2007 SOUTHEASTERN NATURALIST 6(1):183–186 Occurrence of Carcharhinus isodon (Finetooth Shark) in Florida Bay Tonya R. Wiley1,* and Colin A. Simpfendorfer1 Abstract - Carcharhinus isodon (finetooth shark) is a migratory shark found in coastal waters of the southeastern United States and is well documented in the waters of north Florida in both the Gulf of Mexico and the Atlantic Ocean. The southernmost reports are from Lemon Bay (27°N), just north of Charlotte Harbor on the west coast and from Port Salerno (27°N) on the east coast. Four C. isodon were captured on bottom-set longline in Florida Bay, just north of 25°N latitude, during routine sampling for Pristis pectinata (smalltooth sawfish). These captures extend the southern range of C. isodon in Florida to approximately 25ºN and increase the likelihood of exchange between the Atlantic and Gulf stocks. Introduction Carcharhinus isodon Müller and Henle (finetooth shark) is a moderate- sized species of shark found in shallow US coastal waters from North Carolina to Florida and in the northern Gulf of Mexico (Bethea et al. 2004, Carlson et al. 2003, Castro 1993). Compagno (1988) reported C. isodon occur from New York south to Florida, and from Cuba in the Gulf of Mexico to Texas and Mexico, also Guyana and southern Brazil. Castro (1983) described the species as inhabiting the Atlantic coast of North America from New York to Florida and the Gulf of Mexico. Carcharhinus isodon inhabits the western Atlantic from North Carolina to Brazil and is common off the southeastern United States, where it spends the summer off Georgia and the Carolinas and winters off Florida (Castro 1993). The species is known to migrate southward in early fall as the water temperatures decrease (Carlson and Brusher 1999, Castro 1993). There are few records from the Caribbean and south Atlantic and no valid records of this species from Cuba (Castro 1993). In Florida, C. isodon have been reported in the Florida panhandle (Bethea et al. 2004, Carlson et al. 2003), Port Salerno (Springer 1950), Lemon Bay (Clark and von Schmidt 1965), Melbourne Beach (Dodrill 1977), and Daytona Beach (Castro 1993). Therefore, the range of C. isodon has been documented as north of 27°N, the latitude of Port Salerno on the Atlantic coast and Lemon Bay on the Gulf of Mexico coast. However, the capture of four specimens just north of 25°N in Florida Bay represents an extension to the southern range of this species in the United States. 1Mote Marine Laboratory, Center for Shark Research, 1600 Ken Thompson Parkway, Sarasota, FL 34236. *Corresponding author - twiley@mote.org. 184 Southeastern Naturalist Vol. 6, No. 1 Methods Longline sets targeting P. pectinata, have been carried out since 2000 from Tampa Bay to the outer Florida Keys. Longlines consisted of an 800-m bottom-set mainline of 8-mm braided nylon rope anchored and marked with buoys at each end. Gangions were constructed of 1 m of 5-mm braided nylon cord and 1 m of stainless steel wire leader. Mustad tuna circle hooks (12/0 to 16/0) were baited with frozen Mugil cepahlus Linnaeus or M. curema Valenciennes (mullet). The date, time, and location of each set were recorded. Physical parameters (water temperature, salinity, and dissolved oxygen) were recorded at each sampling location midway between the surface and bottom using a YSI 85 water-quality meter. Sharks caught during surveys were identified and sexed. Four measurements of length to the nearest 0.5 cm were taken when possible: precaudal (PCL), fork (FL), total (TL) and stretched total length (STL). Locations of longline sets and C. isodon captures were plotted using a geographic information system (ArcView 3.3). Results During 1233 longline sets from Tampa Bay to Key West, four C. isodon were captured in Florida Bay (Fig. 1). Three were captured on 12 February Figure 1. Locations of 1233 longline sets (x), three Carcharhinus isodon captured 12 February 2003 (A), and one captured 10 February 2005 (B). 2007 T.R. Wiley and C.A. Simpfendorfer 185 2003 just south of East Cape Canal (25°7'24"N, 80°3'59"W) and the fourth on 10 February 2005 near Curry Key (25°7'54"N, 80°57'48"W) west of Flamingo. The first three were caught at a depth of 1.3 m and the fourth at 1.1 m. Despite extensive year-round sampling, the four C. isodon were all captured in February in water temperatures of 22.2 and 23.5°C. The two male C. isodon were 111 and 130 cm TL, and the two females were 115 and estimated 100 cm TL. Discussion The water temperatures and timing of the C. isodon captures was consistent with Castro's (1993) reporting that they leave the Carolinas in early fall and migrate southward as the surface water temperature decreases below 20°C. Dodrill (1977) reported the species to be found off Melbourne Beach from November to April. Similarly, Carlson and Brusher (2004) reported that young of the year and juveniles remain in Apalachicola Bay, in the Florida Panhandle, until fall, when they emigrate offshore into the Gulf of Mexico. It is unknown if the four C. isodon captured in Florida Bay are from the Gulf of Mexico or Atlantic Ocean populations; however, the timing of the captures coincides with the southward winter migration of both populations. Given that differences in reproductive biology have been reported between C. isodon occurring in the Atlantic and Gulf of Mexico, it is possible that there is little or no interaction between these two stocks. Castro (1993) reports males reach maturity at about 130 cm and females at 135 cm TL for animals from the Atlantic Ocean. Carlson et al. (2003) reported males reach maturity between 100 and 129 cm TL and females between 118 to 124 cm TL in the northern Gulf of Mexico. This separation of the Atlantic and Gulf stocks may be mediated with thermal preference, which restricts this species from reaching the Florida Keys where interaction may occur. The occurrence in Florida Bay at the southern tip of Florida increases the likelihood that there is greater movement or interaction between these two stocks than indicated by previous information on their distribution. The results of this finding extend the southern range of C. isodon in Florida to approximately 25ºN and increase the likelihood of exchange between the Atlantic and Gulf stocks. The low level of catch, despite extensive sampling in southwest Florida, suggests that their incidence in southern Florida is rare and only occurs when water temperatures are low during winter. Further research on the movements of this species, both in the Gulf of Mexico and along the Atlantic coast will be required to understand the seasonal movement patterns and extent of their southern movements. Acknowledgments We thank J. Castro for help with C. isodon reports and distribution. Funding from NOAA Fisheries Office of Protected Resources, National Fish and Wildlife 186 Southeastern Naturalist Vol. 6, No. 1 Foundation, National Geographic Committee for Research and Exploration, The Disney Wildlife Conservation Fund, and Florida Fish and Wildlife Conservation Commission supported this research. Literature cited Bethea, D.M., J.A. Buckel, and J.K. Carlson. 2004. Foraging ecology of the early life stages of four sympatric shark species. Marine Ecology Progress Series 268:245–264. 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