Movement and distribution of young bull sharks Carcharhinus leucas in a variable estuarine environment

The space utilization and distribution of young (<2 yr old) bull sharks Carcharhinus leucas within a 27 km stretch of the Caloosahatchee River estuary in Southwest Florida was examined using an array of acoustic monitors to define influences of environmental variables. A total of 56 young sharks from 3 cohorts (2003, 2004, 2005) were fitted with acoustic tags and monitored for up to 460 d. Sharks did not remain within the estuary continuously, but on average approximately onethird were present at any one time from each cohort. Salinity and freshwater inflow showed greatest influence on shark distribution, with temperature appearing to play a limited role. Although individuals occurred in salinities from 0.1 to 34, electivity analysis indicated that they generally avoided areas with salinity <7 and had an affinity for areas with salinities from 7 to at least 20. There were significant relationships between the mean location of a cohort within the estuary and salinity, with sharks occurring further up river when the river was more saline. These relationships were more pronounced for the youngest sharks, and strength of the relationship decreased with age. Since bull sharks are euryhaline, these results suggest that they may select environmental conditions via movement, possibly to reduce energetic costs associated with osmoregulation.