1. Shark social preference experiments
Here we recorded the time spent and visits made to zones adjacent to compartments with different stimulus groups (i.e. shark vs. no shark). Sharks demonstrated:
- Active preferences to be social versus solitary.
- Preferred to associate with conspecific lemon vs. heterospecific nurse sharks.
- Spent more time with groups consisting of size-matched individuals.
These findings indicate that social preferences might well play an important role in the formation and composition of free-ranging groups of juvenile sharks.
2.Wild lemon shark social interactions and leadership
Observations of sharks were made from towers placed in a mangrove inlet, where it was possible to record group compositions of colour-code tagged wild juvenile lemon sharks. In two years we completed 83 observations days, re-sighting 38 sharks and resulting in:
- 1800 groups being recorded with 80% of these being pairs lasting <10mins in duration.
- Non-random social structure indicating repeated social interactions between sharks, explained by body length and possibly preference for relatives but not for sex.
- Evidence that some sharks lead more groups than others and usually larger sharks take up these dominate positions.
FACEBOOK for SHARKS
Shark daily social network: Nodes are individual sharks, lines between them means they were observed in a group together. Size of node relates to size of shark and black nodes (females) and white (males).
3. Predation risk influences aggregation formation and habitat selection
Field observations of juvenile lemon sharks were conducted in a tidally influenced mangrove-inlet. Acoustic tracking determined the movement patterns of juvenile lemon sharks and their predators (sub-adult lemon sharks) across the tidal cycle, results showed:
- Greater numbers of juvenile lemon sharks used the inlet for longer time periods at deeper and warmer high tide depths. This coincided with an increased presence of potential predators in the surrounding areas. See below figure showing tracking locations of predators at high (left) and low tide (right).
- Smaller juvenile lemon sharks visited the inlet more often and spent longer.
- Tracking data also revealed a tidally-influenced pattern, with both juvenile and sub-adult lemon sharks detected at locations inshore over the high tide and offshore during the low tide.
We concluded that the mangrove lake served as a ‘refuge’ for juvenile lemon sharks aggregating over the high tide, providing safe habitat when inshore areas become accessible to large predators. It is likely that these decisions are updated as the sharks grow and also with daily fluctuations in abiotic factors, such as water depth.
4. Novel on-shark acoustic receivers for investigating social behavior
Here we developed and tested the use of a novel technology, an animal-borne acoustic proximity receiver that records close-spatial associations between free-ranging sharks by detection of acoustic signals emitted from transmitters on other individuals. Results showed that:
- Receivers performed well in both captive and field experiments, are small enough to be carried by juvenile sharks (>80cm), able to detect other animals with transmitters at high resolution (0-4m), have adjustable detection range and variable battery power.
In combination with network theory this could be a particularly powerful approach for studies examining shark social structure and space use. We are collaborating with researchers in Australia to do just that on Port Jackson sharks.