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Triakis Megalopterus) from the Southeast Coast of South Africa

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Triakis Megalopterus) from the Southeast Coast of South Africa Age validation, growth, mortality, and demographic modeling of spotted gully shark (Triakis megalopterus) from the southeast coast of South Africa Item Type article Authors Booth, Anthony J.; Foulis, Alan J.; Smale, Malcolm J. Download date 03/10/2021 23:04:43 Link to Item http://hdl.handle.net/1834/25378 101 Abstract—This study documents vali- Age validation, growth, mortality, dation of vertebral band-pair forma- tion in spotted gully shark (Triakis and demographic modeling megalopterus) with the use of fluoro- chrome injection and tagging of cap- of spotted gully shark (Triakis megalopterus) tive and wild sharks over a 21-year from the southeast coast of South Africa period. Growth and mortality rates of T. megalopterus were also estimated and a demographic analysis of the Anthony J. Booth (contact author)1 species was conducted. Of the 23 OTC Alan J. Foulis1 (oxytetracycline)-marked vertebrae 2 examined (12 from captive and 11 Malcolm J. Smale from wild sharks), seven vertebrae Email address for contact author: [email protected] (three from captive and four from 1 Department of Ichthyology and Fisheries Science wild sharks) exhibited chelation of Rhodes University the OTC and fluoresced under ultra- P.O. Box 94 violet light. It was concluded that a Grahamstown, 6140, South Africa single opaque and translucent band pair was deposited annually up to at 2 Port Elizabeth Museum least 25 years of age, the maximum P.O. Box 13147 age recorded. Reader precision was Humewood, 6013, South Africa assessed by using an index of aver- and age percent error calculated at 5%. Department of Zoology No significant differences were found Nelson Mandela Metropolitan University between male and female growth pat- P.O. Box 77000 terns (P>0.05), and von Bertalanffy Port Elizabeth, 6013, South Africa growth model parameters for com- bined sexes were estimated to be L∞ =1711.07 mm TL, k=0.11/yr and t0= –2.43 yr (n=86). Natural mortal- ity was estimated at 0.17/yr. Age at maturity was estimated at 11 years Spotted gully shark (Triakis mega- Demographic modeling has been for males and 15 years for females. lopterus, Smith, 1839), is one of five conducted on many elasmobranch Results of the demographic analysis Triakis species and is endemic to populations when there are insuf- showed that the population, in the southern Africa (Compagno, 1988). ficient catch, effort, and abundance absence of fishing mortality, was Its distribution range extends from data available to conduct a full stock stable and not significantly different northern Namibia (although anec- assessment (Simpfendorfer, 1998; from zero and particularly sensitive dotal information indicates that it Romine et al., 2009). Demographic to overfishing. At the current age is caught as far north as Angola) modeling is a popular approach be- at first capture and natural mortal- ity rate, the fishing mortality rate southward around the coast to Coffee cause it provides the best available required to result in negative popu- Bay in the Eastern Cape Province, description of the population being lation growth was low at F>0.004/ South Africa (Compagno, 1988; Com- studied given several life history yr. Elasticity analysis revealed that pagno et al., 2005). It is a shallow parameters. Demographic modeling juvenile survival was the principal water (<50 m) (Compagno et al., therefore provides a compromise factor in explaining variability in 1989; Smale and Goosen, 1999), between simple life history tables population growth rate. demersal species that is recreation- and more detailed stock assess- ally important to shore and ski-boat ment models. Demographic models anglers. With the exception of some became popular in the 1990s and information pertaining to its repro- are now the most widely used popu- ductive and feeding biology (Smale lation models used to assess shark and Goosen, 1999) there is little populations (Simpfendorfer, 2005). information available to guide its A fundamental requirement for the management. Given its narrow dis- application of age-structured demo- tribution range and small popula- graphic models is that ages of sharks tion size, it could be vulnerable to are available. overexploitation in a manner similar Correctly determining the age of Manuscript submitted 14 May 2010. Manuscript accepted 3 November 2010. to its congener, T. semifasciata, that fish, particularly elasmobranchs, is Fish. Bull. 109:101–112 (2011). has declined in abundance and is crucial if (unbiased) time-based life now carefully managed (Smith and history rates such as growth, ma- The views and opinions expressed Abramson, 1990; Cailliet, 1992). An turity, and mortality are to be esti- or implied in this article are those of the author (or authors) and do not necessarily equivalent analysis is required for mated. Although numerous fish age reflect the position of the National Marine T. megalopterus to