Cape fur seal white shark research EDNA Science Dyer Island Conservation Trust

Infographic: How seals use subsurface structures to sneak by white sharks

Written by Michelle Jewell.  Follow her on twitter: @ExpatScientist

If you have ever avoided parking on a risky-looking street or taken a different route between classes to avoid a bully, your behaviour has been altered due to the perceived presence of ‘predators’.  In the wild, prey animals also change their behaviour when they think predators are around, and these altered anti-predator behaviours can often influence other species, and then influence more species, and eventually change the entire ecosystem.  This is an example of indirect effects predators have on ecosystems.

My research has focused on these principles of predator/prey interaction in the ocean, and a great place to study oceanic predators and their prey are Cape fur seal colonies in South Africa.  Every summer in the southern hemisphere (November), Cape fur seals give birth to thousands of pups.  For example, the seal colony Geyser Rock has a population of about 60,000 Cape fur seals and every year they give birth to 10,000 seal pups!  By winter (April – September) these 6 month old ‘young-of-the-year’ seals begin to venture off the colonies and swim offshore with the adults to the fishing grounds.  These young-of-the-years are typically slow, plump from months of a mostly fat milk diet, and – most importantly – naïve.  White sharks take advantage of this naivety and aggregate closely around seal colonies every winter to catch leaving/returning seals.  Young-of-the-year pups are forced to learn how to avoid sharks quickly or suffer some rather permanent consequences!  This means that during a full year, every seal colony goes through a period of high white shark presence (winter) and very low to no white shark presence (summer).  Therefore, we can see how seals act ‘normally’ during the summer when there are no/very few sharks and how they change their behaviour in the winter to avoid white sharks.

Dyer Island Geyser Rock Shark Alley South Africa Seal Island False Bay

A) The Western Cape of South Africa with B) Seal Island, False Bay and C) Dyer Island & Geyser Rock.

Also, there are many different kinds of seal colony islands along the coast, which lets us ask more questions about how seals use their environment to avoid sharks.  I conducted my study at the Dyer Island/Geyser Rock system, which is home to ‘Shark Alley’ as well as many shallow reefs, kelp forests, and shipwrecks.  About 100km to the east is another seal colony called Seal Island, which is a world-famous spot to see white sharks predate on seals, but this island system lacks the abundant nearby structures/reefs/kelp forests that are present at Geyser Rock.  By looking at these two different kinds of islands, we can also examine how structures – or ‘refugia’ – may alter how seals avoid white sharks at Geyser Rock from how seals avoid white sharks at Seal Island.

And here’s what we found…

Agents of seal cape fur white shark research dyer EDNA Science

Do all of these structures and anti-predatory tactics of Cape fur seals change white shark movements around Geyser Rock?  Most definitely!  Check out that study (and infographic!) here.

This project was funded by the Dyer Island Conservation Trust and supported by Marine Dynamics shark tours, Volkswagen South Africa, and OCEARCH.  The infographic was designed by EDNA Science.  You can read the detailed scientific publication on Behavioral Ecology & Sociobiology by clicking here.

Have a question?  Leave it in the comments and I will answer!

White sharks in the Northwest Pacific Ocean

Written by Heather Christiansen

White sharks are one of the most protected shark species globally. However, unlike other well-known aggregations in the Northeastern Pacific Ocean, South Africa, and Australia, relatively little is known about white sharks in the Northwest Pacific Ocean. Unfortunately, this region also has little white shark specific protection measures in place.

mapMap of study region

 White shark’s behavior can vary depending on where they live. In order to determine the best way to protect white sharks in the Northwest Pacific Ocean it is important to gather details on what habitat they are using, if habitat use varies seasonally, how large and fast individuals grow, migration patterns and information on reproduction (mating, birthing areas, number of young per mother etc).

In order to collect as much information as possible on white sharks in the Northwest Pacific Ocean we worked with regional scientists and gathered all records and observations of white sharks in the area since 1951. Records were obtained from a variety of sources including scientific literature, newspapers, news websites and museums. There were a total of 240 records of white shark occurrences from Russia in the north down to Vietnam in the south. Individual sharks weighed between 35 to 5578 lbs (about the weight of an average pickup truck!). The size of the white sharks were wide ranging from young of the year at just over 4′ total length (measured from it’s snout to the end of its tail) up to the largest white shark on record at 19′ 9″ total length!

White Shark size comparison

Size of the largest and smallest white sharks in this study compared to the size of an average 10 year old girl.

We found that white sharks live in this region year round, but were absent from northern waters (near Russia and Republic of Korea) during autumn and early winter and southern waters (near China, Taiwan, Vietnam, and the Philippines) in July and August. White sharks have been recorded in a wide range of water temperatures, but this data indicates a preferred temperature range.

Based on the number of observations over the study period we estimated the trend in the relative abundance of white sharks in this region. We determined that the population was relatively stable until recently (approximately the last 10 years) where the relative abundance has started to decline slightly. We need to be careful interpreting these results though because they are based on reports from observers and not focused monitoring. These results indicate that there is a regional population of white sharks and further monitoring is required to estimate the number of animals living in the region.

One of the most interesting results of this study was the number of pregnant females recorded. There have only been 26 pregnant white shark females reported worldwide, 11 of which were found in this study. We were able to estimate females in this region are pregnant for 20 months and had up to 10 pups per litter.  Pregnant females were also recorded in more southern waters around Taiwan and Okinawa early in their pregnancies and around mainland Japan towards the end of their pregnancies. We don’t know very much about the reproductive strategies of white sharks worldwide so this study gave us a unique look at what pregnant females are doing. Additionally, we recorded young of the year white sharks in four countries indicating they may be using multiple countries as nursery areas.

Created with GIMPCreated with GIMP

Left: ovary from early term pregnant white shark, Right: egg cases from same shark  approx. 4″ long

This study provides important information that helps fill in gaps in our knowledge for white sharks in the Northwest Pacific Ocean. This study will be used to guide future research and determine what conservation measures are necessary to protect white sharks in the Northwest Pacific Ocean. Formal monitoring programs both nationally and internationally will help improve biological knowledge and assess future population trends.

If you would like more information on the observation records of white sharks in the Northwest Pacific Ocean you can look at the full publication here:

Christiansen HM, Lin V, Tanaka S, Velikanov A, Mollet HF, et al. (2014) The Last Frontier: Catch Records of White Sharks (Carcharodon carcharias) in the Northwest Pacific Ocean. PLoS ONE 9(4): e94407. doi:10.1371/journal.pone.0094407

available at: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0094407