Research with Bite: Ocean Research’s Project Great White Shark in 2013

It was another exciting year in 2013 at Oceans Research for Project Great White Shark where research continues in the beautiful Mossel Bay. As we approach the exciting months of winter a.k.a “breaching season”, it seemed the perfect time to reflect on the past year of research and what our staff and interns have observed so far in 2014.

The aim of Project White Shark is to identify the spatial patterns in relative abundance, and investigate the size composition to define the population structure for white sharks in Mossel Bay. Oceans Research also aims to establish an index of abundance for white sharks in this area to better our estimates for their global population size. In simpler terms, we want to know how many sharks are in the bay at different times of the year, what areas they are concentrated in, what sizes are found in the different areas across seasons, and also, if individuals are staying in the bay year-round or moving away for months – even years – at a time.

In 2013, Oceans Research interns collected data for Project Great White Shark across 314 sampling trips to the various sites in Mossel Bay (34˚ 11’ S, 22˚ 09’ E; Image 1) totalling a massive 782 hours of sampling effort! Each trip averaged 2 hours and 49 minutes of sampling and with a total of 1,547 recorded white shark sightings, each trip averaged 1.96 shark sightings per hour (sharks per unit effort; SPUE).

Image 1: Sampling sites for Project Great White Shark within Mossel Bay, South Africa.

The below graph (Image 2) represents the average number of sharks observed per unit effort (hour; SPUE) across each of the four seasons in 2013, with the winter months corresponding to the highest value of 1.97 SPUE. In comparison to the average SPUE of 1.83, this peak in the winter months is correlated to the movement of white sharks to Seal Island (approximately 800 metres from shore) where the Cape Fur seal pups begin to enter the water for the first time since their birth in November/December. The lack of experience of these seal pups in identifying and avoiding strikes from their large predators makes them easy targets in their new aquatic environment during this time of year, thus providing the perfect feeding opportunity for the larger marine-mammal consuming sharks in Mossel Bay.

 

Image 2: Average number of sharks per unit effort (hour; SPUE) observed across seasons in Mossel Bay in 2013.

White sharks ranging from 125 – 474 cm total length were observed in 2013, with just under 75% of recorded individuals being placed in the 175 – 324 size range. Larger sharks (325 – 474 cm total length) comprised 17% of the recorded individuals, with sharks ranging from 125 – 174 cm total length making up the last 9% of observed white sharks for the year. Already in 2014, two of these larger individuals have already been sighted with total lengths estimated to be 470 and 500 cm! It will be very interesting to see if these sharks hang around until later in the year!

In terms of white shark activity for 2013, Seal Island and Blue Houses sites were observed to be the focal areas (areas of highest activity) in the seasons of winter and summer respectively in Mossel Bay. Image 3 presents the average SPUE observed across all of our study sites for 2013, and illustrates the higher SPUE for Seal Island (1.88) and Blue Houses (1.43) in comparison to the average of 0.86 SPUE and this pattern has been observed for the last few years. The peak SPUE value at Seal Island is a result of the previously discussed movement of white sharks to the area for winter’s ‘breaching season’, as the young seal pups of the year enter the water for the first time and provide easy high-energy prey targets. The high SPUE witnessed at Blue Houses is correlated with this site being the focal area for white shark activity in the summer months. 

Image 3: Average number of sharks per unit effort (hour; SPUE) observed at the six sampling sites in Mossel Bay in 2013.

A lack of inexperienced seal pups entering the water in summer in comparison to in the early winter months drives the white sharks to move across the bay to the reef systems of the Grootbrak area - the Blue Houses site in particular - to utilise the fish stocks that reside there. The river systems and mouths located between these two focal sites (Hartenbos, Kleinbrak, and Grootbrak rivers) provide resting areas for the sharks between hunting trips, which is a potential reason why we see less sharks in these areas as they could be less attracted to our baits due to the fact that they might have already hunted and fed. The dynamic nature of the white shark population across the sampling sites for Oceans Research in Mossel Bay is illustrated in Image 4 below, which shows how SPUE vales for each of the locations changes throughout the year. The movement of Mossel Bay’s white sharks from Blue Houses to Seal Island and back as winter comes and passes can also be seen below.

Image 4: Number of sharks per unit effort (hour; SPUE) observed throughout 2013 at each of the sampling sites for Project Great White Shark in Mossel Bay.

In previous years of this study (2013 included), the movement of white sharks from Blue Houses to Seal Island has been observed to occur around the end of March – early April however, in 2014 this movement was noted to have occurred in early March. The precise reason for this early movement is not currently known but possible explanations include changing fish stock supplies and varying environmental cues.

Another interesting observation made by Oceans Research across the summer of 2013 and 2014 was the predominance of “red-tide” algal blooms that entered the bay quite frequently; some days reducing the visibility in the water to less than one metre! It is unknown what is driving these intense algal blooms and which species of algae are causing this problem, however we have seen that these events have the potential to reduce the number of shark sightings made on sampling trips (even in focal areas).

