NOAA Exploitation and Reproduction of the Bullnose Ray (Myliobatis
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14 4 NOAA First U.S. Commissioner National Marine Fishery Bulletin established 1881 of Fisheries and founder Fisheries Service of Fishery Bulletin Abstract—Species of the genus Myli- obatis have been poorly assessed Exploitation and reproduction of the bullnose because of a lack of available in- ray (Myliobatis freminvillei) caught in an formation regarding their capture and life history. We provide valuable artisanal fishery in La Pared, Margarita data based on the commercial land- ings of the bullnose ray (Myliobatis Island, Venezuela freminvillei), which we studied dur- ing 2 separate time periods (October Alejandro Tagliafico (contact author)1,2 2005–December 2007 and January– 1 December 2013). A total of 187 in- Nicolás Ehemann dividuals were analyzed: 85 females María Salomé Rangel1,2 (24.0–96.0 cm in disc width [DW]) Néstor Rago3 and 102 males (22.8–118.0 cm DW). There was no difference in the over- Email address for contact author: [email protected] all sex ratio (females to males: 4:5); however, differences were found be- 1 tween the annual sex ratio in 2005, Escuela de Ciencias Aplicadas del Mar 2006, and 2013. Estimated capture Núcleo de Nueva Esparta per unit of effort for this species was Universidad de Oriente 0.8 individuals/trip (standard de- Calle La Marina viation [SD] 1.3) or 2.9 kg/trip (SD Boca de Río, 6304 Isla de Margarita, Venezuela 5.5), showing an increase in effort 2 National Marine Science Centre through time and significant differ- Southern Cross University ences between years. Approximately 2 Bay Drive 25% of both sexes were shorter than Coffs Harbour, New South Wales 2450, Australia the estimated median DW at ma- 3 Universidad Nacional de Costa Rica turity. No pattern in the reproduc- Escuela de Biología tive cycle was identified because of Avenida 1, Calle 9 the lack of landings during several Heredia 86-3000, Costa Rica months; however, mature individuals were observed frequently through- out the study period; gravid females were observed on only 2 occasions. This study provides baseline biologi- Elasmobranch overfishing has been dangered, near threatened, or have cal information on the life history of documented since the late 1940s not been assessed to date. The bull- the bullnose ray for necessary fish- (Castro, 2013) and recent analyses nose ray (Myliobatis freminvillei) is ery management. have demonstrated that population included within the data-deficient diminution has occurred around the group, as a result of the lack of avail- world, with at least one in 4 species able information required to evalu- being actually threatened with ex- ate population trends; as a conse- tinction (Dulvy et al., 2014). None- quence, the need for further investi- theless, numerous species of rays and gations of its biology and the fishery skates have been poorly studied, pre- are required to re-assess the status venting definition of their status and of this species in the IUCN Red List identification of threats (Molina and (Stehmann, 2009). Lopez, 2015). Such is the case for the The bullnose ray is a benthic–pe- genus Myliobatis, which comprises 11 lagic eagle ray widely distributed in Manuscript submitted 14 January 2015. species (White, 2014). Of those spe- the western Atlantic and captured Manuscript accepted 23 December 2015. cies, 6 have been documented as data mainly with artisanal long lines, gill- Fish. Bull. 114:144–152 (2016). deficient by the International Union nets, and industrial shrimp trawls Online publication date: 21 January 2016. for Conservation of Nature (IUCN) (Cervigón et al., 1992; Stehmann, doi: 10.7755/FB.114.2.2 in the IUCN Red List of Threatened 2009; Froese and Pauly, 2015). In Species, vers. 2015.4, available at Brazil, a similar species, the south- The views and opinions expressed or implied in this article are those of the website, accessed December 2015), ern eagle ray (M. goodei), is discarded author (or authors) and do not necessarily 2 have been classified as species of as bycatch of beach seining (Velasco reflect the position of the National least concern, and the remaining 3 et al., 2011). However, in many Ven- Marine Fisheries Service, NOAA. species have been categorized as en- ezuelan coastal communities, the Tagliafico et al.: Exploitation of Myliobatis freminvillei off the coast of Venezuela 14 5 bullnose ray is commercially valuable and is consumed size in this article refer to DW. Sexes were differenti- fresh or salted (Cervigón et al., 1992; Cervigón and Al- ated by the presence of claspers in males and their calá, 1999). Between 2006 and 2007, 13,000 kg of bull- absence in females (Conrath, 2005). Differences in the nose ray were landed in the state of Nueva Esparta proportions of sexes were tested with a chi-square test in Venezuela, representing 0.03% of the total landings (c2) (Zar, 1996). from 176 commercially