|Scientific Name:||Rhinobatos horkelii|
|Species Authority:||Müller & Henle, 1841|
Rhinobatos horkeli Müller & Henle, 1841 [orth. error]
|Taxonomic Notes:||Published records of Rhinobatos percellens in southern Brazil (Chao et al. 1982 in Vooren et al. 2005) are due to problems with one of the criteria used for separating R. horkelii and R. percellens. From measurements of only four specimens (two juveniles of R. percellens and two juveniles of R. horkelii, all from Rio de Janeiro), Bigelow and Schroeder (1953) used the relative size of the nasal groove as a diagnostic criterion for separating R. horkelii and R. percellens. However, the value of this morphometric measurement as stated by Bigelow and Schroeder (1953) does not permit the correct identification of specimens of all sizes and from all areas where these species occur. All 9,853 specimens of Rhinobatos from southern Brazil examined according to other criteria described by Bigelow and Schroeder (1953) (adult body size and colour) since 1972 by Lessa (1982) and Sadowsky (1973) in Vooren et al. (2005) were R. horkelii. Therefore all fishery data for Rhinobatos from southern Brazil refer to R. horkelii (Vooren et al. 2005). This aspect of taxonomy and identification is important for the enforcement of laws for the protection of R. horkelii in Brazil.|
|Red List Category & Criteria:||Critically Endangered A2bd ver 3.1|
|Assessor(s):||Lessa, R. & Vooren, C.M.|
|Reviewer(s):||Valenti, S.V. & Kyne, P.M. (Shark Red List Authority)|
A guitarfish endemic to the Southwest Atlantic, recorded from Brazil to Argentina but with its centre of distribution in southern Brazil and scarce elsewhere. The abundance of this species has decreased by >80% since 1986, following intensive fishing with annual landings of up to >1,800 tons. It is extremely vulnerable to trawl fishing in the coastal waters where pregnant females and adult males congregate for parturition and mating, and where small juveniles occur throughout the year. In 2004, monitoring of beach-seine catches and reports from fishers confirmed that R. horkelii is currently only rarely captured. Given large documented declines and continuing fishing pressure across its coastal inshore range, the species is assessed as Critically Endangered. Without protection against all fishing activities, this guitarfish may well become extinct in about ten years and measures are urgently required to protect it, particularly in areas of critical habitat.
|Previously published Red List assessments:||
|Range Description:||Rhinobatos horkelii was named the Brazilian guitarfish by Bigelow and Schroeder (1953) for its wide distribution along the Brazilian Coast and further south to Mar del Plata (Argentina). These authors recorded the southern guitarfish Rhinobatos percellens as having the same distribution, but Refi (1973) concludes that R. percellens does not occur on the Argentinean coast, and that these Argentinean records of R. percellens refer to R. horkelii (Devincenzi 1920, Lahille 1921, Devincenzi and Barattini 1926, Lopez 1946 in Refi 1973). Specimens examined by all those authors are deposited at the Museo de La Plata, Argentina. Fishery statistics of R. horkelii show important commercial catches of the species only in southern Brazil between latitudes 28 to 34°S. This is evidence that the species has its centre of distribution in southern Brazil and is scarce elsewhere (Vooren et al. 2005).|
|FAO Marine Fishing Areas:||
Atlantic – southwest
|Range Map:||Click here to open the map viewer and explore range.|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||This is a coastal species. Its seasonal migration and breeding cycle in southern Brazil are described below, from data published by Lessa (1982), Lessa et al. (1986) and Vooren et al. (2005). In southern Brazil the adults migrate to coastal waters with depths of less than 20 m from November to March. At that time artisanal fisheries operate from the beaches, and the guitarfish catches are 98% pregnant females. Adult males reach the beach fishing grounds at the end of February. Parturition and mating take place in March. Soon after, the males and females return to deeper waters and disperse to depths of 40 to 150 m over the continental shelf. Newborn pups and juveniles remain in shallow waters throughout the year. The smallest pregnant females found were 91 cm TL. The proportion of pregnant females increases with size, and 100% pregnancy occurs in females of >119 cm TL. Litter size is 4 to 12 pups, the number increasing with the size of the mother. Pregnancy is divided into two stages: a) Period of dormancy (embryonic diapause). This occurs from April to November, while the pregnant females are in deeper water (40 to 150 m), at temperatures of 13 to 18°C. Ovulation occurs in April when the fertilized eggs are enclosed within a common shell (candle). However, they remain dormant in the uterus with no embryonic development. b) Period of embryonic development. This requires higher summer temperatures in shallow waters and does not start until females return to shallow waters in November. From November to March, bottom temperature at depths of 10 to 20 m near the coast are 20 to 25°C. Embryonic development starts when the common shell (or candle) breaks up in December and ends with birth in February. Embryos are found only from December to late February. Their size increases from 1 cm TL in December to 29 cm TL in February, when birth takes place. A von Bertalanffy growth