River blackfish are common throughout their geographic range. However, due to human influence, river blackfish populations have been significantly reduced in certain areas. Competition with introduced trout and overfishing have put a strain on river blackfish, causing their numbers to decline. In addition, human influence has reduced stream debris (removing essential shelter), and construction has led to an increase in sedimentation, degrading aquatic habitats. Such an impact, along with their low fecundity, means that populations are vulnerable to endangerment or extinction and should be monitored closely in the future.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
Anti-predator adaptations differ between large and small members of the species. Juvenile river blackfish are most susceptible to predation by large carnivorous fish found in deeper waters. In order to avoid this predation, smaller individuals tend to stay in shallow waters under the cover of leaf litter and aquatic plants. Young river blackfish spend much time under leaf litter and little time out in the open in order to avoid predation by dragonfly nymphs (Odonata) and freshwater crayfish (Parastacidae).
Larger blackfish are not prey to fish in deep waters. Instead they are the primary targets of larger, terrestrial predators that dive and wade for food, such as herons. Larger fish have limited mobility in the confined space of shallow waters, and they are more susceptible to terrestrial predation when they are exposed so close to the surface. Therefore, large blackfish tend to remain in deeper waters in order to minimize the risk of terrestrial predation.
Known Predators:
Anti-predator Adaptations: cryptic
Average length and weight are 30 cm and 450 g, respectively. Body size varies geographically in river blackfish. River blackfish in southern Victoria are significantly larger than those found in northern Victoria on average. They also show great diversity in terms of dorsal and ventral body color. Lateral and dorsal coloration is variable and can be mottled yellow, brownish-green, grey, dark brown, or light blue. These background colors are peppered with many dark, irregular blotches. Ventral color may be light blue, yellow, or purple.
River blackfish are elongated and round, and the body is covered in small scales. The dorsal and anal fins are long and found low on the body. The fish is covered in a thick layer of slime, a characteristic which has earned river blackfish alternative nicknames such as slippery or slimy blackfish. River blackfish have muscle composed almost entirely of white muscle, which is especially conducive to short, quick, burst movement, rather than consistent steady swimming. Although the abundance of white muscle is extremely beneficial in terms of capturing prey, the low endurance that results from a severe lack of red muscle reduces the ability of river blackfish to escape natural predators and human anglers.
River blackfish have hemoglobin in their blood with low oxygen affinity relative to that of other aquatic fish. However, they exhibit thermal acclimation, which allows them to adjust their blood’s oxygen affinity depending on temperature. Fish acclimated to temperatures of 20°C have hemoglobin that favors increased release of oxygen to muscle tissues, whereas fish acclimated to temperatures of around 10°C have hemoglobin that is more efficient at oxygen uptake from the surrounding environment.
Average mass: 450 g.
Average length: 30 cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike
The lifespan of Gadopsis marmoratus has not been documented in nature or in captivity.
Although river blackfish can be found in both slower and faster flowing waters, they prefer to stay in low-velocity (0 to 20 cm/s), highly sheltered pools of lowland rivers. Shelter in the pools includes rocks and woody debris, along with other inputs from the terrestrial landscape, which help to slow the flow of the river. The waters that the fish inhabit are variable in temperature, ranging from 5 to 28 °C, and contain a large amount of aquatic vegetation. Tagging studies have shown that adult river blackfish swim at deeper depths relative to stream surface (40 to 60 cm) and that depth preference is positively correlated with the size of the individual fish. Oxygen-rich bodies of water are most suitable for river blackfish survival. Smaller, immature blackfish are usually not found at deeper depths due to the predation risk posed by larger carnivorous fish such as adult brown trout (Salmo trutta). These immature fish tend to remain in shallow waters, burrowing themselves under leaf litter.
Average depth: 0.4 - 0.6 m.
Habitat Regions: freshwater
Aquatic Biomes: rivers and streams
River blackfish (Gadopsis marmoratus) have a relatively limited range, inhabiting freshwater rivers in southeastern Australia. They are found both north and south of the Great Dividing Range, a mountainous geographic barrier that divides Victoria into two separate regions.
Biogeographic Regions: australian (Native )
River blackfish are carnivorous, ambush predators. They prefer to ambush prey areas of cover in order to most efficiently use their short, quick bursts of speed. They have a diverse diet. Prey items include insects, mollusks, crustaceans, small fish, and terrestrial invertebrates that fall into the water. Of all their prey items, larval mayflies are most important, comprising 50% of the diet. Terrestrial insects make up the second greatest portion of the blackfish diet, 10% of their overall intake.
