Search

Recently, the abundance of young Atlantic bluefin tuna ( Thunnus thynnus) tripled in the northwestern Mediterranean following effective management measures. We investigated whether its predation on sardine ( Sardina pilchardus) and anchovy ( Engraulis encrasicolus) could explain their concurrent size and biomass decline, which caused a fishery crisis. Combining the observed diet composition of bluefin tuna, their modelled daily energy requirements, their population size, and the abundance of prey species in the area, we calculated the proportion of the prey populations that were consumed by bluefin tuna annually over 2011-2013. To assess whether tuna could alter the size structure of the three small pelagic fish populations (anchovy, sardine, and sprat ( Sprattus sprattus)), the size distributions of the consumed prey species were compared with those of the wild populations. We estimated that the annual consumption of small pelagic fish by bluefin tuna is less than 2% of the abundance of these populations. Furthermore, size selectivity patterns were not observed. We thus concluded that tuna predation is unlikely to be the main cause of major changes in the small pelagic fish populations from this area. [ABSTRACT FROM AUTHOR]

Sardine populations worldwide can fluctuate drastically over short time periods, in terms of both biomass and biological characteristics. Fluctuations might be amplified by pathogens, but such hypotheses have never been considered in the absence of clear macroscopic symptoms. In the Gulf of Lions (NW Mediterranean), an enduring severe decrease in sardine ( Sardina pilchardus) size, condition and age has been observed since 2008, resulting in a strong decline in landings. This situation might have been caused or aggravated by diseases, especially as other drivers such as fisheries are not expected to be important. Therefore, we developed and performed a general veterinary study, aimed at detecting a wide range of potential pathogens, including parasites, viruses and bacteria. We explored which infectious agents are most likely to produce a causal relationship with sardine health, important information for future infection experiments. Among about 1300 sardines sampled during June 2014-July 2015, microscopic parasites (often trematodes and coccidians) and bacteria Tenacibaculum and Vibrio spp. were found. However, no clear damage to tissue was observed and there was generally no link between the agents' presence and host size or condition, so that no strong indications of pathogenicity were present. Nonetheless, 54 % of the sardines analysed in 2015 had elevated quantities of melano-macrophage centres (macrophage aggregates), indicating stress on the fish that might potentially be related to starvation and/or pollution. [ABSTRACT FROM AUTHOR]

Long-term time-series of bluefin tuna catches from ancestral Mediterranean and Atlantic trap fisheries are presented and analysed. The trap is a passive gear, little modified for centuries, that catches bluefin tuna Thunnus thynnus during their annual spawning migration. These features, together with preliminary analyses, lead us to suggest that long-term fluctuations in trap catches could reflect those in true abundance if they vary in the same manner all around the western Mediterranean and adjacent Atlantic. To test this hypothesis, we investigated 54 time-series more than 20 years long (the longest ones spanning four centuries) of trap catches along the western Mediterranean coasts of Italy, Sicily, Sardinia, Tunisia, Spain, and Morocco, and the adjacent Atlantic coasts of Spain, Portugal, and Morocco. Trends and cycles were identified using Eigen Vector Filtering and spectral analysis, and the synchrony between short- and long-term fluctuations in trap catches was studied with the modified correlogram of Koenig and Knops. The magnitude of fluctuations in trap catches is large, periods of great abundance being up to seven times bigger than those when abundance was low. More interesting was the occurrence of 100-year-long periodic fluctuations as well as 20-year cycles. These medium- to long-term fluctuations, representing more than 50% of the total variability in the time-series, were synchronous all around the western Mediterranean and adjacent North Atlantic. In contrast, short-term variability was synchronous at a local scale only. It is argued that long-term fluctuations in trap catches could be considered as a proxy for those of true abundance, and a synthetic time-series has been computed to depict them. Biological and ecological processes that could cause such long-term fluctuations are also discussed. [ABSTRACT FROM PUBLISHER]