Newsroom: Do Sharks Abide by the Tide?

Animals move across a landscape for a variety of reasons – to find food, avoid prey, or reduce energy. However, it can be difficult to collect real-time information on the factors in the environment that may shape animal movement patterns (i.e. environmental context). In 2019, researchers at UC Davis explored the use of a new tool to provide context to organism movement, specifically in marine ecosystems. Alexandra McInturf and Dr. Damien Caillaud, along with several co-authors, employed a hydrodynamic current simulation model (a visualization of current movements) to examine the influence of local tidal patterns on broadnose sevengill sharks (Notorynchus cepedianus) in the San Francisco Bay Estuary.

*Sevengill sharks (*N. cepedianus) are common visitors to the waters of the San Francisco Bay Estuary. (Source)

Hydrodynamic models are typically used to model sediment transport, erosion, or the trajectory of oil spills. However, in this study, the UC Davis research team generated a model of the estuary that not only projected its current flow, but also incorporated fine-scale shark movement data (Figure 1). The goal of this model was to provide a greater understanding of whether current flow was a major driver of shark movement patterns in this habitat. The resulting visualization offered key insight into how frequently the sharks moved with the tide, particularly through the high-flow area underneath the Golden Gate Bridge. While the sharks appeared to “go with the flow” more often than not, their displacement throughout a given tidal cycle was not affected by current strength; that is, stronger tides did not elicit a greater distance traveled by the animals.

Figure 1. A visualization generated by the hydrodynamic current simulation model. Sevengill sharks were tagged with a transmitter and actively tracked (followed by boat) for 3-4 days each (see Figure 2). This movement data was then combined with the hydrodynamic current simulation model to visualize how sharks responded to the current. Here, the shark’s track is indicated in yellow (orange points were interpolated, as the shark was likely lost by the boat). In the background, each small red arrow (vector) indicates the strength and direction of the current in that location. Brighter red corresponds to areas of stronger current flow (McInturf et al. 2019).

The study concludes that there are likely numerous other environmental factors, in addition to the current, that affect shark movement in this area. For instance, the San Francisco Bay Estuary is a known foraging ground for this species, which could explain why sharks did not always move according to the prevailing current flow. More broadly, these results suggest that hydrodynamic current simulation models can be effective in providing context to animal movement data, particularly when collecting environmental information is logistically challenging or impossible. Such models could also be used to predict how animals may respond to infrastructure that affects current flow in coastal areas, such as through turbines or dredging.

**Figure 2. Maps of the San Francisco Bay Estuary, displaying movement data for each tagged sevengill shark**. Sharks were captured, tagged, and released northwest of Alcatraz Island (marked with a star), and tracked at three separate time periods in 2008. The red line overlays the Golden Gate Bridge. The inset in the upper left shows the location of the study in California, USA (McInturf et al. 2019).

For more information:

McInturf, A.G*., A.E. Steel*, M. Buckhorn*, P. Sandstrom*, C.J. Slager, N.A. Fangue*, A.P. Klimley*, D. Caillaud*. 2019. Use of a hydrodynamic model to examine behavioral response of broadnose sevengill sharks (Notorynchus cepedianus) to estuarine tidal flow. Environmental Biology of Fishes, 102(9), 1149-1159.

*Denotes UC Davis ABGG Affiliated Authors

Cover Photo Source: Wikimedia Commons