1. INTERACTION OF MARINE TURTLES WITH trammel nets
IN THE GULF OF GABES (TUNISIA)
Imed JRIBI(1), Malak BEN FATMA(1) and Mohamed Nejmeddine BRADAI(2)
(1) Sfax Faculty of Sciences P.O. Box 1170, 3000, Sfax, Tunisia
(2) National Institute of Sea Sciences and Technologies, P.O. Box 1035, 3000 Sfax, Tunisia .
Imed JRIBI
National Institute of Sea Sciences
and Technologies
35th Annual Symposium on Sea
Turtle Biology and Conservation
Sfax faculty of Sciences
Sea turtle by-catch data in the Mediterranean indicate high number of captures reaching about per year, with more than incidental deaths per year, while many others are killed intentionally. Small vessels using set net, demersal longline or pelagic longline represent most of the Mediterranean fleet and likely cause more incidental or intentional deaths than large vessels typically using bottom trawl or pelagic longline (Casale, 2011). In Tunisia, the studies to date have considered benthic trawls (Jribi et al., 2007), longlines (Jribi et al., 2008) and gillnets (Echwikhi et al., 2010) mainly in the gulf of Gabès, a wintering and foraging ground in the Mediterranean for the loggerhead turtles. They showed also a high level of interaction. In order to complete these studies and to have a more comprehensive view on the interaction with different fishing gears in the area, we assessed the level of the interaction of trammel net with loggerhead.
Introduction
Published data on sea turtle by-catch in the Mediterranean indicate high number of captures reaching up to /year, with more than incidental deaths per year, while many others are killed intentionally. Small vessels using set net, demersal longline or pelagic longline represent the most common Mediterranean fishing fleet and likely to cause more incidental or intentional deaths than large vessels that typically use bottom trawl or pelagic longline (Casale, 2011). In Tunisia, the studies to date have considered bycatch of turtles by benthic trawls (Jribi et al., 2007), longlines (Jribi et al., 2008) and gillnets (Echwikhi et al., 2010) mainly in the gulf of Gabès, a wintering and foraging ground in the Mediterranean for the loggerhead turtles. They showed also a high level of interaction. In order to complete these studies and to have a more comprehensive view on the interaction with different fishing gears in the area, we assessed the level of bycatch of loggerhead turtles in trammel net fishing gear.
Material and methods
Fig. 1: Gulf of Gabes (Tunisia)
The fieldwork took place in two important ports in the gulf of Gabes (Fig. 1), those of Sfax and Chebba. Interviews
with fishermen have been conducted. Data collected concerned the boats (length, power ...), the crew (number, experience...), the fishing gear used (mesh sizes, net heights, Mesh panel shape, gear depth ...), the technical description of the fishing activity (fishing periods, soak time, Length of the net…)and the environmental characteristics of the fishing ground (depths, bottom type, distance from the coast). The interviews also focused on the specific composition of catches of target species and discards of non target species.
To measure the impact of fishing on sea turtles, fishermen were interviewed on the incidental catch of sea turtles, the size of captured turtles and the seasonal variations of the catches.
The average annual catch of sea turtles is estimated as follows: = mean turtle catch by the gear =
- Ti is the average of captured turtles / year declared by a fisherman interviewed
n is the number of interviewed fishermen
The total annual catch of turtles is estimated by multiplying the average annual catch by the number of boat using the gear.
The average annual mortality of sea turtles is estimated as follows: = mean of turtle catched dead by the gear =
Mi is the average of turtles captured and found dead/year declared by a fisherman interviewed
The total annual mortality of turtles for the gear is estimated by multiplying the average annual deaths by the number of boat using the gear.
Results and discussion
In the study area as well as the rest of Tunisian waters, the fleet is very dynamic and changes often the fishing methods as well as gear structures depending on the season and target species. Trammel nets (Fig.1) are used throughout the year, however, they can be changed by another gear at any time. They consist on three nets in one: one net with a fine mesh made of fine flexible (usually braided) sandwiched between two others, of larger mesh and stronger thread. A fish, in attempting to pass through the larger mesh, pushes the fine-mesh net (which is also much less stretched than the larger-mesh net) through the larger mesh forming a bag on the other side in which the fish is entrapped. Trammel nets are generally set at sunset and the hauling operation starts just before sunrise. The spectrum of the target species is very wide including mollusks (Octopus, Sepia...), shrimp and fish (mainly Sparidae).
Fig. 2: Trammel net
Tab. 1: Estimations of catch rate, mortality rate and total catches and mortality of sea turtles
Gear
Trammel net
Catch rate (turtle/year/boat)
3.95±1.65
Mortality rate (dead turtle /year/boat)
0.2±0.1
Number of boat using trammel net in the study area
6559
Total annual catch ±
Total annual mortality
1311.8±655.9
A total of 20 skippers were interviewed during 2012: 11 at the Port of Chebba and 9 at the port of Sfax. Concerning the estimation of the by-catch, all fishermen declared an interaction of trammel nets with marine turtle. The answers indicated the capture of at least 1 turtle with an average of 3.95 turtles/year/boat. Concerning the mortality, the study indicates a low rates (0.2 turtles/year/boat), however, the total mortality is relatively high because the large number of boats using trammel net (n=6559). Table 1 presents a summary of the catch rate, mortality rate and total catches and mortality.
Acknowledgements This study has been made thanks to all fishermens interviewed. Thanks, also, to the symposium President Pr. Yakup Kaska, to Lobna Ben Nakhla (RAC/SPA), to the Sea Turtle Symposium organisers and all sponsors of the symposium for financial support to attend the symposium.
References
Casale P. (2011). Sea turtle by-catch in the Mediterranean. Fish Fish. 12:299–316.
Echwikhi K., Jribi I., Bradai M. N. and A. Bouain. (2010). Gillnet fishery -loggerhead turtle interactions in the Gulf of Gabes, Tunisia. Herpetol. J. 20: 25-30
Jribi I., Bradai M.N., and Bouain, A. (2007). Impact of trawl fishery on marine turtles in the Gulf of Gabès, Tunisia. Herpetol. J., 17, 110–114.
Jribi I., Bradai M. N. and Bouain A. (2008). Incidental captures of sea turtles by longline in the Gulf of Gabès (South Tunisia): Comparative study between bottom longline and surface longline. Sci. Mar., 72(2):