Bach, Pascal

Cotel, P.

Filippi, T.

Lamoureux, J.P.

Martin-Baillet, V.

Rabearisoa, N.

Horizon / Pleins textes

  
 
 
Marine mammal depredation and pelagic longline 
fishery: assessment of the efficiency of a scaring 
mitigating device in Saint-Paul Bay (Reunion Island) 
 
 
 
 
 
 
 
 
 
 
 
 
Rabearisoa N., T. Filippi, V. Martin-Baillet, J.P. 
Lamoureux, P. Cotel & P. Bach 
 
 
- August 2010  -
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Citation : 
Rabearisoa N., T. Filippi, V. Martin-Baillet, J.P. Lamoureux, P. Cotel & P. Bach, 2010 - 
Marine mammal depredation and pelagic longline fishery: assessment of the 
efficiency of a scaring mitigating device in Saint-Paul Bay (Reunion Island). 
IRD/SWIOFP Report, 12 p.  
 1
 
1. Introduction 
 
Depredation is defined as the removal of fish or bait from fishing gear by marine 
predators such as in priority sharks and toothed whales  and is opposed to predation, 
which is the catch of free ranging fish (Donoghue et al., 2003). It is documented 
worldwide and is known in many fisheries but opposite to bottom longline fishery 
targeting toothfish (Dissostichus eleginoides), pelagic longline fisheries targeting tuna 
(Thunnus spp) and swordfish (Xiphias gladius) received less interest from the 
scientific community regarding this issue. In the Indian Ocean, this phenomenon is 
characterized by a lack of data. Only a few papers deal with this problem (Nishida 
and Shiba, 2004 ; Sivasubramanian, 1964 ; Poisson et al., 2001 ; Romanov et al., 
2007) 
In tropical areas, depredation on pelagic longline capture involves false-killer whales 
(Pseudorca crassidens), pilot whales (Globicephala macrorynchus) and pelagic 
sharks, and depredation on bait involves small delphinids, such as spinner dolphins 
(Stenella longirostris) or Risso’s dolphins (Grampeus griseus).  
The monitoring of the extent and magnitude of depredation is of a great importance 
since it leads to many negative consequences affecting commercial aspects 
(expenditure of extra money when fixing damaged gear and/or moving away to 
avoid areas of high depredation rate, loss of fish), biological aspects (change in 
hunting behavior of cetaceans, risks of injury or mortality when interacting with 
fishing gear, threats by fishermen) and assessment aspects (increase of fishing 
effort, fish loss not taken into account in stock analysis) (Donoghue et al, 2002).  
Many mitigation measures have been tested so far but none of them proved to be 
efficient speaking of long term (Jefferson and Curry, 1995). Most research are 
currently focusing on the use of active and passive acoustic means to deter 
depredation from cetaceans. They can be efficient at short term but are found to 
create opposite effect at medium term as they are used as an acoustic attractor by 
cetaceans (Mooney et al., 2009 ; Brotons et al., 2008 ; Franse, 2005). 
There are good evidences that cetaceans use their sight to locate the gear and/or 
the boat, follow them and depredate the fish caught. In order to mitigate 
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depredation events and then reduce interactions between toothed whales and 
longline close to the gear, we propose to develop the physical protection of capture. 
The goal of this study is to test the efficiency of scaring devices protecting physically 
catches and frightening predators. As a first step of those experiments, we chose to 
work at a small scale by conducting surveys in Saint-Paul Bay, located in the south-
western of La Reunion Island. We tested those devices on a coastal species of small 
delphinids, the bottlenose dolphin (Tursiops aduncus), which is known to be resident 
within the study area (Dulau-Druot et al, 2008).       
 
2. Materials and methods 
 
Study area and biological material 
 
Our surveys started on July 19th, 2010, four days a week (and are still going on). 
These surveys aimed at searching for dolphins and assessing the efficiency of our 
scaring devices on them. The study area extended from the Saint-Gilles harbor to 
Saint-Paul Bay, and was restricted to the coastal waters (up to 60 m deep). Surveys 
started at 07.00 am and ended at 11.30 am.  
Before testing those depredation mitigating devices at a larger scale on pelagic 
longliners operating in open waters, we chose to work at a small scale with resident 
dolphins. In the case we obtain positive results, we will develop those experiments at 
a larger scale onboard fishing vessels and in real fishing conditions on the species 
involved in catch depredation. 
 
