Potential Sources of Survey Bias Associated with Hand‐Retrieved Longline Catches of Subadult Sharks in Georgia Estuaries

Abstract

AbstractLunar phase, tide stage, and bait type were examined as potential sources of survey bias affecting catch rates of subadult sharks on hand‐retrieved longlines in Georgia estuaries. Sampling occurred during April–September from 2001 to 2003 in eight of Georgia's nine estuaries; this sampling period coincides with the pupping seasons for a variety of shark species. The effects of lunar phase and tide stage on catch rates of sharks were evaluated during 2001 and 2002. The effect of bait type (i.e., squid Loligo spp. or spot Leiostomus xanthurus) on shark catch rates was evaluated during 2003. During 2001 and 2002, 420 subadult sharks representing two families (Carcharhinidae and Sphyrnidae) and nine species were caught during 212 longline sets. Catch rates were evaluated for the four most commonly occurring shark species: the Atlantic sharpnose shark Rhizoprionodon terraenovae, bonnethead Sphyrna tiburo, blacktip shark Carcharhinus limbatus, and sandbar shark C. plumbeus. Neither lunar phase nor tide stage affected catch rates of any species other than blacktip sharks. In 2003, longline sets (n = 80) resulted in the capture of 177 subadult sharks from seven species. Overall catch rates were higher with squid bait than with spot, and bait type also affected shark species catch composition. Mean sizes of Atlantic sharpnose sharks were significantly different between the two bait types, indicating a possible ontogenetic shift in bait preference. For example, catch of neonate Atlantic sharpnose sharks was higher on hooks baited with squid than on hooks baited with spot, whereas catch of juveniles was higher on hooks baited with spot. Although timing of sampling events did not influence catch rates of subadult sharks, bait choice could bias estimates of abundance for certain life stages and affect the probability of detecting some species.