Fact Sheet: Dhufish

West Australian dhufish

Glaucosoma herbraicum

illustration: R.Swainston/animafish.com

There are good reasons why the West Australian dhufish is a WA fishing icon – it grows to a great size and tastes superb.

Exclusive to WA waters

Dhufish are endemic to WA, which means they do not live anywhere else.

Dhufish have been found off the Recherche Archipelago near Esperance and as far north as Shark Bay. However, they are most common along WA’s lower west coast between Kalbarri and Augusta.

Range of West Australian dhufish

West Australian dhufish belong to the Glaucosomatidae family and is one of four species in this pearl perch family.

Home-lovers

Adult dhufish live around rocky outcrops and ledges. They can usually be found in water 20 to 50 metres deep, however sometimes they have surprised fishers by turning up in water just 3 metres deep.

Tagging and otolith chemistry studies have shown that they are generally sedentary – that is, they do not travel too far from home.

An important characteristic of dhufish is they are demersal, which means they live near the seabed.

As with other demersal fish, dhufish are susceptible to barotrauma, the result of gases expanding in the fish’s body. This species also has an unusual blood chemistry which impairs recovery from the exhaustion associated with capture.

Note: Not all barotrauma is visible. Internal injuries can include blood clotting and haemorrhaging.

dhufish showing signs of barotrauma

The most obvious signs of barotrauma in a fish are a bloated stomach, bulging eyes and the stomach pushed out through the mouth or gills. A fish with these symptoms may not survive when put back and may simply float on the sea surface. WA dhufish are not suited to catch and release fishing, so if you do have to return a dhufish to the water, use a release weight, which must be on board all vessels fishing for demersals in WA waters.

A release weight consists of a barbless hook with a weight attached that is inserted through the fish’s lip. The fish is then lowered into the water. When it reaches the depth from which it came, it can be released by a gentle tug on the line.

juvenile dhufish ready to be returned to the water with a release weight

Predators and prey

‘Dhuies’ have big eyes and cavernous mouths, with which they prey on other fish, crustaceans and molluscs such as squid and octopus. As larvae and juveniles they may be prey to a wide range of other sea creatures.

Adult dhufish have a distinctive vertical dark line called a chevron running through their eye that fades once they die and which is also fainter on females. Juvenile dhuies have black stripes along their body.

At the larval stage, dhufish have huge eyes relative to their head size. This makes them very well adapted to seeing in low light.

3-month-old juvenile dhufish, just over 3 cm. Photo: Julia Shand, UWA

Spawning season

Dhufish on the lower west coast of WA can spawn between October and May, with a peak spawning period extending from December to March. Females can spawn close to daily during this spawning season. Larger individuals are known to spawn for longer durations, producing many more, and larger batches of eggs than smaller fish.

The reproductive strategy of dhufish is complex, and individuals have been found to form small social hierarchies during the spawning season. Spawning occurs in pairs or small groups of individuals, with large males competing to fertilise eggs by controlling access to females.

Male dhufish have relatively small testes and produce lower volumes of sperm compared to polygamous species like snapper that spawn in large aggregations.

Gender difference

Male dhufish grow bigger than female dhufish. An easy way to tell the difference between male and female adult dhufish is the presence of an elongated filament on the dorsal fins of males.

The importance of big dhuies

Dhufish can live for more than 40 years, reach more than a metre in length and weigh more than 25 kilos. Individuals of this species grow most quickly when they are young. Their growth slows down considerably after about 12 years, and they reach close to their maximum size at about 20 years. Like all animals, individual dhufish may be larger or smaller than their peers at the same age.

Research has showed that dhufish reached sexual maturity at around 3 to 4 years old, or when they are 30 to 35 centimetres long. A recent study indicates that, even though dhufish are reproductive at this time, smaller females produce fewer eggs and do not spawn every month of the spawning season.

