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Using an unbaited stationary video system to investigate the behaviour and interactions of bull sharks (Carcharhinus leucas)

Authors:
Analyses
Using an unbaited stationary video system
to investigate the behaviour and interactions of bull sharks
(Carcharhinus leucas) under an aquaculture farm
*Loiseau, N., Kiszka, J.J., Bouveroux, T., Heithaus, M.R., Soria, M. & Chabanet, P. (2015) Using an unbaited stationary video system to investigate the behaviour and interactions of bull sharks (Carcharhinus leucas) under
an aquaculture farm. African Journal of Marine Science.
The bull shark (Carcharhinus leucas) is a large predator that is able to exploit a diversity of marine, brackish and freshwater habitats. Very
little is known about the behaviour of adult bull sharks (most studies have focussed on juveniles on their nursery habitats), including their
social interactions and grouping patterns.
Active fish farms have been documented to attract wild predatory fishes, including bull sharks. In order to better understand the behaviour
and interaction of bull shark with the aquaculture farm of Saint-Paul bay (la Réunion, France), we deployed an experimental unbaited
stationary video camera beneath of a floating aquaculture cage.
We aimed to investigate short-term site attachment, behaviour, and grouping patterns of bull sharks during daylight hours.
Loiseau N.*, Kiszka J.J., Bouveroux T., Heithaus M.R., Soria M., Chabanet P.
The aquaculture farm was established in 1999 to breed red drums and goldlined sea breams
in Saint-Paul bay (sandy embayment located off the northwest coast of the island).
Videos viewed at 8x speed until sharks were detected.
One sighting presence of a shark on the frame.
A re-sighting a second observation of an identified individual within 15
minutes.
Associated sharks 2 individuals simultaneously seen and within sample
duration of less than 15 minutes
Using identification data, we obtained the number of sightings for each identified individual
and assessed the occurrence per day for each shark. To investigate associations among
sharks we used the half-weight index (HWI) to calculate coefficients of association (CoAs),
following the equation:
CoAs = X/[X+ 0.5(Ya + Yb)]
where X, the number of times both individual a and b were seen together; Ya, the number of times individual
a was seen without the individual b, and Yb, the number of times individual b was seen without individual a.
CoAs = 0 => 2 individuals never seen together.
CoAs = 1 => 2 individuals always observed together.
To determine whether the patterns of associations between individuals were different from
random, an association matrix from calculated CoAs between sharks was built with a daily
sampling period. We used the permutation test to determine whether sharks associated
preferentially with other individuals and/or avoided one another In order to test null
hypothesis of no preferred companions between sharks we randomly permuted associations
9000 times until the p-value was stabilizing within the same period.
218 observations 217 bull sharks and 1 tawny
nurse shark (Nebrius ferrugineus).
Sharks were mostly observed in the afternoon.
Short-term patterns of site attachment of bull sharks to the aquaculture farm for several identified individuals.
Fish farms may punctually serve as “landmarks” for bull sharks moving along the west coast of La Réunion.
The combination of association patterns and observed social behaviour suggests that some bull sharks are more gregarious than others.
Potential drivers of these associations may include improved foraging efficiency or male mating behaviour avoidance (all sharks were females).
The advantage of video sampling is to observe the behaviour and swim of sharks, alone or in association with conspecifics. Recorded behaviours included intraspecific social
interactions such as synchronized swimming.
Future studies that incorporate unbaited video technology will greatly enhance our current understanding of behaviour of sharks
and dynamics of social interactions within sharks.
Sharks swimming alone: 179 times, 82%
Sharks swimming in pairs: 35 times, 16%
Groups of 3, 4 and 5 individuals: 1 time each
Straight-line synchronised swim (see above): 13 times
Milling: 25 times
Larger individuals not systematically in lead position.
A - Location of the study area and B -
Saint-Paul bay and location of the
aquaculture farm where the study has
been conducted (white dot).
High-definition video camera (SONY HDCAM 750) on a stainless-steel tripod of 3 m height.
Cabled to a housing (150 cm x 70 cm) mounted on the buoys of the aquaculture cages.
