| River blackfish | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Perciformes |
| Family: | Percichthyidae |
| Genus: | Gadopsis |
| Species: | G. marmoratus |
| Binomial name | |
| Gadopsis marmoratus Richardson, 1848 |
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The river blackfish, (Gadopsis marmoratus) is an Australian native freshwater fish of the Murray-Darling river system, found from southern Queensland through to central Victoria. It is also found in some eastern and southern flowing coastal rivers. Found primarily in upland and "midland" habitats, though early records of fish fauna suggest it was originally far more extensively distributed and was found in some lowland habitats as well. Originally, river blackfish co-inhabited many of its lowland and "midland" habitats with species such as Murray cod and golden perch, and its upland habitats with species such as trout cod and Macquarie perch.
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River blackfish are elongated with a rounded body, distinct snout and large mouth, and small to moderate sized eyes. The caudal fin, soft dorsal fin and anal fin are rounded. The spiny dorsal fin is low, weak and blends into the soft dorsal fin. Colour varies but is usually either yellow or green with dark green mottling, or a very dark purple/black colour.
It is a carnivore that takes crayfish, shrimp, the occasional small fish and occasional surface prey. It is strongly nocturnal.
Murray-Darling river blackfish show a maximum size of around 30 cm and < 0.8 kg.
It is a highly underrated sportsfish, particularly the Southern form (see below). It is fast and strong, and has been shown to take flies and lures, even surface lures, in addition to the more usual worm baits.
Endemic to south-eastern Australia, river blackfish like many Murray-Darling native fishes have managed to cross the Great Dividing Range via natural river capture events and so are found in south-flowing coastal rivers and streams in the eastern half of Victoria. These fish are now often referred to as southern river blackfish. This population shows a far great maximum size of 60 cm or more and about 5.5 kg. Specimens this large were likely to be very old — 30 years or more — and are rarely seen now. Logic suggests that due to these dramatic size differences, as well as isolation from the parent Murray-Darling species, genetic drift and natural selection, southern river blackfish are separate species. The taxonomy has not been reflected to update this.
River blackfish continue the trend present in Murray-Darling native fish of speciating into primarily lowland species and upland species, with the upland species being in this case the two-spined blackfish, Gadopsis bispinosus. The two-spined blackfish is a more specialised upland inhabitant, and is found in the strongly flowing, cobble-bottomed sub-alpine rivers and streams of northeast Victoria, southeast New South Wales, and the Australian Capital Territory. Though with the blackfish species perhaps you would say the split is lowland/midland and upland as there are overlaps in their range, and river blackfish are found in many upland habitats.
There are also indications that river blackfish populations in the southern and northern halves of the Murray-Darling river system may represent two distinct species or sub-species, with genetic and other differences.
The similarities between the Gadopsis blackfish and the Maccullochella cods (such as the Murray cod, Maccullochella peelii) are striking. There are some grounds for believing that Blackfish may be in effect a smaller version of Murray cod, inhabiting similar niches to Murray cod but in habitats that are too small for Murray cod (having said this, their distributions originally overlapped substantially), and there are grounds for believing that blackfish and Murray cod have drawn heavily on common genes at the family level. FishBase, for instance, has scrapped the family Gadopsidae and listed the blackfishes as members of the temperate perch family, Percichthyidae. A mitochondrial DNA study has confirmed a relationship between the Blackfish and the Percichthyidae but the exact relationship was not resolved in that study (Jerry et al., 2001). Further study is needed to resolve the relationship between the blackfish and the Percichthyidae.
Blackfish have a recruitment method similar to Murray cod, but with more specialisation to upland habitats. Blackfish spawn in spring and lay a very limited number of large, adhesive eggs (<1000) on sunken timber (snags), or in the case of two-spined blackfish on submerged rocks. Similar to Murray cod, the male guards the eggs until they hatch. Upon hatching the larval blackfish are, uniquely, attached to their ruptured egg case by a tether until the yolk sac is largely used and the larvae are ready to commence exogenous feeding. (This unique structure is presumably an adaptation to upland river/stream habitats, to prevent larvae being swept away in currents.)
Blackfish have declined very seriously due to overfishing, stream siltation and snag removal, and predation and competition by introduced species, particularly introduced trout species. Recent bushfires in south-eastern Australia (2003–2006) have filled many blackfish rivers with large quantities of silt, and infilled the interstices ("gaps") between larger rocks that blackfish normally use as a refuge from predatory alien trout species. The presumed result will be increased levels of alien trout predation on blackfish and the long term future of blackfish species is now of some concern. The blackfish species are very low in fecundity, slow-growing and long lived, and have low migratory tendencies, so are extremely vulnerable to overfishing and localised extinctions.