Cape Cod White Shark Predation Study

An investigation into the predatory behavior of white sharks off Cape Cod, MA

ABSTRACT

For over three decades, the predatory interactions between white sharks and pinnipeds have been studied at well-known white shark aggregations sites, such as the South Farallon Islands (California) and Seal Island (South Africa). These areas are characterized by sharp drop-offs in water depth, which are thought to be crucial for successful predation. Recent observations of white sharks hunting off the coast of Massachusetts, however, suggest that sharks in this region have predatory success in much shallower water, without the tactical advantage of large changes in depth. By utilizing decoy targets in this area, the behavioral tactics of white sharks hunting pinnipeds in this region will documented and analyzed relative to the environmental conditions (e.g., depth). This information, augmented with natural predation accounts, will be compared to current scientific knowledge of predatory behavior found in other locations. This will further our understanding and provide new insight into the behavior of white sharks in the western north Atlantic.

INTRODUCTION

Predator-prey interactions are of central importance in any species’ ecology and have implications for population dynamics, management, and conservation (Walter, 1999: Musick, 1999). Very little is known about white sharks (Carcharodon carcharias) off the east coast of North America, but recent studies have begun to better understand this enigmatic predator. Every May through December, white sharks come to the coast of New England to hunt grey seals (Halichoerus grypus), an event rarely observed given the relative size of this coastline and seemingly rapid duration of the event.

While white shark predatory behavior is likely prey- and site-specific, these sharks are generally thought to hunt pinnipeds near sharp drop-offs, such as those described off the coasts of California, Mexico, and South Africa. However, the underwater bathymetry is radically different where grey seals are commonly found in New England. The outer coast of Cape Cod features broad shallow areas featuring sandbars and sandy terrain. Here, the white shark has returned to successfully hunt the grey seal, but unlike all other well-studied research sites, without the tactical advantage of deep drops-offs.

Published literature from other locations suggests that white sharks need a minimum depth of 23 ft. to remain undetected, as well as have enough vertical distance to build momentum for a successful pinniped predation. At Seal Island in South Africa, white sharks patrol in average water depths of 39-46 ft. However, attacks on pinnipeds occur more frequently in a bottom depth range of 85-100 ft. (Fallows et al. 2012).

Initial data from the first year of study off Cape Cod suggests that white sharks hunt grey seals in much shallower water. Sixteen decoy predations were observed in 2015. The average depth was 13.0ft with the median depth 12.4ft. There was a 9.7ft range with the minimum depth at 8.7ft, while the deepest decoy predation was at 18.4ft. The average water temperature for decoy predations was 62.9F, with the median water temperature 64.3F. The temperature range was 7.4 degrees with the lowest water temperature recorded at 58.8F.

Preliminary observations imply that white sharks off Cape Cod may prefer locations for hunting pinnipeds based on conducive environmental conditions and bathymetry, such as channels created by sand bars to facilitate their ambush attack. These potential “hunting grounds” may be frequented because of the underwater conditions and topography rather than solely on the presence of large aggregations of pinnipeds such as haul outs. Some of these locations may also be areas of high human density and activity.

KEY QUESTIONS

Potential questions answered through data collection and analysis include:

What is the size range of white sharks hunting pinnipeds?

What is the preferred depth range for pinniped predation?

What is the predation distance range from shore?

How do the following environmental variables influence predatory behavior: water depth, distance from shore, time of day, water visibility, water temperature, swell height and direction, light levels, wind direction, barometric pressure, tidal state?

What is the “high risk zone” for this region (factors where an attack could be considered highly likely)?

What are the approach patterns? Do the patterns utilize features of underwater topography such as sand bars?

What are the initial attack tactics - surface lunge, lateral snap, etc.?

What is the average water burst speed during predation?

Where are the anatomical locations of bites on the prey?

Are there social interactions between sharks during predation?

MATERIALS AND METHODS

In the 1980’s, researchers including Scott Anderson developed an innovative method for studying white sharks using decoys. The predatory behavior of the white shark off the coast of Cape Cod can be studied much more effectively by using decoys rather than relying solely on infrequently witnessed natural predations. This results in significantly more data for analysis and help ensure an adequate sample size.

During the period from June 31st - November, 2016, we will present decoys to white sharks to examine their behavior. All work will be conducted in cooperation with the Division of Marine Fisheries and coordinated with DMF biologist Gregory Skomal. Under direct guidance of spotter aircraft, several decoy versions will be deployed from the study vessel (see vessel list). Most of the decoys are being developed in conjunction with mechanical engineers from the Woods Hole Oceanographic Institute. Manufactured with 3D printing technology, these decoys are outfitted with an array of data sensors including accelerometers and bite pressure sensors. A few of these models will also have the capability to dive to specific depths facilitating mid-water data collection.

Spotter aircraft will provide an aerial perspective and data. Unlike natural predations, information on initial strike patterns and approaches can be obtained on each white shark investigation or attack on a decoy. Spotter aircraft and drones provide a unique vantage of white shark behavior before an initial predation attempt. Simultaneous high definition photographs and video footage will be captured from aerial, vessel, decoy, and underwater perspectives for analysis. When possible still images will also be obtained to aid in individual identification.

To ensure an adequate sample size, decoy targets will be used to collect the majority of data, but time will also be spent observing for natural predations, and gathering information to collaborate decoy attained data such as: location of attack (depth, visibility, distance from shore, etc.); seal species and size class; solitary, small group, or large group targeting; seal distribution; prey mass; duration of attack; size of shark responsible for attacks. Coordinates for all predations will later be combined with satellite overlays for geospatial analysis.

With the potential for increasing white shark encounters with humans in this region, the risk of white shark attacks on humans is also likely increased. The first recommendation for a shark-attack minimization strategy is to study the area and the sharks within it (Curtis et al. 2011). In addition to studying white shark predatory behavior, this study will quantify and delineate the “high risk zone” for this area, taking into consideration factors such as depth or distance from shore where an attack could be considered highly likely. We hope that our findings will therefore also further public safety information and practices for this region.

LITERATURE CITED

Curtis T, Bruce B, Cliff G, Dudley S, Klimley P, Kock A, Lea R, Lowe C, McCosker J, Skomal G, Werry J, West J, 2011. Responding to the Risk of White Shark Attack. Updated Statistics, Prevention, Control Methods, and Recommendations. In: Global Perspectives on the Biology and Life History of the White Shark, ed. Michael L. Domeier, Chapter 31, CRC Press, Boca Raton, FL

Fallows C, Martin RA, Hammerschlag N. 2012. Comparisons between white shark-pinniped interactions at Seal Island (South Africa) with other sites in California (United States). In: Global Perspectives on the Biology and Life History of the White Shark, ed. Michael L. Domeier, Chapter 9, CRC Press, Boca Raton, FL.

Martin RA, Hammerschlag N, Collier R, Fallows C. 2005. Predatory Behaviour of White Sharks (Carcharodon carcharias) at Seal Island, South Africa. Journal of the Marine Biological Association of the UK, 85: 1121–1135.

Martin RA, Rossmo DK, Hammerschlag N. 2009. Hunting patterns and geographic profiling of white shark predation. Journal of Zoology, 279: 111–118.

Walters, C (1997) Challenges in adaptive management of riparian and coastal ecosystems. Conserv Ecol 1:1