Bycatch of large elasmobranchs
in the traditional tuna traps (tonnare) of Sardinia
from 1990 to 2009
The species composition and extent of large elasmobranch bycatch in the Sardinian tuna trap fisheries
have received minimal attention in the primary literature.
Introduction
The Sardinian Large Elasmobranch Database (SLED), a non-governmental project, collects data from direct observations, fishing industry sources and literature reviews on the bycatch of large elasmobranchs in the six active tuna fisheries in Sardinia (Saline, Capo Testa, Carloforte, Portoscuso, Calasetta and Portopaglia) over the period 1990-2009. During this period, 42 incidental capture events were documented, including 15 Mobulid rays and 27 large sharks belonging to ten species and seven families (23% of the total elasmobranch species reported in Sardinian waters)
Although the low number of elasmobranchs captured in this study does not represent a conservation concern, the species captured include some that are considered rare and/or declining in this region. The most abundant species caught was Mobula mobular, while the most frequently caught shark species was Alopias vulpinus. There were differences in catch rates between the six sites, with the highest recorded at Saline, Carloforte and Porto Paglia.
Although the impact of traditional tuna traps on elasmobranchs is less than other fishing gears in the Mediterranean, long-term monitoring of bycatch is essential to assess the presence, biodiversity and conservation of elasmobranchs in previously unstudied areas.
Schematic diagram of traditional tuna traps. The arrows indicate the tuna’s path through the individual net sections 1. Camera della morte, 2. Camera di ponente, 3. Bastarda, 4. Bordonaro, 5. Grande, 6.Bordoneredda, 7. Camera di levante, 8. Pedale and 9. Coda.
Methods
During 2004-2009, the SLED project collected information through direct observations, fishing industry sources and literature reviews to assess the extent and composition of bycatch in the six traditional tuna fisheries in Sardinia. Historical data from 1990 to 2003 were obtained mainly through interviews with fishermen and analysis of written sources.
The catch per unit effort (CPUE) rate was calculated for each site based on the number of fishing days per year. The historical period was compared with previous data from Camogli and Baratti, two of the few other Italian areas for which similar studies are available.
Results
Between 1990 and 2009, 42 incidental capture events of large elasmobranchs were recorded, comprising 15 Mobula mobular and 27 sharks belonging to nine species and six families. The two most notable events involved the aggregation of three M. mobulars in 1993 and twelve specimens in 2007.
The most frequently caught shark species was Alopias vulpinus (26% of the total), followed by Cetorhinus maximus (10%) and Carcharodon carcharias (5%). Isurus oxyrinchus, Carcharhinus brachyurus, Carcharhinus obscurus, Prionace glauca, Sphyrna zygaena and Hexanchus griseus were also reported.
The CPUE analysis showed that historical catch rates were significantly higher than the current period, suggesting a decline in elasmobranch populations in Sardinian waters. However, the impact of bycatch in tuna fisheries remains lower than in other fishing methods.
Discussion
Information collected from the SLED database has improved knowledge on the bycatch of elasmobranchs in Sardinian tuna fisheries, providing new data on their presence and abundance. The reduction in CPUE over time could indicate a decline in elasmobranch populations in the Mediterranean, as reported in previous studies.
Although tuna traps do not pose a significant threat to elasmobranch conservation compared to other fishing methods such as longlines and drift nets, long-term monitoring of bycatch remains essential to better understand marine biodiversity and promote sustainable management policies.
Acknowledgments
This paper has been carried out thanks to the contributions of the Centro di Educazione Ambientale e alla Sostenibilità Laguna di Nora and its Chairman Giuseppe Ollano. The authors thank all those who collaborated in the collection of data, including local fishermen, researchers and SLED project volunteers.
Related post
In silico investigation on the antigenic properties of Dolphin morbillivirus haemagglutinin
complexed to sperm whale (Physeter macrocephalus) SLAM and nectin-4 receptors
Cetacean morbillivirus (CeMV) is the most pathogenic virus for marine mammals and has caused deadly epidemics in the last 30 years.