fished species (Marval and Cer- Maturity was determined by macroscopic observa- vigón, 2009). Although no fishery specifically targets tion of reproductive organs at the landing site. Females this species, it is often landed as bycatch. were considered mature or immature on the basis of The results of this study provide insight on the life the presence or absence of fully developed ova in the history and landings data of the bullnose ray captured ovaries. Gravid females were recognized by the pres- as part of a small-scale, artisanal fishery in Margarita ence of embryos in the uterus, and post-gravid females Island—baseline information that would be useful for were recognized by the presence of a well-developed, management and conservation of this little known and large, and highly vascularized uterus (Conrath, 2005). potentially vulnerable species. For males, maturity was determined by inspecting and manipulating claspers. Organisms were considered ma- ture when claspers were strongly calcified and could be Materials and methods easily rotated around the base (Conrath, 2005). They also had to have an extensible distal portion (Conrath, La Pared is a small and isolated fishing community lo- 2005) and show the presence of seminal fluid (Bizarro cated on the northern coast of Macanao Peninsula, Mar- et al., 2007). Male bullnose ray that lacked these char- garita Island, in the state of Nueva Esparta in north- acteristics were considered to be immature. eastern Venezuela (11°03′32.53″N, 64°18′47.25″W). At The proportion of males to females at different re- this place, fishermen manage the resources, rotating productive stages was examined monthly to identify target species and using different gears depending the reproductive cycles of the species in the study area. on the time and abundance of fish (Tagliafico et al., Other measurements, such as follicle diameter, uterus 2013a). width, clasper length, gonad weight, and liver weight All bullnose ray used in analysis for this study were could not be obtained consistently because of the speed sampled from fish landed with bottom gillnets at a site at which animals were cut and sold on arrival at the in this community. Descriptions of the boats and fish- landing site. ing gear of this fishery can be found in Méndez-Arocha The median size at maturity was estimated for males (1963), Ginés et al. (1972), Iriarte (1997), Suárez and and females through the use of the logistic function: Bethencourt (2002), and González et al. (2006). –a(L –b) Sampling was carried out weekly from October Mf =1/1 + exp i , (1) 2005 through December 2007 and, 5 years later, from where Mf = the fraction of mature individuals; January through December 2013. The number of fish- a = the change in slope of Mf as a function of the ing boats, number of individual bullnose ray landed, size intervals (Li); and and the total weight landed (kilograms) were recorded b = the DW at 50% maturity (DW50). during each visit. Catch per unit of effort (CPUE) was defined as the number of individuals caught per trip Parameter estimates for a and b were obtained by using and the total kilograms caught per trip. To identify the least squares method with the statistical software statistical differences in the CPUE between years and R, vers 3.1.1 (R Core Team, 2014). A covariance analy- months (except for 2005, because sample data were sis of these logistic regressions by sexes was performed. collected over only 3 months during that year), homo- geneity of variances were tested with Levene’s test, followed by 2-way fixed-effects analyses of variance Results (ANOVA), based on permutations; all of these analy- ses were conducted with Primer 61 (PRIMER-E Ltd., For this study, 187 bullnose rays were analyzed. Gen- Ivybridge, U.K.; Clarke and Warwick, 2006) and PER- eral characteristics of these specimens are summarized MANOVA+ add-on software for Primer (Anderson et in Table 1. The size ranges for all organisms analyzed al., 2008). When statistical differences were detected, a were 22.8–118.0 cm DW and 0.2–19.0 kg. Both average Tukey’s honestly significant difference (HSD) test was DW and average weight increased slightly in recent performed. years (Table 1). The size-frequency distribution shows To determine the size-frequency distribution of the that, although the fishery-captured specimens were of bullnose ray that were captured and analyzed, the several size classes, the greatest number of individu- disc width (DW), of each individual was measured in als were between 55.0 and 60.0 cm DW (Fig. 1). The centimeters; therefore, all subsequent references to largest recorded individuals were males; however, only 4 males were larger than 75.0 cm DW, whereas 25 fe- males were found above that size. 1 Mention of trades names or commercial companies is for identification purposes only and does not imply endorsement As summarized in Table 2, during the 39-month by the National Marine Fisheries Service, NOAA.