It appears that dhufish only reach their full reproductive potential as they become larger and thus older.

large female dhufish spawn a greater number of egg batches each spawning season

Older and larger female dhufish spawn more eggs per batch and have a longer spawning period than smaller females. This means big female dhufish are very important for the overall health of a dhufish population.

the larger the female, the longer their spawning season

Boom years

‘Recruitment’ is a term used by researchers to describe the natural addition of fish to a population, either by reproduction or migration. While it sometimes refers simply to young fish joining a population, in some contexts it may instead refer to fish attaining a size at which they can be captured by fishing gear and enter the fishery.

Dhufish eggs and larvae are dispersed by currents and this may explain a high level of variation in dhufish recruitment between years, particularly in the southern parts of its range. Water temperature and the availability of food are also thought to affect spawning and survival of dhufish eggs, larvae and juveniles.

In some years with very favourable conditions, dhufish recruitment is very successful (‘boom’ years) while in others, recruitment is lower. Researchers have found that in the last 20 years, there have only been a few boom years.

The variation in dhufish recruitment between years is an important consideration when managing their stocks. When a lot of dhufish, resulting from a boom year, recruit into the fishery, it can give the impression that fish stocks are fine. However, if these fish – which can make up much of the breeding stock – are fished down quickly, the population may struggle to recover if recruitment following this boom year is low.

33-day-old larval dhufish less than 1 centimetre long, photo: Julia Shand, UWA

How old is this dhufish?

Working out the age of sampled dhufish helps researchers understand the ‘age structure’ of a dhufish population (how many fish there are of different ages). This tells them whether fishing is reliant on a particularly successful year class of fish (fish spawned during a certain year) – or whether recruitment has also been good in other years and whether there are enough older fish in the population.

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 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.

section of a dhufish otolith

Researchers extract the otoliths and cut thin sections from them using a high-precision saw. Under magnification, the alternating opaque and translucent bands can be seen and counted. This is done to thousands of fish otoliths in the Department of Primary Industries and Regional Development (DPIRD) research laboratories each year.

Researchers can also find out about the environmental conditions in which a fish lived by chemically analysing otoliths. Stable isotopes of oxygen and carbon are deposited in otoliths from the water in which the fish lived. This ‘isotope signature’ often varies between locations.

juvenile WA dhufish

Fishy science

Along with the demand for seafood by WA’s population provided by commercial fishers, the number of recreational boats and the level of fishing activity in nearshore waters has grown in WA, leading to boats travelling further to fish than in the past.

In addition, gear such as soft plastic lures, and fishing technology has improved and become more widely owned, including high quality colour echo sounders and global positioning systems (GPS).

These technologies have increased the ability of both recreational and commercial fishers to accurately locate, record and return to reefs and other prime fishing grounds.

To manage fishing activity so that stocks remain sustainable into the future, two types of data are analysed. The first type includes the species’ biology, life cycle and evidence about the age structure of the population being fished.

The other type of data include catch, effort and ‘catch rates’ – information about how many fish are being caught compared to the past. This is considered in relation to the amount of time that people are spending fishing.

For a long-lived species such as dhufish that has few natural predators once fully grown, important signals that populations are being overfished may include a decrease in the maximum age of fish sampled and a decrease in the proportion of older fish.

The oldest dhufish caught and aged by scientists was 41 years of age (Hesp et al. 2002). This occurred in the late 1990s. These days, due to fishing pressure, it would be very hard to find a dhufish that old.

Since 2010, West Coast demersal stocks have been under a recovery plan. It is important to follow fishing rules, which have been set to support recovery and sustainability of these important species. By following the rules and good catch care practices, fishers can contribute to the recovery.

Glossary

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

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

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

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.

Isotope signature: The ratio of stable or unstable isotopes (chemical forms) of particular elements.

Legal size: Size (usually a minimum length) at which it is legal to catch and keep a fish.

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

Otolith: Fish ear bone.

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

Release weight: A weighted barbless hook connected to a fishing rig attached to a fish to be released.

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

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