Giant guitarfish

For the giant guitarfish in the genus Glaucostegus, see common shovelnose ray.

Giant guitarfish
Conservation status
Critically Endangered  (IUCN 3.1)
CITES Appendix II (CITES)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	Rhinopristiformes
Family:	Rhinidae
Genus:	Rhynchobatus
Species:	R. djiddensis
Binomial name
Rhynchobatus djiddensis Forsskål, 1775

The giant guitarfish (Rhynchobatus djiddensis), also known as the whitespotted wedgefish, is a large species of guitarfish in the family Rhinidae. It is restricted to the Red Sea, Persian Gulf, and western Indian Ocean, but was formerly considered more widespread due to confusion with its relatives.

Taxonomy and range

The giant guitarfish was previously believed to range throughout a large part of the Indo-Pacific, but recent evidence has shown that it, as traditionally defined, actually was a species complex consisting of four different species. In addition to the giant guitarfish, this complex includes the white-spotted guitarfish, the broadnose wedgefish and possibly the smoothnose wedgefish. With these as separate species, the giant guitarfish has a relatively restricted range; it is found only in the Red Sea and Persian Gulf, and the western Indian Ocean as far south as South Africa. Ranging across the western Indian Ocean, it is common to misidentify the separate species of guitarfish within a similar range.

Description

A large fish reaching up to 3.1 m (10 ft) long and weighing as much as 227 kg (500 lb), it is brownish or greyish above with a variable pattern of white spots. Juveniles have a black spot above each pectoral fin, but in adults it is a dusky ring or absent. There is a dark, bar-like pattern between the eyes (on top of the head), which separates it from the other Rhynchobatus species in its range. In addition to this, the guitarfish can also be characterized by the large black eyespots on the pectoral bases and a distinct pointed snout.

Behavior

Breeding

The giant guitarfish is considered ovoviviparous, where the embryos initially feed on the yolk and then receive more nourishment from uterine fluid enriched with mucus which is obtained by means of indirect absorption. Because of their lower fecundity, the giant guitarfish are known to only produce, on average, four pups per litter during the summer.

Diet

The giant guitarfish feeds on bivalves, crabs, lobsters, squid and small fish. Despite not having any cutting teeth, this species is a known predator of stingrays. One specimen from the Philippines had more than 20 stingray barbs stuck in its jaw, apparently unaffected by the stingray's venom.

Ventral view.

Habitat

It is a shy fish, found at depths of 2–50 m (6.6–164.0 ft), inhabiting areas with a sandy sea floor. These are generally around coastal reefs or reef flats, but they will sometimes venture into the brackish waters of estuaries.

Population

Giant guitarfishes have been exploited as incidental catch, and as a result, it has led to severe declines, reduced population size, and disappearances. Though the giant is labeled as critically endangered on the IUCN Red List, there is no specific data for the species that could be used to calculate the number of individuals in the decreasing population. Species-specific reporting and identification issues are the two major components as to why no accurate data is kept. Although there's an insufficient data record kept on the exact giant guitarfish population; the harvest rates infer that the species is substantially decreasing while the fishing rates remain stable or even increase. As the human population along the coastlines increase, fishing technology is improvised, and the expansion of the fish markets ultimately lead to an increase in fishing effort on a global scale. The highest increases in the fishing industry have occurred around the regions of Asia.

Regarding the catch databases of the giant guitarfish, a research survey from the Gulf of Thailand a ninety-three percent decline in the catch rates of Rhinobathidae, which includes the numerous species of wedgefishes and guitarfishes. From this respective data, the peak catch rate occurred in 1968 to a low in 1972. Additionally, the Indonesian Aru Islands wedgefish gillnet fishery had rapidly expanded their industry from the mid 1970s to its peak in 1987, operating with more than five hundred boats. Shortly afterward, the catch rate of the fish declined significantly and only a hundred boats remained in the area by 1996. By 1992, a wedgefish fishery in the Arafura Sea had been withdrawn because the area had been significantly overfished and the Indonesian investors only obtained limited returns from their investment.

Concerning the catch rates at varying levels of taxonomy, five contemporary datasets are available for accounted data ranging from Iran, Pakistan, India, and Indonesia. Though it's uncertain, these datasets most likely include various species of giant guitarfishes, and each probable species is listed within the collected data. Raje and Zacharia 2009, an included dataset, doesn't include the count of giant guitarfishes but rather represents the landing data of myliobatid rays including stingrays, butterfly rays, devil rays, and eagle rays. Although this dataset doesn't specifically account for the giant guitarfish population, the collected data can be utilized to infer the decline of giant guitarfishes considering they share the same distributions, habitat, and susceptibility to capture as the several species of myliobatid rays. Overall, the collected data is utilized to calculate proportional declines, annual change in population, and the reduction of the giant guitarfishes over the three generations of complementary data.

One of the primary landings data for the "giant guitarfish" category are available from Iran including data ranging from 1997 to 2016. This specific dataset most likely includes all rhinids and glaucostegids occurring locally. From this dataset, it shows that the landings declined by sixty-six percent over the twenty years, which is approximately ninety-one percent of a population reduction over the last three generations of larger glaucostegid species. In addition to the data collected from Iran, Pakistan includes the landing data for the "rhinobatid" category spanning from 1993 to 2011, covering the country's two coastal provinces. Data from the Sindh province showed a seventy-two percent decrease from peak landings in 1999 to a low in 2011. In addition to the Sindh province, data showed an eighty-one percent decrease from landings in 1994 to a low in 2011 in the Balochistan province. These decreases are the equivalent of ninety-eight to ninety-nine percent population reduction over the last three generations of larger glaucostegid species.

Interaction with man

The giant guitarfish is harmless to humans. It is listed as critically endangered on the IUCN Red List as its population is believed to have declined significantly due to unregulated high levels of exploitation for its flesh and fins, the latter for shark fin soup. Although finning has been prohibited in the eastern Pacific, fins are still traded when the guitarfish is landed. Its low fecundity and presumed slow growth rate make it highly vulnerable to unsustainable exploitation.

Conservation

As of today, there are limited conservation efforts or management regulations established for the giant guitarfish. The practice of finning is now prohibited in all administrations in Australia. Along with this, general fisheries management and protected areas will most likely benefit the giant guitarfish in Australia.