812 



Fishery Bulletin 98(4) 



daily doses of lOmg/kg for elasmo- 

 branchs (Stoskopf, 1993). Despite its 

 antibiotic properties it has been dem- 

 onstrated to have toxic effects on some 

 teleosts (e.g. Marking et al., 1988). 

 Although widely used, the impact 

 of OTC injection on the growth of 

 elasmobranchs has been poorly stud- 

 ied. Tanaka ( 1990) demonstrated that 

 injection of OTC did not affect the 

 growth of juvenile Orectolobus japon- 

 icus at normal dosages. Gelsleichter 

 et al. (1998) reported that OTC injec- 

 tion did not affect the growth of nurse 

 sharks (Ginglymostoma cirratum ), but 

 that it may have some level of hepato- 

 toxicity. In the only reported study of 

 a wild elasmobranchs, Natanson et al. 

 (1999) reported that four Galeocerdo 

 cuvier specimens injected with OTC 

 did not have growth rates different 

 from those not injected with OTC. 



The dusky shark, Carcharhinus 

 obscurus, is a large species of coastal 



shark that occurs in tropical, subtropical, and temperate 

 oceans world-wide (Compagno, 1984). They are born at 

 70-100 cm total length (TL), mature at approximately 280 

 cm TL, and grow to at least 365 cm TL (Last and Stevens, 

 1994). Natanson et al. (1995) provided estimates of growth 

 parameters for C. obscurus from the western North Atlantic, 

 using length-at-age data and length-frequency data. These 

 data indicate that C. obscurus is slow growing, reaches 

 maturity at approximately 19-21 years, and possibly lives 

 to 45 years. Natanson and Kohler (1996) estimated growth 

 parameters for C. obscurus from the southwest Indian 

 Ocean, using length-at-age data that indicated growth sim- 

 ilar to that from the western North Atlantic. 



A demersal gillnet fishery in southwestern Australia 

 targets neonate and small juvenile C. obscurus. mostly 

 from February to June. The fishery has operated since 

 the 1940s, but significant catches of C. obscur'us were not 

 taken until the mid 1970s ( Simpfendorfer and Donohue, 

 1998). Increasing fishing effort in the 1980s lead to con- 

 cerns about the status of this resource and prompted a 

 research project that included a tag and release study to 

 estimate mortality (Simpfendorfer, 1999), movement, and 

 growth parameters for C. obscui-us. The aims of this paper 

 are 1 ) to estimate growth rates and growth parameters for 

 C. obscurus based on tag-recapture data using four differ- 

 ent methods, 2) to investigate differences in growth due to 

 sex or injection with OTC, and 3 ) to estimate growth vari- 

 ability and measurement error 



Materials and methods 



114°E 116°E 118°E 120°E 122°E 124°E 126°E 



128°E 



Tagging 



Figure 1 



Numbers of juvenile Carcharhinus obscurus released in one-degree geographic 

 blocks off southwestern Australia. 



net vessels operating in southwestern Australia. These 

 vessels operate in continental shelf waters in depths of 

 7-100 m. The gill nets are constructed of 16.5-cm and 

 17.8-cm (stretched) mesh monofilament netting that is 1.5 

 m to 2.0 m deep. Nets are 3-7 km in length and are set on 

 the sea floor for periods of 7-24 hours. 



Individual C. obscurus were measured (fork length [FL): 

 the distance from the tip of the snout to the caudal fork 

 measured as a straight line) to the nearest centimeter, the 

 sex of each fish was determined, and each fish was exam- 

 ined for the presence of an open umbilical scar, and tagged 

 with an individually numbered Jumbo Rototag (Dalton 

 Supplies, Woolgoolga. New South Wales, Australia) in the 

 first dorsal fin. Approximately one in every three animals 

 was injected with OTC (25 mg/kg) to mark the vertebral 

 centra for age validation studies. 



A total of 2199 juvenile C. obscurus were released from 

 March 1994 to June 1996 between the Western Australian 

 and South Australian border (129°E) and Kalbarri (29°S) 

 (Fig. 1). Tagged sharks ranged in size from 53 cm FL to 

 108 cm FL but most were 70-81 cm FL (Fig. 2). One thou- 

 sand seven hundred and thirty (78.7%) had open umbilical 

 scars; 1062 were female, 1100 were male, and 37 were of 

 unknown sex; 879 were injected with OTC and 1320 were 

 not injected. Information on tag-recaptures, including date 

 and location of recapture, and length were reported by 

 commercial fishermen and research observers operating 

 in the demersal gillnet fishery. Fishermen were provided 

 with tape measures and trained to measure fork length in 

 an attempt to improve the accuracy of recapture data. 



Analysis 



Neonate and juvenile C. obscurus were caught between 

 March 1994 and June 1996 by commercial demersal gill- 



Growth rates of five gi'oups of juvenile C. obscurus — males, 

 females, OTC injected, non-OTC-injected, and all recap- 

 tures combined — were estimated by using four different 