assess its vulner- and growth studies have been under- Fisheries Service, NOAA. ability to fishing pressure. taken, remarkably few have included 102 Fishery Bulletin 109(1) validation (Campana, 2001; Cailliet and Goldman, OTC. All 12 display sharks and 11 tagged wild sharks 2004). Campana (2001) defines validation as either the were sacrificed for vertebral analysis. validation of the periodicity of growth increments, or as In addition, a total of 129 spotted gully sharks were the validation of the age estimate made by reader(s). collected opportunistically from ski-boat fishermen, fish- In a recent review, Campana (2001) noted that of the ing competitions, and research cruises over a 21-year 372 papers in which age validation was reported, only period (1984–2009) between Cape St Francis and Cof- 15% of the papers actually incorporated validation of fee Bay, South Africa. Vertebrae were collected from a the absolute age of wild fish. Therefore, when conduct- subsample of 96 sharks. Total length (TL) and sex were ing an age and growth study it is necessary to specify recorded for all these sharks. whether one or both validation goals are met and which validation method is used. Age determination Three methods of validation are typically used. These are edge analysis (also known as marginal increment Between five and eight vertebrae were removed from or zone analysis), carbon dating, and mark-recapture of the trunk region in the vicinity of the first dorsal fin, tagged fish injected with a calcium-chelating fluoresc- soaked in 4.5% sodium hypochlorite for 15–45 minutes ing chemical such as the antibiotic, oxytetracycline to remove excess connective tissue, and were either hydrochloride (OTC). All of these methods have been stored in 70–80% ethyl alcohol or frozen (Yudin and used to validate elasmobranch ages (Campana et al., Cailliet, 1990). Cleaned vertebrae were embedded in 2002; Smith et al., 2003; Cailliet and Goldman, 2004; polyester casting resin and sectioned with a diamond- Ardizzone et al., 2006; Chen et al., 2007). The most bladed saw along the sagittal plane to a thickness commonly used and most reliable validation method is of 0.6 mm (Natanson et al., 2006; Rizzo et al.1), and tagging with OTC (Campana, 2001). mounted on glass slides with DPX mountant (Lasec, In studies of elasmobranchs by OTC-injection, sharks South Africa). are measured, injected with OTC, tagged, and released. Band pairs, defined as one optically opaque and The date of release is then noted. Once the shark is one optically translucent band were counted by us- recaptured, the number of vertebral band pairs distal ing a dissecting microscope with transmitted white to the fluorescent mark is compared with the time be- light (460–490 nm). OTC-injected specimens were also tween release and recapture of the shark. This experi- viewed with an Olympus BX60 microscope (Olympus, mental approach can be problematic if recapture rates Johannesburg, South Africa) under ultraviolet trans- are low. Validating elasmobranch age estimates can be mitted light (510–550 nm). Each specimen was aged time consuming because recapture rates are generally twice, three weeks apart by a single reader, without low and there is a long period of time between collect- prior knowledge of the length or sex of the specimen. ing a sufficient number of samples and analyzing the Counts were accepted only if both counts were in agree- vertebrae. ment. If the estimated number of bands differed by We developed a Leslie matrix-based demographic two or less, the specimen was recounted and the final model to evaluate the ability of T. megalopterus to sus- count was accepted as the agreed upon number; if not, tain increased levels and patterns of fishing pressure. the specimen was discarded. If the third count did not We also validated the age of T. megalopterus using concur with one of the previous two counts, the sample oxytetracycline to estimate growth and mortality rates, was rejected. important demographic model parameters. An age-bias plot was used to graphically assess the readings and their associated agreement (Campana, 2001; Natanson et al., 2006). A t-test was conducted on Materials and methods the slope of the age-bias plot (the linear regression of the second against the first age readings) to test the General overview null hypothesis that the slope was equal to one. Com- parisons of reader accuracy for each age were made by In 1994, a tagging program incorporating researchers using a paired t-test, and a c2-test of symmetry was and trained volunteer fishermen was initiated at the used to test for systematic bias in the determination of Port Elizabeth Museum in order to obtain age valida- age (Hoenig et al., 1995). tion, movement studies, and population dynamics of the The variability of the within-reader age estimates spotted gully shark. A total of 402 wild sharks (113 male, was estimated with an index of average percent error 230 female, and 59 of undetermined sex) were tagged, injected with OTC at a dosage of 50 mg/kg (Tanaka, 1990), and released.
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