Danielle M. Knip

Habitat use and spatial segregation of adult spottail sharks Carcharhinus sorrah in tropical nearshore waters

An array of acoustic receivers deployed in Cleveland Bay, north Queensland, Australia, passively tracked 20 adult spottail sharks Carcharhinus sorrah over 2 years (2009-2010) to define patterns in movement and habitat use. Individuals were present in the study site for long periods, ranging from 8 to 408 days (mean = 185). Size and location of home ranges did not vary over time. A high level of segregation occurred among C. sorrah, with individuals using different types of habitat and showing strong attachment to specific regions. The depth of habitat individuals used varied between sexes. Males tended to use a narrow range of habitat depths within the study site (2·8-6·0 m), whereas females used shallower habitats (1·4-6·2 m) and displayed a seasonal shift in the depth of habitat used. Mean monthly habitat depth used varied by as much as 2 m for females, with individuals using shallower habitats during the winter months. Long-term presence and consistent home ranges suggest that Cleveland Bay provides important habitat for C. sorrah. By defining patterns in the use of nearshore habitats for C. sorrah, this study improves the understanding of the movement and habitat use of smaller-bodied coastal sharks and may help provide guidance for the management of their populations.

Wet-season effects on the distribution of juvenile pigeye sharks, Carcharhinus amboinensis, in tropical nearshore waters

Tropical nearshore environments are highly dynamic systems owing to extreme freshwater flow and flooding episodes that occur in wet-season months. We hypothesised that juvenile sharks in tropical nearshore waters respond to seasonal freshwater inflow by moving away from areas of strong flow. An array of fifty-eight acoustic receivers deployed in Cleveland Bay, north Queensland, Australia, passively tracked thirty-two juvenile pigeye sharks, Carcharhinus amboinensis, throughout two wet seasons from 2008 to 2010. Influences associated with wet seasons appeared to play a role in habitat use by juvenile C. amboinensis in this region. Home ranges and distribution of individuals showed distinct changes, with individuals moving north away from sources of freshwater inflow during high flows. The location of individuals within the bay was strongly influenced by freshwater inflow in both years. Although juvenile C. amboinensis moved in response to freshwater inflow, home-range sizes remained stable, and the amount of space individuals used did not change in relation to freshwater inflow. By defining the response of juvenile sharks to highly variable freshwater flow events, this research provides useful information for understanding species behaviour in a dynamic and changing climate, and contributes towards effective management of tropical river systems.

A comparison between traditional kernel-based methods and network analysis: an example from two nearshore shark species

Understanding how marine species use their environment has become increasingly important in management and conservation. Acoustic monitoring allows long-term tracking of marine animal movement that is traditionally analysed using kernel-based home range estimators. These traditional methods, however, are limited because they do not examine movement pathways within activity spaces. Network analysis (NA) provides an alternative approach to traditional home range analysis that treats acoustic receivers as network nodes and analyses movement between nodes. To investigate the utility of NA in identifying core use areas and compare the results with traditional analysis, a case study using acoustically monitored coastal sharks was conducted. To make direct comparisons with static traditional analysis a temporal scale was not explicitly explored. Comparison of traditional analysis and NA demonstrated that both methods provided similar results for identifying core use areas (50% kernel utilization distribution (KUD) equivalent), but that NA tended to overestimate general use areas (95% KUD equivalent) compared to kernel-based methods. Furthermore, frequent bidirectional movements within core use areas were identified by NA, indicating the importance of movement corridors within or between core areas. Movements between acoustic receivers outside core use areas were less frequent and unidirectional suggesting transiting movements. Therefore, NA may be a practical alternative to traditional home range metrics by providing useful data interpretation that allows for a comprehensive picture of animal movement, including identifying core use areas and pathways used.