us but, as with most wrasses, baldchin groper begin adult life as females and later turn into males!

Baldchin groper first mature as females at around 4 years of age and 30 centimetres in length. They then produce eggs for a number of years before changing sex to male at around 11 years of age and 50 centimetres long.

The scientific term for this is ‘functional protogynous hermaphroditism’. Another term for this reproductive behaviour is ‘monandric’ – that is, a specific type of protogynous hermaphrodite that will first become sexually mature as a female before developing into a male. What factors trigger females to change sex to male is unknown.

example of age frequency distribution of male and female baldchin groper

A study of a related tropical fish, the Venus tuskfish (Choerodon venustus), found that sex change occurred at a smaller size in heavily-fished populations than in moderately-fished populations. Researchers concluded that the trigger for female-to-male sex change in that species was the absence or low number of males in the population. More research is required to find out if this type of social trigger causes baldchin groper to change sex.

It is very important to consider this fascinating and complex life cycle when managing baldchin groper stocks. If all large baldies are caught by fishers and removed from the population there may be a shortage of males – and therefore sperm – for reproduction.

Another reason why it is a good idea to preserve large baldchin is that the large females produce more eggs than smaller ones – so keeping them alive helps keep the stock sustainable.

Spawning in groups – a risky time

While often solitary, baldchin have occasionally been observed to gather in groups of to spawn.

The larger size of males and small size of male testes relative to female ovaries suggest that a male fertilises the eggs of a single female at a time.

Spawning of baldchin groper occurs throughout its range. At the Abrolhos Islands, spawning usually occurs from early spring to mid-summer with the peak period being October to December.

Female baldchin groper are serial spawners, releasing eggs in a series of batches over one breeding season.

Bottom-dwellers

Baldchin groper are ‘sedentary’ – which means they may not travel far from home.

They are also bottom-dwelling fish and, as is the case for many other demersal fish, they are extremely susceptible to barotrauma if caught in waters as shallow as 10 metres deep.

Barotrauma is the expansion of gases in the fish’s body caused by rapid changes in pressure, similar to ‘the bends’ in humans. The most obvious symptoms include a bloated stomach, bulging eyes and the stomach pushed out through the mouth or gills.

The mortality of baldchin groper that are returned to the water after capture is 90%, even if captured in relatively shallow water.

baldchin with distinctive white chin

Fishing pressure and fishy science

The level of fishing activity by WA’s recreational and commercial fishers has grown along with population growth and the development of fishing technology.

In particular, widespread ownership of fish sounders and GPS (global positioning system) means fishers can more easily locate fish and return to good fishing spots.

Researchers use 2 main methods to work out the current fishing pressure on baldchin groper stocks. The first includes analysing historical and current fishery data. Catches (the number or weight of fish caught) must be considered in the context of how many people are fishing and how many days they are going out fishing (called ‘effort’). ‘Catch rates’ refer to data about both catch and effort combined – that is, the amount of fish caught in relation to fishing effort.

The second type of information used by researchers is biological data on the length, age, sex and reproductive status of fish. This provides valuable information on a species’ growth rate and life cycle, and allows comparisons of the age structure of the fished population over time.

Important signals that a population of fish is experiencing too much fishing pressure include a decrease in the maximum age of fish caught or a decrease in the proportion of older fish caught.

If too many mature fish are removed from a population this may affect the ability of that population to reproduce, reducing its resilience to fishing.

In the case of baldchin groper, researchers think that ‘recruitment’ – in other words, the addition of young fish to the population – can also decline if heavy fishing causes a shortage of males.

Ear bones – what tales they tell

Working out the age of sampled baldchin groper helps researchers understand the ‘age structure’ of a population – in other words, how many fish there are of different ages – as a way to monitor the health of stocks.

The ear bones of fish, called otoliths, contain a detailed record of their age. Each year as a fish grows, tiny bands of calcified material are laid down in the ear bone, similar to growth rings in a tree. When growth is faster, translucent or clear bands are laid down. When growth is slower, the bands are milky or opaque. Research has shown that in baldchin groper at the Abrolhos Islands, milky or opaque bands are laid down during spring and summer months. Researchers extract the otoliths and cut thin sections from them using a high precision saw. The bands are counted under a microscope – one opaque band for each year – to work out the age of the fish.

Researchers can also find out about the environmental conditions in which the fish lived – and therefore where it may have spent major parts of its life – by analysing the chemical composition of otoliths.

A section of an otolith from a baldchin groper

Glossary

Abundance: Number or mass of fish in a stock or population.

Age structure: The number of fish of different ages within a population.

Aggregation: A gathering of fish in one area, for example during spawning.

Barotrauma: Expansion of gases in the fish’s body due to a decrease in pressure, similar to ‘the bends’ in humans.

Biodiversity: Number and variety of life forms, including different species, the genes they contain and the ecosystems they form.

Catch rate: The amount of fish caught in relation to fishing effort.

Demersal: Bottom-dwelling, or living near the seabed.

Effort: The amount of time spent fishing by a given group of fishers.

Endemic: Restricted to, or only found in, one place.

Maturity: Stage at which a fish can reproduce or breed.

Monandric: Sex change occurs after female sexual maturity is reached.

Mortality: Frequency of death.

Otolith: Fish ear bone used to work out age.

Protogynous hermaphrodite: Female to male sex change.

Recruitment: Addition of fish to a stock or population as a result of reproduction.

Sedentary: Non-migratory, or tending to stay in one location.

Fish illustrations © R.Swainston/www.animafish.com