As well as heading out into the field to conduct research for Project Great White Shark, Oceans Research also aims to educate and involve the community in our research. Numerous meetings have been held with the surfing community of Mossel Bay since the beginning of 2014 to explain how we execute our research and discuss the highly contentious issue of chumming. These meetings aim to explain to the public the science behind why we chum, and the minimal to negligible impact our methods and protocols pose to shark behaviour and water-user safety. It also provides the perfect opportunity for people to ask questions or raise concerns directly to the staff of oceans, and even to make suggestions about how we can improve how we communicate our research and actions to the public. A great example of how communications between Oceans Research and the public is increasing is the initiation of a “chumming flag” system during research trips to study the white sharks. When chumming on these trips, a white flag with a black profile of a shark (Image 5) is flown from the research vessel so that it can clearly be seen from land. This allows people on the beach to know if our vessel is chumming or not (in the latter case, this means that research is being carried out for other projects in the bay).

 

Image 5: Oceans Research intern, Michele Donihe, and Field Specialist, Curtis Young, on the look-out for white sharks during a chum trip. The chumming flag (white flag and black shark) is flown from a Oceans Research vessels whenever chumming is being performed.

It’s been an exciting year so far on the waters of Mossel Bay in early 2014 for Project Great White Shark, and with breaching season just around the corner, everyone is very excited because....

...WINTER IS COMING

Lauren Peel

Oceans Research 

PI

Mimicry - Survival through deception

Human–shark interactions have become more and more common over the last 10 years due to an increase in human population, and a higher number of ocean users. Fisherman, divers, swimmers and surfers all enter the realms of the shark when pursuing these activities and unfortunately although incredibly uncommon, shark-human conflicts do occur.

My research aims to investigate potential shark deterrents to help increase protection for surfers in “sharky” waters.  There have been a number of different shark deterrents investigated in the past to try and protect ocean users.  Electrical barriers, acoustic playbacks and chemical deterrents have all shown potential.  However none of these methods have been completely successful or sustainable and can have a negative effect on the environment and non-target species.

Shark nets were introduced in South Africa in 1952 in Kwa-Zulu Natal to protect bathers. However, the nets are not a species specific defense and kill huge numbers of sharks and other marine species such as whales, dolphins and sea turtles. Other deterrents such as the shark shield have been developed for individual safety of swimmers and surfers but when tested with large sharks such as the white shark it was less than successful (it was eaten).  Therefore it is important that more sustainable methods of protection are investigated in order to help conserve shark species and protect water users.

I am currently looking at whether or not the visual sign stimulus of orca whales (Orcinus orca) creates a natural fear and avoidance behaviour in white sharks (Carcharodon carcharias) and if this fear can be utilized as a shark deterrent to increase surfer protection.

The idea behind this theory originates from observations of orca whales feeding on elasmobranchs, including white sharks all over the world.A fascinating event occurred in the South Farallon Islands (a well-known white shark aggregation area) in 1999 where two orcas were seen eating a large white shark. Furthermore,white sharks were not seen in the area for almost 2 months after this event occurred. This reaction from the white sharks suggests that they may have a natural fear of orca whales and take precautionary measures to avoid them.

There are two possible explanations for this behavioural reaction to another species.  It could be an innate behavioural response where sharks have a“hard wired”genetic instinct to be afraid of orcas and react to their warning signals without any previous experience.  Alternatively it could be a learnt behaviour where the sharks have seen orca whales before and have experienced the specific visual, olfactory and acoustic cues of the orcas so they can avoid them as early as possible in the future. Either way it makes sense for white sharks to be able to recognize and respond to the threat of orca whales.

Orcas have a very striking black and white pattern on the ventral side of their body.  This unambiguous colouration and pattern could be a distinct warning signal to white sharks triggering a cautious response and signalling its threat. This could be valuable for juvenile orcas when they are still small and vulnerable to attack from large sharks if they get separated from their family pod.

Interestingly there is a small dolphin species found off the Southern African coast called the Heaviside dolphin (Cephalorhynchus heavisidii). These dolphins share this distinctive ventral pattern and also live in white shark occupied waters, but this dolphin poses no threat to sharks. However it could be possible that this dolphin species has evolved a similar pattern to the orca whale to increases its protection from becoming preyed on by large sharks such as great.  If the sharks recognise this pattern and it triggers a cautious or even a withdrawal response immediately, then the dolphin will be less likely to be attacked. This defense tactic these dolphins seem to have evolved is known as Batesian mimicry.

Source: interesting facts.org.   