curve was established on the basis of annuli counts in vertebrae of 289 individuals from 36 to 123.1 cm TL. Annulus formation occurred in September, and growth parameters obtained for observed length-at-age were: L8=135.51 cm TL; K=0.1940 and To=-1.0785. Full maturity is reached at nine years for females and six years for males. The fact that the females grow to a larger size and reach greater ages (longevity 28 years) than the males (longevity 15 years) reflect differences in mortality rates between the sexes. From the analysis of catch curves (Ricker 1975) carried out on a large sample collected in 1983 in southern Brazil, it was observed that females recruited to the fishery from the age of four years, with complete recruitment at the age of nine years. A total mortality coefficient of Z=0.72 was calculated for females. In contrast, males entered the fisheries at the age of four years, and recruitment was complete at the age of six years. A mortality coefficient Z=0.98 was determined for males.|
|Movement patterns:||Full Migrant|
Rhinobatos horkelii was formerly abundant in southern Brazil, where in the 1980s it was the only economically important species of batoid caught in the area. In southern Brazil the main fishery ports are Rio Grande and Itajaí. Rhinobatos horkelii is fished by otter trawl, pair trawl, shrimp trawl, beach seine and gillnet (Klippel et al. 2005). Statistics on the annual catch and effort of the Rio Grande fleet of otter trawl and pair trawl provide a record from 1975 onwards. Total landings at Rio Grande by fishery methods combined increased from 842 t in 1975 to 1804t in 1984 and then declined continuously to 157 t in 2001. The average trawl CPUE of R. horkelii in southern Brazil over the years 1993 to 1999 was 17% of that observed during 1975 to 1986, indicating a decline in abundance of >80% since 1986 in southern Brazil (Miranda and Vooren 2003, Vooren et al. 2005).
Catches increased slightly after 2000 to 0.3 tons/trip in 2002, when trawl fleets from Itajaí and Rio Grande exploited a refuge area for a part of the Rio Grande do Sul population (Martins and Schwingel 2003, Vooren et al. 2005). Following this, CPUE fell again by 31% from 2002 to 2003 and the overall population is still considered to be at critically low levels (Vooren et al. 2005). Monitoring of beach-seine catches and reports from fishers in 2004 confirmed that R. horkelii is now scarce in coastal waters, and in a trawl survey in February 2004 of 62 trawl stations at depths of 7 to 20 m along 700 km of southern Brazil coastline, only 23 individuals were caught, 17 of which were juveniles (Vooren et al. 2005).
Across the rest of the species range, where it is scarce, inshore fishing pressure is generally high. For example, a coastal multispecies demersal trawl fishery operates off Quequén, Argentina (38°37'S, 58°50'S), in which bycatch of batoids fluctuates seasonally between 44.5% and 67.5% of total catch (Tamini et. al. 2006).
|Conservation Actions:||Rhinobatos horkelii has been listed as Critically Endangered on the Brazilian federal law of Threatened and Overexploited Aquatic Species since 2004 (Vooren and Klippel 2005). Permits for directed fishing of the species are no longer issued, transport and sale of the species are prohibited, and incidental catches of the species must be discarded at sea. This law is gradually becoming more effectively enforced. Also the prohibition of trawl fishing within three nautical miles from the coast of southern Brazil is now being enforced satisfactorily. However, the species is still caught as bycatch in the legally permitted coastal gillnet fisheries and offshore trawl and gillnet fisheries. For the conservation and recovery of the stocks of R. horkelii and several other critically threatened elasmobranchs, Vooren et al. (2005) recommend fishery exclusion areas (no-take areas) on the continental shelf of southern Brazil (latitudes 28°30'S to 34°30'S) as follows: one continuous area along the entire coastline, within 5 to 10 nautical miles from the coast, for the protection of the coastal nursery; three large shelf areas, one starting from the coastline, two from mid-shelf, and all three extending to the shelf edge. The latter three protected shelf areas are critical wintering areas of R. horkelii. The four proposed no-take areas constitute 46% of the total shelf area. The Law of Threatened and Overexploited Aquatic Species determines that actions for the recovery of these species must be taken. In this context, steps will be taken towards implementing the proposed no-take areas on the shelf of southern Brazil.|
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Lessa, R., Vooren, C.M. and Lahaye, J. 1986. Desenvolvimento e ciclo sexual das femeas, migrações e fecundidade da Viola, Rhinobatos horkelii (Müller and Henle, 1841) do Sul do Brasil. Fundação Universidade Federal do Rio Grande. Atlântica 8: 5-34.
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Vooren, C.M., Lessa, R.P. and Klippel, S. 2005. Biologia e status de conservação da viola Rhinobatos horkelii. In: C.M. Vooren and S. Klippel (eds). Ações para a conservação d e tubarões e raias no sul do Brasil, pp. 33-56. Porto Alegre: Igaré.
|Citation:||Lessa, R. & Vooren, C.M. 2007. Rhinobatos horkelii. The IUCN Red List of Threatened Species 2007: e.T41064A10396152. http://dx.doi.org/10.2305/IUCN.UK.2007.RLTS.T41064A10396152.en . Downloaded on 10 October 2015.|
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