Fish in focus - Western Blue Groper

Glenn Moore's blog | Created 4 years ago

Continuing our series of endemic fish species, here is a species that is endemic to the south west of Western Australia and along the south coast to Victoria.

The charismatic Western Blue Groper
Achoerodus gouldii (Richardson, 1843)

Few SCUBA divers can resist the charms of a Western Blue Groper due to their inquisitive and docile nature. This particular female was apparently swimming around me for some minutes while I was photographing corals, unaware of her. Fortunately my buddy alerted me to her presence, so I quickly changed subject! The yellow-green body colour indicates that this is a female.

Profile of a female Western Blue Groper

Female Western Blue Groper
Image copyright WA Museum

The males are deep blue and always larger, like the one below (image Barry Hutchins).

Profile of a male Western Blue Groper

Male Western Blue Groper
Image copyright WA Museum

The female posed and showed me her beautiful thick lips and peculiar powerful peg-like white teeth. Western Blue Groper also have unusual pharyngeal (throat) teeth that are used to bite and crush the crustaceans, molluscs and echinoderms they favour in their diet.

Female Western Blue Groper viewed directly front-on

Female Western Blue Groper
Image copyright WA Museum

Tooth of the Western Blue Groper washed up on a beach

Pharyngeal teeth of the Western Blue Groper
Image copyright WA Museum

The Western Blue Groper, is a Labrid (wrasse) which is the second most diverse family of marine fishes. It is the largest bony reef fish in temperate (cool) Australian waters, reaching a maximum of 40 kg weight and 1.75 metres in length. It is the second largest wrasse after the Humphead Maori Wrasse. It is found between Portland in Victoria and the Houtman Abrolhos Islands in WA to depths of 40 metres.

Only about 40 years ago ichthyologists (fish scientists) discovered that sex reversal occurs in a number of reef fishes. In fact this appears to be the norm for many of them. Wrasses demonstrate this in a spectacular way – they are hermaphrodites (possess both male and female reproductive organs) and are also sexually dichromatic (each sex has a different colour pattern). There are many instances of the males and females being described as different species in the wrasse family. There can be up to three colour phases – juvenile (not sexually active), initial (usually female), terminal (mature male) phases.

Female Western Blue Groper swimming through a reef

Female Western Blue Groper
Image copyright WA Museum

Male Western Blue Groper viewed in profile.

Male Western Blue Groper
Image copyright WA Museum

The Western Blue Groper is a protogynous hermaphrodite which means that it starts off life as a female (bright green) and can change to a male (blue) as it grows. It lives in small social groups consisting of one large male, two to three smaller females (when 15-20 years old, around 650 mm) and several immature individuals. If the dominant male dies, the largest dominant female takes over his role, and changes to a functional male. Unlike some other wrasses, however, sex change in blue groper only occurs after they have bred as females. Sex change happens at lengths over 820 mm when they are 30-35 years old. Within a couple of hours of the disappearance of the male, the dominant female will start behaving in a masculine manner. A couple of days later, the female’s colouration will change to that of the male and within about 14 days her sex will have changed to male. As a result of this change, the other females will have moved up one position in the social order of the harem, with a vacant position left at the bottom for a new female member.

Western Blue Groper swimming on a reef

Male Western Blue Groper
Image copyright WA Museum

This long-lived, slow-growing species takes about eight years to reach a length of 400 mm, and at the maximum size, is estimated to be at least 70 years old. Stocks of this excellent eating quality fish have been over exploited, but fish restrictions are now in place to protect it.

All photos by Sue Morrison unless otherwise acknowledged.