Description of the scaring device 
 
The general principle of our mitigating system is the scaring effect it could have on 
marine mammals. We designed them based on the tori lines towed behind longliners, 
attached from a high point at the stern and consisting of a backbone from which 
streamers hang down at regular intervals. Those tori lines aim at scaring marine 
birds, preventing them from accessing the baited hooks and getting caught on the 
longline (Keith, 1999).  
 3
Two similar square PVC frameworks of one meter side, which we will call “devices”, 
were maintained at the sea surface by four buoys at each corner and equipped with 
four baited branchlines of five meters long (Fig 1.a and 1.b). The “scaring” consists 
of six or eight streamers of one meter long made up of tarpaulin material and fixed 
on a PVC tube. The branchline was inserted into the PVC tube of the scaring device, 
and the bait was fixed at its end by a knot (no hook was used to avoid risks of 
injury on dolphins). The lower strands of the frightener were leaded so that they 
covered the bait. The upper ones were equipped with water balls so that they can 
move more freely, causing a scary effect on the dolphins and preventing them from 
taking the bait (Fig. 1.f).  Both devices were equipped with a video camcorder, 
enclosed in a waterproof housing and fixed at the center of the framework, recording 
the behavior of the dolphins interacting with the bait (Fig 1.c and 1.d). The 
branchline can be baited with unprotected fish (Fig. 1.e) or protected one (Fig. 1.f). 
 
Experimental protocol    
 
Surveys started by an area prospection to search for dolphins. When we found a 
group, we put the devices in the water (Fig. 2.a). During the first surveys, both 
devices were unequipped with scarings in order to observe the interaction between 
dolphins and unprotected baits, and assessing whether the general configuration of 
our devices don’t scary them. Then, we put one baited device equipped with scarings 
and another one without scarings (both in the vicinity of the group). That allowed us 
to compare the dolphins’ behavior towards both of them. With a kayak, a diver went 
near the devices and swam around them with a camera to take some underwater 
photos of the dolphins and the devices (Fig. 2.b and 2.c).  
Data collection consisted in recording the geographical position of the study area, the 
characteristics of the dolphins group (size, species and physical particularities of 
some individuals…), the current strength, and the state of the bait before and after 
their interaction with the dolphins (whether they were damaged or not). A minimum 
of 3 and a maximum of 6 people embarked onboard the boat.  
 
 4
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1.a 1.b 
1.c 
1.d 
1.e 
1.f 
Fig. 1.a: PVC frame equipped with a camrecorder Fig. 1.b: Underwater device equipped 
with buoys  
Fig. 1.c & 1.d: PVC frame equipped with a camcorder, 4 branchlines and 4 buoys 
Fig. 1.e & 1.f: Devices equipped with 4 unprotected baits (left) and 4 “frighteners”-protected 
baits (right)  
 5
 
 
2.b 
2.c 
Fig. 2.a: IRD team putting the device into the water 
Fig. 2.b: Diver (on the middle) filming a dolphin (on the right) evolving near the device (on the 
left) 
Fig. 2.c: Intern checking the device 
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Designation Quantity
Unit price 
(euros)
Total price 
(euros)
Boat rental 14 100,00 1400
Fuel 14 30 360,00
2 Kayaks (90 euros / week) 4 90 360,00
Camcorder rental + insurance 1660,25
Camcorder  purchase + battery 1346,22
PVC bend  = 40 m/m 12 0,95 11,4
PVC Tube =40 m/m 3 10,00 30
PVC Tube = 16 m/m 2 1,50 3
PVC Tube  6m 1 18,12 18,12
Bait, tools and diverse materials 528,22
TOTAL 5717,21
 
Expenses report 
8000 Euros were allocated to those surveys within the framework of the component 
4 of the SWIOFP project. To date, we spent about 5700 Euros (devices construction, 
camera hire and purchase, boat and kayak rental…) (Tab.1 ) 
 
 
 
 
 
 
 
 
 
 
 
Tab. 1 Expenses report 
 7
3. Preliminary results  
 
To date, we conducted 14 surveys (representing four surveys a week, and one week 
without any survey due to climatic conditions). We made 16 observations of T. 
aduncus and 3 observations of S. longirostris. Based on those observations, we were 
able to put our devices into the water 13 times during the whole study period (Fig. 
3).  
 