Source: arkive.org

Batesian mimicry is a broad and somewhat complex area of evolutionary biology.  It was first brought to light by an English naturalist called Henry Walter Bates, whose work on butterflies in the Amazon rainforest in the 1800’s pioneered the breakthrough of this natural phenomenon.

Batesian mimicry is most commonly seen in insects, fish and snake species, but found in a number of terrestrial and marine species.  It is based on predators learning from past experience and used as a form of defense by less dangerous or harmless species in order to increase their survival rate.

The two main strategies of mimicry are Batesian and Mullerian mimicry.  There are three groups involved with this protective tactic. The species copying the signal is the “mimic”, the species being mimicked is known as the “model” and the predator the mimic is intending to deceive is the signal receiver.

Batesian mimicry works on the basis that the predator the species is trying to avoid has encountered the unpalatable or more dangerous model previously and learnt that the particular signal (i.e. colouration, smell or sound) means the animal is an unprofitable one and therefore is deterred from feeding on individuals that emit that particular signal, this could explain the similar colouration in the Heaviside dolphin.For this reason Batesian mimicry is most beneficial to mimics when the model species are in greater abundance than the mimic species. If there are more mimics than models the predators will not experience the negative reaction from the individual and therefore not learn to avoid the warning signs. This is known as negative frequency dependent selection.

One of the most famous and fascinating examples in Batesian mimicry in the marine world is the mimic octopus (Thaumoctopus mimicus). This highly intelligent cephalopod is capable of mimicking other marine animals for protection when moving across open areas of sand on the ocean floor. It has been known to mimic sea snakes, lion fish, sole fish and even sting rays. 

Source: Wikipedia

Mimicry is a very specific phenotypic evolutionary process, which once the initial transition from cryptic colouration to an aposematic one has been perfected the benefits for the mimic are obvious.  However the process from changing from a cryptic animal to an unambiguous one was confusing for biologists. If the process was a gradual change, such as in the case of most evolutionary processes, with each generation becoming slightly less cryptic and more unambiguous, then the species would suffer a significant fitness loss. 

One hypothesis made by evolutionary biologists Clarke and Shepherd (1960) was that this step consists of one large mutation using “super genes” where there is a sudden phenotypic change. This theory states that the first generation of this modification are not perfect mimics, but carry a remote likeness to the signal enough for survival and to gain selection for future evolution, where the mimicry is refined and perfected.

To test whether or not mimicking the visual signal of an orca whale will reduce attack rates on potential prey items at the surface I am towing foam decoys around Seal Island in Mossel Bay.  These decoys will simulate potential prey items at the surface and induce seal hunting behaviour from the white sharks.  There are 4 decoys towed in pairs, separated 15 m apart in order to give enough space between them to make them independent tests due to the low visibility of the water.

The two pairs will consist of a plain black decoy paired with an orca decoy, which will have the ventral black and white pattern of an orca whale, and a plain black decoy paired with a black and white chequered decoy as a control against the specific biological pattern.  If white sharks are afraid of the visual sign stimulus of orca whales it is expected that the orca decoy will be attacked less frequently than the other decoys. 

The chequered decoy will test if the sharks are avoiding the orca decoy because it views it as a threat i.e. an orca, in which case the chequered decoy will be attacked significantly more than the orca decoy, or purely for the reason that it doesn’t look like the shark’s intended target i.e. a Cape fur seal, due to the contrasting colours and therefore both the chequer and the orca decoy will have a similar attack rate.

Underwater activity will also be recorded using GoPro cameras to show any withdrawal at the decoys that cannot be seen from the surface.  If the orca pattern is seen as a deterrent by white sharks, it is expected that there will be a higher withdrawal rate on the orca decoy than the other decoys.

All four decoys towed at the same time to test the possible aposematic qualities of the ventral pattern of an orca whale in the same environmental conditions.

Environmental conditions also play a large part of white shark hunting behaviour.  It is assumed that sharks hunting in clear calm waters will be able to distinguish the difference between a real prey item such as a seal, and a foam decoy.  Therefore fewer attempts on the decoys in high visibility are anticipated.  However, when there is a disturbed sea surface due to wind chop and the water visibility is poor due to increased debris and wave movement, sharks appear to mistake a decoy for a prey item more regularly, as witnessed in previous research.  This is presumably down to a greater margin of error in identifying prey at the surface.  My study will investigate whether there is any correlation between the frequency of attacks on the decoys and the abiotic factors they are towed in, specifically to the Mossel Bay area.

The overall outcome of this research project is to investigate whether it is possible to reduce surfer’s risk of attack from white sharks by replicating the ventral pattern of an orca whale on the bottom of surfboards to act as a deterrent.  Furthermore, by recording the weather conditions and analysing correlations with attacks on the decoys, we can create better guidelines for swimmers and surfers specifically in the Mossel bay area (and also comparing it with other areas) making people more aware of higher risk times to be in the water.

Mike Barron  - Msc. Candidate