Bach, Pascal

Cotel, P.

Lamoureux, J.P.

Rabearisoa, N.

Horizon / Pleins textes

 
 
 
 
Marine mammal depredation and pelagic longline fishery: 
Second trials to assess the efficiency of a scaring 
mitigating device in Saint-Paul Bay (Reunion Island) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Rabearisoa N., J.P. Lamoureux, P. Cotel & P. Bach 
 
- March-May 2011  - 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Dolphin trying to attack a protected bait 







 

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Citation : 
Rabearisoa N., J.P. Lamoureux, P. Cotel & P. Bach, 2011 - Marine mammal 
depredation and pelagic longline fishery: Second trials to assess the efficiency of a 
scaring mitigating device in Saint-Paul Bay (Reunion Island), March – May 2011. 
IRD/SWIOFP Report, 11 p. 
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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 and their echolocation 
abilities to locate the gear and/or the boat, follow them and depredate the fish 
caught. In order to mitigate depredation events and then reduce interactions 
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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. The 
development of those devices ensue from two preliminary surveys undertaken in 
Seychelles in 2007 and 2008, and aiming at testing the efficiency of two older 
devices regarding physical protection of capture on a commercial longliner 
(Rabearisoa et al, 2010). In August 2010, as a first step of those experiments, we 
worked at a small scale by conducting surveys in Saint-Paul Bay, located in the 
south-western of La Reunion Island (Rabearisoa et al, 2010). In March and May 
2011, we improved our devices and we worked at a larger scale in the same study 
area. We tested those devices on two coastal species of small delphinids, the 
bottlenose dolphin (Tursiops aduncus) and the spinner dolphin (Stenella longirostris), 
which are known to be resident within the study area (Dulau-Druot et al, 2008).  
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Our surveys started on March 2nd, 2011, lasted 20 days and ended on June 1st, 2011. 
These surveys aimed at searching for dolphins and assessing the efficiency of our 
devices on them. The study area extended from L’Hermitage to Saint-Paul Bay, and 
was restricted to the coastal waters (up to 300 m deep). Surveys started at 07.00 am 
and ended around mid-afternoon.  
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. 
 
 
 
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Surveys were undertaken onboard an artisanal fishing boat, Le Rapace (Fig 1.a). 
They started by baiting the branchlines with mackerel (Fig 1.b) (40 ones were 
protected by devices, 40 others were not), together with an area prospection to 
search for dolphins. Using both types of branchlines allowed us to compare the 
dolphins’ behaviour towards the presence and the absence of devices, and assess 
their efficiency regarding bait protection.  
We then set the 80 baited branchlines on a 500 meters long experimental longline 
(Fig 1.c). A branchline was set every 5 meters and a buoy was set every 4 
branchlines to maintain the line at the water surface (Fig. 1.d). We set alternatively 
20 protected and 20 unprotected branchlines. A camera was used to record some 
underwater images of the devices. 
Data collection consisted in recording the geographical position of the set, the 
characteristics of each dolphins group (size and species) we encountered (whether 
they interacted with the line or not), and the status of each bait after the retrieval of 
the branchline  (whether it was damaged or not).  
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The device consists of twelve 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 device, and the bait was fixed at its end by a simple knot (no hook was used 
to avoid risks of injury on dolphins). The lower strands of the device were 
leaded so that they covered the bait. The upper ones move more freely in the water 
column, causing a scary effect on the dolphins and preventing them from taking the 
bait (Fig. 1.e) 
 
 
 
 
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Fig. 1.a: Le Rapace, the artisanal fishing boat rented for the experiments 
Fig. 1.b: Baiting of the branchlines 
Fig. 1.c: Setting of a protected branchline on the experimental longline 
Fig. 1.d: General configuration of the experimental longline  
Fig. 1.e: General configuration of the depredation mitigation device 




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11,000 Euros were allocated to those surveys within the framework of the 
component 4 of the SWIOFP project. At the end of the surveys, we spent about 
10,200 Euros (Tab.1). 
 
    
 
 
 
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From March 2nd, 2011 to June 1st, 2011 we conducted 20 surveys in St-Paul Bay (one 
line setting per survey and per day) (Fig. 2). We made 15 observations of T. aduncus 
(we encountered the same group 9 out of 15 times) and 3 observations of S. 
longirostris during the whole experiment. There were 9 interactions between our 
experimental line and some dolphins groups (meaning that they damaged one or 
more baits on the line). 6 of those interactions occurred with T. aduncus (Fig. 3.a & 
3.b), and 3 occurred with S. longirostris. We were able to say that the observed 
damages were due to dolphins when they were observed evolving in the vicinity of 
the line and stopping near it for a while. Another particularity of dolphins damages 
was the successive nature of those attacks on the set (as opposed to the random 
nature of other fish attacks). This particularity is also common when toothed whales 
depredation on baits or capture occurs on pelagic longliners.  
Tab. 1 Expenses report 
 	
 












 
     
Based on those observations, we only considered the 9 sets which interacted with 
dolphins in our analysis (Tab. 2). A total of 339 protected and 330 unprotected baits 
were set. 47 protected baits and 103 unprotected ones were damaged (partially or 
fully) (Fig. 3.c). For 8 sets out of 9, there were more damaged unprotected baits 
than protected ones. The ratio between damaged protected and unprotected baits 
ranged between 0.11 and 0.73, except for a set on which we observed more 
damaged protected baits. For this particular set (survey #19), which interacted with 
a group of about ten T. truncatus, there were 1.95 times more damaged protected 
baits than unprotected ones. Nevertheless, those results showed that our devices are 
efficient in protecting the bait from dolphins attacks.   
 