On the housing a solar-recharged battery (12V-75A) that supplied power to the camera and
a player/recorder (SAMOURAI-© Atomos 2013) that stored clips.
Data transmitted to the surface through an HDMI video cable and clips recorded on several
0.5-1.0 TB hard drives.
Continuous record during daylight hours between 9:00am to 6:00pm from March 6th to April
6th 2012.
Strong associations: sharks#1 and #2 (CoA=0.55),
sharks#4 and #6 (CoA = 0.46).
Some sharks avoid others since the proportion of non-zero
association indices is lower in the real dataset (proportion=
0.57) than in the random one (proportion= 0.63, p = 0.047).
Introduction and objectives
Materials and methods
Results
Discussion
Data analysis
Individuals Size Sex Maturity Sightings Days Time(sd) Df–D
l
Shark1150200 F Immature 4513 14:58
(37:16)
31
Shark2200250 F Adult 33 9 17:47
(43:41)
24
Shark3250300 F Adult 20 11 29:48
(43:42)
24
Shark4200250 F Adult 21 7 22:49
(47:30)
17
Shark5250300 F Adult 20 9 14:31
(14:47)
10
Shark6250300 F Adult 11 6 32:05
(65:42)
14
Shark7250300 F Adult 8 6 92:18
(106:41)
26
Shark8250300 F Adult 3 3 179:05
(58:07)
15
Days: N days where each individual was detected;
Time mean time in hours between recaptures (SD);
Df–D
lnumber of days between the first and the last
observation
Residency Associations Behaviours
* nicolas.loiseau1@gmail.com
Preprint
The global expansion of aquaculture has raised concerns about its environmental impacts, including effects on wildlife. Aquaculture farms are thought to repel some species and function as either attractive population sinks (‘ecological traps’) or population sources for others. We conducted a systematic review and meta‐analysis of empirical studies documenting interactions between aquaculture operations and vertebrate wildlife. Farms were associated with elevated local abundance and diversity of wildlife, although this overall effect was strongly driven by aggregations of wild fish at sea cages and shellfish farms (abundance: 72×; species richness: 2.0×). Birds were also more diverse at farms (1.1×), but other taxa showed variable and comparatively small effects. Larger effects were reported when researchers selected featureless or unstructured habitats as reference sites. Evidence for aggregation ‘hotspots’ is clear in some systems, but we cannot determine whether farms act as ecological traps for most taxa, as few studies assess either habitat preference or fitness in wildlife. Fish collected near farms were larger and heavier with no change in body condition, but also faced higher risk of disease and parasitism. Birds and mammals were frequently reported preying on stock, but little data exist on the outcomes of such interactions for birds and mammals – farms are likely to function as ecological traps for many species. We recommend researchers measure survival and reproduction in farm‐associated wildlife to make direct, causal links between aquaculture and its effects on wildlife populations.
Article
The global expansion of aquaculture has raised concerns about its environmental impacts, including effects on wildlife. Aquaculture farms are thought to repel some species and function as either attractive population sinks ('ecological traps') or population sources for others. We conducted a systematic review and meta-analysis of empirical studies documenting interactions between aquaculture operations and vertebrate wildlife. Farms were associated with elevated local abundance and diversity of wildlife, although this overall effect was strongly driven by aggregations of wild fish at sea cages and shellfish farms (abundance: 72x; species richness: 2.0x). Birds were also more diverse at farms (1.1x), but other taxa showed variable and comparatively small effects. Larger effects were reported when researchers selected featureless or unstructured habitats as reference sites. Evidence for aggregation 'hotspots' is clear in some systems, but we cannot determine if farms act as ecological traps for most taxa, as few studies assess either habitat preference or fitness in wildlife. Fish collected near farms were larger and heavier with no change in body condition, but also faced higher risk of disease and parasitism. Birds and mammals were frequently reported preying on stock, but little data exists on the outcomes of such interactions for birds and mammals-farms are likely to function as ecological traps for many species. We recommend researchers measure survival and reproduction in farm-associated wildlife to make direct, causal links between aquaculture and its effects on wildlife populations.
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