New Zealand smooth skate

New Zealand smooth skate
Drawing by Dr Tony Ayling
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Order: Rajiformes
Family: Rajidae
Genus: Dipturus
Species: D. innominatus
Binomial name
Dipturus innominatus
(Garrick & Paul, 1974)

The New Zealand smooth skate, Dipturus innominatus, is a skate of the genus Dipturus, found around New Zealand at depths of between 15 and 1,300 m. Their length has been reported up to 2.4 m.[1]

Description

D. innominatus males grow up to 93 cm in pelvic length (PL).[2] PL is a measure from the tip of the animals’ snout to the outer margin of the pelvic fins.[2] Contrasting the male, the female smooth skates grow to a larger size of approximately 112 cm PL. The largest recorded female smooth skate measured in at 158 cm PL.[2] Smooth skates are the largest known skate in the world and can weigh up to 70 kg at maturity.[3] The smooth skate has a dark grey upper side with darker grey to black spots and an under side that is lighter grey to white in color. Some adults have small prickles or whiskers on their snouts or noses.[4] Smooth skates are commonly mistaken for rough skates but are larger in size and lighter in color.

Range and Habitat

Dipturus innominatus is endemic to New Zealand.[2] They are found in the coastal waters of New Zealand's North, South and Stewart islands, and are also found in the Snares Islands shelf and Chatham Rise.[2] D. innominatus are found at shorelines to depths of roughly 1200 metres below sea level, although it is rare to find them exceeding depths of 800 metres.[2] This species inhabits soft sea floor areas on the continental shelf and its upper slope: they are most numerous on the mid outer continental shelf.[2]

Diet and Foraging

Dipturus innominatus live in the dermal zone near the sea bed.[3] The feeding habits of smooth skate have been researched specifically and results have shown the effects of the commercial fishing industry on this species feeding habits. Smooth skate are bottom feeding predators (benthic) and will scavenge for food eating dead fish and scraps from larger predators[3] There is no difference between the diet of the individuals regarding the depth of the water or the sex of the individual.[3]

Though there is no change in the diet between sex and depth, there is a change in diet when it comes to the maturity of the fish.[3] When the fish are young most of their diet is made up of small crustaceans and as the individuals mature they start to consume larger crustaceans and small fish.[3]   When reaching maturity they consume larger fish and the left over fish from predators and commercial fishing.[3]  Mature Dipturus innominatus no longer eat crustaceans that once were the majority their diet when they were younger.[3]  This change of diet in regards to size/maturity is also show in the skate species Sympterygia acuta, which follow a very similar diet pattern to New Zealand smooth skate.[5]

The diet of mature Dipturus innominatus are largely made up of shrimps, prawns and fish coming from 17 different families.[3] The most common of these species were found to be hoki, deep water cod species and many different species of discarded fish from the commercial fishing industry.[3]

Predation

There is little information on the predation, parasites and diseases effecting Dipturus innominatus. Marine mammals, especially seals, are known to feed on other species of skate, so the assumption that New Zealand marine mammals would do the same is not unrealistic.[6] Skates are part of seals diets, though this may be underestimated in the data as skates do not have boney parts that are easily identified when seal stomachs are surveyed.[7]

Predation on skate egg cases by gastropods have recorded to effect some skate species.[8]

Fishing has an impact on New Zealand smooth skate populations, therefore humans act as predators. New Zealand smooth skate is often caught as by-catch in the commercial fishing industry targeting Hoki.[3]

References

  1. ↑ "Dipturus innominatus summary page". FishBase. Retrieved 2017-05-16.
  2. 1 2 3 4 5 6 7 Francis, M (2003). "Dipturus innominatus". IUCN Red List of Threatened Species. SSG Australia & Oceania Regional Workshop. Retrieved April 4, 2017.
  3. 1 2 3 4 5 6 7 8 9 10 11 Forman, J. "Diet and scavenging habits of the smooth skate Dipturus innominatus". Journal of Fish Biology. 80: 1546–1562.
  4. ↑ Garrick, J; Paul, L. "The Taxonomy of New Zealand Skates (Suborder Rajoidea), with descriptions of three new species". Journal of the Royal Society of New Zealand. 4: 345–377.
  5. ↑ Barbini, S. "Big fish (and a smallish skate) eat small fish: diet variation and trophic level of Sympterygia acuta, a medium‐sized skate high in the food web.". Marine Ecology. 37: 283–293.
  6. ↑ Swain, D. "Spatial distribution of fishes in a Northwest Atlantic ecosystem in relation to risk of predation by a marine mammal.". Journal of Animal Ecology. 84: 1286–1298.
  7. ↑ Swain, D. "Assessing threats to species at risk using stage‐structured state–space models: mortality trends in skate populations.". Ecological Applications. 19: 1347–1364.
  8. ↑ Hoff, G. "Skate Bathyraja spp. egg predation in the eastern Bering Sea". Journal of Fish Biology. 74: 250–269.
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