Observations on the Reproductive Seasonality of Atlantoraja Platana (Gunther, 1880), an Intensively Fished Skate Endemic to the Southwestern Atlantic Ocean

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Abstract

Specimens of the La Plata skate (Atlantoraja platana) were collected monthly from commercial fishing landings at Guaruja, Sao Paulo State, Brazil, from March of 2005 to April of 2006. One hundred males ranging from 13.1 to 70.0 cm and 88 females ranging from 12.5 to 76.0 cm of total length were collected and their gonads analysed to determine maturity stages. Gonadosomatic and hepatosomatic indexes did not significantly vary among seasons between the sexes. Ovulation and egg-laying were continuous throughout the year. These observations suggest an annual cycle with eventual -though not well delimited-peaks in the reproductive activity. This pattern has been reported for skates of the same genus and for other species. Atlantoraja platana is intensely exploited, though as a non-target species, and retained for exportation over the South and Southeast Brazilian coast. For these reasons the species is already considered 'vulnerable' by the IUCN Red list of Threatened Species. Present data may be the base for future studies in order to protect the populations of A. platana from local disappearance.

Introduction

Genus Atlantoraja Menni, 1972, is endemic to the Southwestern Atlantic Ocean (McEachran and Aschliman 2004). The La Plata skate's (Atlantoraja platana) (Gunther 1880) distribution ranges from Sao Paulo's littoral area to Argentina, and is common in Rio Grande do Sul State in Southern Brazil (Figueiredo 1977). In Southern Brazil, it is found at depths of 40-100 m (Vooren 1997), though Marcal (2003) recorded its occurrence at up to 231 m deep. In the Southeastern Brazilian continental shell A. platana is commonly caught in the range of 20-120 m deep (Oddone and Amorim 2007).

The assessment of chondrichthyan populations requires a quantitative approach to the study of reproduction (Walker 2005). Oddone and Amorim (2007) reported the size at maturity of male and female A. platana in Southeastern Brazil. Data on the reproduction of A. platana were also provided by Marcal (2003), Oddone et al. (2008) and Oddone and Vooren (2008). However, so far, the trend in the seasonality of the reproduction of this species is unknown. But Vooren and Klippel (2005) demonstrated that intensive fisheries in the South-western Atlantic have led to overexploitation of several species of demersal elasmobranchs, such as the congeneric Atlantoraja castelnaui and A. cyclophora, already 'endangered' and 'vulnerable' species according to the IUCN Red list of Threatened Species. (Hozbor et al. 2004, Massa et al. 2006).

Over the past thirty years, catches of rajoids (skates) have increased in the western Atlantic, mainly as a by-catch of bony-fish target fisheries, yet, sustainable catch rates are completely unknown (Frisk et al. 2002). This situation is also valid for the Brazilian continental shell where bottom trawling fisheries affect populations of A. Platana, which is incidentally captured. Thus, the knowledge of all the events compounding the reproduction of a species needs to be known in order to make decisions on stock management of chondrichthyan fishes.

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The conservation status of A. platana is considered 'vulnerable' by the IUCN Red list of Threatened Species (San Martin et al. 2007). This is a matter of concern for a commercially exploited species because when life history characteristics are coupled with the selective removal of large individuals (as is the case for this species) of a given population subjected to intense fishery pressure, such a population may become highly susceptible to overexploitation and even disappearance, as has been the case for several rajoids (Brander 1981; Hoenig and Gruber 1990). Specimens of Atlantoraja spp. are commonly landed and sold in Santos and Guaruja (Sao Paulo state, Brazil), especially the largest individuals (Oddone, unpublished data). In the present paper, we aim to analyze the trend of the reproductive variables for A. …