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
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