Animal Foods: fish; insects; mollusks; aquatic crustaceans
Primary Diet: carnivore (Insectivore )
River blackfish compete with introduced brown trout (Salmo trutta) for resources. These two fish species have similar diets, the main difference being that river blackfish consume significantly more caddisfly larvae. River blackfish control insect populations through predation.
River blackfish are a common angling fish in Australia and are considered excellent for human consumption. However, the restricted range of river blackfish, especially compared to game fish such as salmon and trout, prevents these fish from contributing to the economy on a larger scale.
Positive Impacts: food
There are no known adverse effects of Gadopsis marmoratus on humans.
Research on river blackfish development has only focused on early development. However, there is no evidence to suggest that they develop in a radically different manner from other bony fish. After they are fertilized externally, the eggs develop for about 14 days. After hatching, the offspring are in the "fry" stage of development. While fry, immature river blackfish carry a yolk sack for nutrient absorption that remains attached to their body until their digestive system finishes developing. The fry stage lasts for approximately 26 days. For 19 of these days, fry remain bound to the substrate upon which they were deposited as eggs. For the last seven days, the fry are able to wiggle free from the substrate and swim freely. River blackfish express none of their characteristic pigmentation while in the fry stage. Pigmentation develops later and is seen to its fullest extent once the fish reach the juvenile stage of development.
Development - Life Cycle: metamorphosis
No in-depth studies regarding communication and perception of river blackfish have been conducted. Like other fish, they are likely to use a combination of vision, chemosensation, and tactile cues to perceive their environment.
Perception Channels: visual ; tactile ; vibrations ; chemical
River blackfish reproduce sexually, but their mating system is unknown. Fertilization of eggs occurs outside of the mother’s body, and eggs are normally laid inside hollow logs. Egg deposition has also been observed inside of man-made structures (e.g., hollow pipes), which may simulate the conditions of hollow logs.
The reproductive season starts in either late spring or summer (October to November in Australia) and is stimulated by water temperatures of 16°C or higher. River blackfish are known for their very low fecundity, averaging approximately 300 eggs per spawning season. This low fecundity is striking, especially when compared to fish such as salmon, which may lay as many as 7,500 eggs in a year. The overall number of eggs laid is positively correlated with the length of a female’s body.
Breeding interval: River blackfish breed once yearly.
Breeding season: Breeding occurs from October to November.
Range number of offspring: 20 to 500.
Average number of offspring: 300.
Average gestation period: 14 days.
Average age at sexual or reproductive maturity (female): 2 years.
Average age at sexual or reproductive maturity (male): 2 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); oviparous
Observations in natural and artificial environments demonstrate that males exhibit parental care in the form of protecting eggs until they hatch and fanning them to keep them well-oxygenated. Females have also been observed lingering around egg sites, suggesting the possibility of female parental investment as well. Parental investment is consistent with their low fecundity; with so few eggs parents must expend energy to ensure maximum survival of offspring.
Parental Investment: male parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male)
The river blackfish (Gadopsis marmoratus) is a freshwater fish endemic to the temperate waters of south-eastern Australia.[2] It is found from southern Queensland through to central Victoria, including in the Murray-Darling river system. 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. It is a popular angling fish in some parts of its range.[2]
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 an ambush predator that takes crayfish, shrimp, small fish and aquatic insects.[2] It is strongly nocturnal.
Murray-Darling river blackfish show a maximum size of around 30 cm and < 0.8 kg, with larger specimens from coastal drainages reaching up to 60 cm (Gellibrand River).
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. There is an unofficial world record caught by Piotr Pomorski on a WildBait Lure, in Victoria Australia that measured 70 cm in length.
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 (12 lb). Specimens this large were likely to be very old — 30 years or more — and are rarely seen now. Southern river blackfish may be a separate species. The taxonomy does not yet reflect to 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. While the split may be between lowland (or midland) and upland, there are overlaps in their range, and river blackfish are found in many upland habitats. There are also indications 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 Gadopsis blackfish and the Maccullochella cods (such as the Murray cod, Maccullochella peelii) are strongly similar. There are some grounds for believing blackfish may be a smaller version of Murray cod, inhabiting similar niches but in habitats 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.[3] Further study is needed to resolve the relationship between the blackfish and the Percichthyidae. In addition, molecular studies have supported the view that G. marmoratus is a species complex of five different species, these include a species in the Murray-Darling Basin and four species in coastal freshwaters.[1] One of these forms from Victoria has been named as Gadopsis gracilis.[4]
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.
River Blackfish are a protected species in South Australia.[2]
The river blackfish (Gadopsis marmoratus) is a freshwater fish endemic to the temperate waters of south-eastern Australia. It is found from southern Queensland through to central Victoria, including in the Murray-Darling river system. 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. It is a popular angling fish in some parts of its range.