Fig. 2: Survey area and spatial distribution of device settings (in red: sets with
dolphins interactions; in yellow: sets without dolphins interactions) 
 

N° survey
Nb of 
protected 
baits set
Nb of 
unprotected 
baits set
Nb of 
damaged 
protected 
baits
Nb of 
damaged 
unprotected 
baits
Species 
involved
4 40 38 6 19 T. aduncus
7 39 40 10 27 T. aduncus
12 40 38 3 9 T. aduncus
13 40 39 3 4 T. aduncus
14 20 20 3 13 S. longirostris
16 40 39 4 7 T. aduncus
18 40 38 1 9 S. longirostris
19 40 39 16 8 T. aduncus
20 40 39 1 7 S. longirostris
 
 
  












 
Tab. 2: Interactions between dolphins and depredation mitigation devices 
Fig. 3.a & 3.b: Group of T.truncatus interacting with the longline 
Fig. 3.c: Depredated baits 
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Those experiments aimed at confirming the results we obtained during the first 
surveys in August 2010 at a larger scale. This objective was fully reached and our 
devices proved to be efficient in protecting the baits since the number of damaged 
unprotected baits was more than twice higher than the number of protected ones. 
Nevertheless, we remain convinced that this protection rate can still be improved by 
digging our work. 
Since August 2010, most of the time, we encountered the same group of T. aduncus. 
In August 2010, during the first surveys, they showed an obvious curiosity towards 
our devices, and were not afraid to come and take the unprotected bait. On the 
contrary, the devices seem to afraid them as the protected bait remained untouched. 
But after that, they showed a complete disinterest in our experiments.  
In March and May 2011, we encountered this particular group 9 times, and 6 times 
out of 9, it interacted with our line. During the first interactions, they damaged more 
unprotected baits, but during the last survey, they damaged twice more protected 
baits than unprotected ones. This result suggests that the behaviour of dolphins 
evolved as time goes by, and that they can get used to the devices, being not afraid 
by them at the end.  
As for the 3 interactions with S. longirostris, there were always more damaged 
unprotected baits than protected ones. These were the first times that this species 
interacted with our devices, so, we could not assess their potential habituation to our 
experiments. 
Thanks to the images we recorded, we were able to observe the underwater 
behaviour of our devices. Many points still need to be improved, but the general 
conception of our device proved to be efficient regarding the protection of the bait. 
The results we obtained are more than positive and encourage us to maintain our 
efforts in this field.  
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Brotons, J. M., Grau, A. M., Rendell, L. (2008) Estimating the impact of interactions 
 
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 
(France). JMBA 88(6): 1263-1272 
Franse, R. Effectiveness of acoustic deterrent devices (pingers). MasterSc of Industrial 
Ecology. Leiden : Universiteit Leiden, 2005, 31 p. 
 
Jefferson, T. A., Curry., B. (1996) “Acoustic methods of reducing or eliminating marine 
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1: 41-70. 
 
Mooney, TA, Pacini, AP, Nachtigall, PE.  (2009) False killer whale (Pseudorca crassidens) 
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 
déprédation dû aux mammifères marins. In « L’espadon : de la recherche à 
l’exploitation durable ». Poisson F., Taquet M. (coord). Programme Palangre 
Réunionnais, Rapport final, pp. 231-247, 2001. 
 
Rabearisoa, N., Bach, P., Lucas, V., Giroux, F., Vely, M., Romanov, E., Tixier, P., 
Guinet, C. 2009. Assessment of the efficiency of the physical protection of fish as 
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WPEB. 12–14 October 2009. 
 
Rabearisoa, N., Filippi, T., Martin-Baillet, V., Lamoureux, J.P., Cotel, P. and Bach, 
P. 2010. Marine mammal depredation and pelagic longline fishery: assessment of the 
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IRD/SWIOFP Report. 12 p. 
 
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longlines in the Indian Ocean: an analysis of the Soviet historical database (1961-
1989) on tuna research . IOTC-2007-DWS-A1-11, 31 p. 
 
Sivasubramaniam. (1964) “Predation of tuna longline catches in the Indian Ocean, by 
killer-whales and sharks.” Bull. Fish. Res. Sin. 17, no. 2: 221-236. 
        


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

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

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