Species Focus: Olive Rockfish (Acanthoclinus fuscus)

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By Paul Caiger.

The largest of at least six species of rockfish found in New Zealand, the olive rockfish (Acanthoclinus fuscus) is also the most commonly encountered, and the only one found in estuarine areas. As a regular rockpool resident throughout New Zealand’s shores, it is one of the few endemic fishes that can be described as mostly intertidal. The slender, sinuous body form, typical of this family, means that it is ideally shaped to squeeze between small crevices and under rocks and boulders – essential for life between the tides.

Living in the intertidal, olive rockfish are well adapted to extreme conditions. Turbulent, pounding waves and huge fluctuations in temperature, salinity and oxygen are part of daily life for this fish. Olive rockfish venture out at night and during high tides, returning to the same rockpools when the tide drops. Even more remarkable is the ability to remain out of water for large periods of time, sheltering under nothing but a moist boulder with a protective mucus coating until the return of the tide. This is also where future generations are born, and in winter and spring nests are constructed beneath boulders, containing thousands of densely-packed eggs. Responsibility of egg tending goes to the male, who locks himself inside the nest chamber, sealing it from the outside world with pebbles and mud, and guards it unabated for up to 10 days until hatching.

The olive rockfish is a member of the family Plesiopidae, otherwise known as longfins, with their distinctive fleshy bent-back fins, and includes the well-known blue devils (Paraplesiops spp.) found in Australia. It can be told apart from other New Zealand rockfishes by its dark colouration (fuscus = dusky, dark) and the bold white stripe usually seen on the head; and for the keener enthusiast, by the division of the lowest lateral line and relatively few anal fin spines. Olive rockfish have a carnivorous appetite, and have been seen ambushing triplefins, though crustaceans and molluscs are their main prey.

Next time you are wandering the rocky shore, and should happen to overturn a boulder or two, keep an eye out for an olive rockfish darting rapidly to the closest shelter. And bear in mind that you are not too dissimilar to another naturalist, the rockpooler none other than Charles Darwin, the first person who collected this fish during his visit to these shores in 1835.

Fast Facts:

  1. The olive rockfish is largest of six species of rockfish in New Zealand, reaching 30 cm in length.
  2. Olive rockfish can be told apart from other New Zealand rockfishes by its dark colouration (fuscus = dusky, dark) and the bold white stripe usually seen on the head.
  3. Olive rockfish are known to home, meaning they return to their same rock hollow or crevice after every foraging mission.
  4. Remarkably, olive rockfish have the ability to remain out of water for large periods of time, sheltering under nothing but a moist boulder with a protective mucus coating until the return of the tide.
  5. The only New Zealand rockfish to be found in estuaries.
  6. Males are the domesticated ones, and seal themselves inside a nest chamber, guarding and cleaning egg until they hatch.
  7. The fabled Charles Darwin was the first person to collect the olive rockfish in New Zealand, in the Bay of Islands during his trip here in 1835.
  8. Rockfishes are carnivorous, and have been seen ambushing triplefins, though the diet predominantly consists of molluscs and crustaceans.
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