 
Devices without  
dolphins interaction 
Devices with  
dolphins interaction 
Fig. 3: Survey area and spatial distribution of device settings  
 
 8
 
 
 
 
During the first survey, the behavior of the encountered dolphins did not allow us to 
work properly.  Instrumented devices were put into the water all the same to 
observe the underwater behavior of the whole device. That allowed us to reconsider 
some points: the buoys size, the floatline and branchline lengths, the tie system of 
the bait…      
During the second survey, we worked in the vicinity of a group of 4 T. aduncus. We 
put the instrumented devices into the water, both with unprotected baits. During the 
first half an hour, dolphins interacted with only one of them, and attacked 2 baits out 
of 4 (Fig 4.c). There was no interaction between the second device and the dolphins, 
4.a 4.b 
4.d 
4.c 
Fig. 4.a & 4.b: Dolphin diving near the device  
Fig. 4.c: Dolphin attacking an unprotected bait 
Fig. 4.d: Dolphin attacking a “frightener”-protected bait 
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letting 4 baits out of 4 intact. We then replaced the damaged baits by intact ones, 
and we put both devices into the water once again (representing a total of 7 intact 
baits). 45 minutes later, 5 baits out of 7 were totally eaten, one was partially 
damaged, and only one was intact. Dolphins were clearly observed swimming and 
diving around the devices (Fig 4.a and 4.b). We then concluded that without 
scarings, dolphins were not afraid to eat the baits. The general configuration of our 
devices (framework, camcorder, branchline…) does not seem to scary them and 
prevent them from coming nearby. 
During the fourth survey, we worked in the vicinity of 12 T. aduncus. We first put the 
two instrumented devices into the water, with all their branchlines protected by 
“scarings” but none of them was attacked (whereas we clearly saw from the sea 
surface that the dolphins turned around the devices). We then removed the 
“scarings” on the second device, and let the first one untouched. 40 minutes later, 
none of the protected bait was attacked, but among the three unprotected baits of 
the second device, one was intact, one was damaged, and one was completely 
eaten. Those first results allowed us to conclude that the scaring devices by evolving 
around the bait, seem to scary the dolphins, and prevent them from attacking the 
baits.   
During the seventh survey, we worked with a group of 10 T. aduncus. One device 
was equipped with “scarings” and the other one was not. No bait was attacked, even 
the unprotected ones. Based on the recorded images, we observed some dolphins 
swimming near the unprotected baits, without attacking them. But only one 
individual (always the same) turned around the protected instrumented device, and 
tried to attack one of the bait. After analysis of the movie, we observed that he 
attempted to attack the only bait which was not correctly protected by the streamers 
(in other words, the only one to which he had access) (Fig 4.d). Those observations 
confirmed the first results we obtained before: the streamers seem to be efficient in 
protecting the fish from the dolphins.  
During the other surveys, we encountered some issues, giving us no way to get 
readable images:  
- the dolphins were already feeding on artisanal nets and were not 
interested in our devices 
 10
- we lost sight of them as we put the devices into the water 
- the dolphins were simply not interested in them, and kept on coming and 
going in the study area without stopping nearby the devices  
- we encountered no dolphins at all 
We also attempted to test the devices on a group of S. longirostris, but their fast 
moving behavior did not allow us to record any images. We summed-up all those 
observations in Tab. 2 
 
 
 
4. Discussion and perspectives 
 
Most of the time, we encountered the same group of T. aduncus, at the same period 
(between 8.30 am and 10 am), and in the same area in St-Paul Bay. During the first 
surveys, they showed an obvious curiosity towards our devices, and were not afraid 
to come and take the unprotected bait (surveys 2, 4). On the contrary, the scaring 
devices seem to afraid them as the protected bait remained untouched (surveys 4 
and 7). But after that (surveys 6, 10, 12, 13, 14), they showed a complete disinterest 
in our experiments. We concluded then that once they “knew” and “understood” 
what those devices were, they were not interested at all in them, and we were not 
able to test them anymore. Moreover, the fact that the surveys lasted less than 5 
hours a day limited the opportunities for finding a group of dolphins which will 
interact with them. 
Tab. 2 : Summary of devices sets undertook 
Date N° survey N° device set Species Size group Comments
19/07/2010 1 1 Devices set without dolphins
19/07/2010 1 2 Devices set without dolphins
20/07/2010 2 3 T. aduncus 4 Dolphins interacted with the unprotected baits
21/07/2010 3 4 T. aduncus Dolphins left the study area as we set the devices
22/07/2010 4 5 T. aduncus 12 Dolphins interacted with the unprotected baits, and let the protected ones untouched
26/07/2010 5 6 T. aduncus 5 Dolphins left the study area as we set the devices
26/07/2010 5 7 T. aduncus 3 Devices set but dolphins feeding on artisanal nets
27/07/2010 6 8 S.longirostris 50? Devices set but dolphins disinterest
29/07/2010 7 9 T. aduncus 10 Dolphins interacted with the unprotected and protected baits
30/07/2010 8 No device set
03/08/2010 9 No device set
04/08/2010 10 10 T. aduncus 5 Devices set but dolphins disinterest
05/08/2010 11 No device set
06/08/2010 1 11 T. aduncus 4 Devices set but dolphins disinterest
17/08/2010 13 12 T. aduncus 5 Devices set but dolphins disinterest
18/08/2010 14 13 T. aduncus Devices set but dolphins disinterest
 11
That means that we can move on to the next step: now that we know how the 
scaring devices behave underwater, we will test them on an experimental longline 
(about 500m long), with one half of branchlines (about 50) equipped with hook timer 
and without “scaring” and a second half on brachlines equipped with both “scarings” 
and hook timers. Hook timers will be deployed to record the time of the attack of the 
bait by predators. We will set it in the St-Paul Bay at sunset, and haul it at dawn. 
That will give us more probability of interactions between dolphins and our mitigating 
system. We will then compare the damage rate of the protected and unprotected 
baits to assess their efficiency and confirm (we hope) the first results we obtained. 
On this fishing ground the abundance of large pelagic predators able to attack the 
bait is low enough to do not introduce biases in the comparative quantitative analysis 
of bait attacks. 
Another stage is the miniaturization of our scaring device in the perspective of its use 
in a longliner. Indeed, it will be essential to produce a small device in order to make 
it easy to operate by fishermen. We will also work out the issue of the automatic 
triggering system of the scaring device: indeed, in the case of the longline fishery, 
we will have to protect the catch (and not the bait, as we did during our surveys in 
Saint-Paul Bay). That means that our system will have to be triggered only when a 
fish will be caught after attacking the bait.              
 
 
 
References 
 
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between bottlenose dolphins and artisanal fisheries around the Balearic Islands. 
Marine Mammal Science 24, no. 1: 112-127. 
 
Donoghue, M., Reeves, R.R., Stone, G.S. (2003) (Eds.). Report of the workshop on 
interactions between cetaceans and longline fisheries, Apia, Samoa: November 2002. 
New England Aquarium Aquatic Forum Series Report 03-1. 45 p. 
Dulau-Drouot, V., Boucaud,V., Rota, B. (2008). Cetacean diversity off La Réunion Island 
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Franse, R. Effectiveness of acoustic deterrent devices (pingers). MasterSc of Industrial 
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echolocation and acoustic disruption: Implications for long-line bycatch and 
depredation. 87: 726-733. Canadian Journal of Zoology. 
 
Nishida, T., Shiba, Y., Report of the predation survey by the Japanese commercial tuna 
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Poisson, F., Marjolet, C., Mété, K., Vanpouille, M. Évaluation du phénomène de 
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Marine mammal depredation and pelagic longline fishery : assessment of the efficiency of a scaring mitigating device in Saint-Paul Bay (Reunion Island)

Marine mammal depredation and pelagic longline fishery : assessment of the efficiency of a scaring mitigating device in Saint-Paul Bay (Reunion Island)

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