Surface habitat modification through industrial tuna fishery practices

Natural floating objects (FOBs) have always been a major component of the habitat of pelagic species. Since the 1990s, the number of FOBs in the open ocean has increased greatly as a result of the introduction of fish aggregating devices (FADs) by the industrial tropical tuna purse seine vessels. These changes, and their potential impacts on the species that associate with FOBs, remain poorly understood. Using fisheries observer data, data from satellite-linked tracking buoys attached to FOBs and Lagrangian simulations, this study quantifies the temporal changes in the density and spatial distribution of FOBs due to the use of FADs in the Indian Ocean (IO) between 2006 and 2018. From 2012 to 2018, the entire western IO is impacted, with FADs representing more than 85% of the overall FOBs, natural FOBs less than 10%, and objects originating from pollution 5%. Results also suggest that both FADs and natural FOBs densities are lower in the eastern IO, but this initial investigation highlights the need for further studies. Our study confirms that FADs have greatly modified the density and spatial distribution of FOBs, which highlights the need to investigate potential consequences on the ecology of associated species.

Fishing for the Virome of Tropical Tuna

While planktonic viruses have received much attention in recent decades, knowledge of the virome of marine organisms, especially fish, still remains rudimentary. This is notably the case with tuna, which are among the most consumed fish worldwide and represent considerable economic, social and nutritional value. Yet the composition of the tuna virome and its biological and environmental determinants remain unknown. To begin to address this gap, we investigated the taxonomic diversity of viral communities inhabiting the skin mucus, gut and liver of two major tropical tuna species (skipjack and yellowfin) in individuals fished in the Atlantic and Indian Oceans. While we found significant differences in the virome composition between the organs, this was totally independent of the tuna species or sex. The tuna virome was mainly dominated by eukaryotic viruses in the digestive organs (gut and liver), while bacteriophages were predominant in the mucus. We observed the presence of specific viral families in each organ, some previously identified as fish or human pathogens (e.g., Iridoviridae, Parvoviridae, Alloherpesviridae, Papillomaviridae). Interestingly, we also detected a ‘core virome’ that was shared by all the organs and was mainly composed of Caudovirales, Microviridae and Circoviridae. These results show that tuna host a mosaic of viral niches, whose establishment, role and circulation remain to be elucidated.

Fishing for the Bacteriome of Tropical Tuna

Background : Although tunas represent a significant part of the global fish economy and a major nutritional resource worldwide, their consumption poses a risk of food poisoning through the development of particular bacterial pathogens. However, their microbiome still remains poorly documented. Here, we conducted a multi-compartmental analysis of the taxonomic composition of the bacterial communities inhabiting the gut, skin and liver of two most consumed tropical tuna species (skipjack and yellowfin), from individuals caught in the Atlantic and Indian oceans. Results : Our results revealed that the composition of the microbiome was independent of fish sex, regardless of the species and ocean considered. Instead, the main determinants were (i) tuna species for the gut and(ii) sampling site for the skin mucus layer, and (iii) a combination of both parameters for the liver. Interestingly, only 4.5% of all ASVs were shared by the three compartments, raising numerous questions about the circulation of microorganisms within the tuna body. Our results also revealed the presence of a unique and diversified bacterial assemblage within the liver, comprising a substantial proportion of histamine-producing bacteria, well known for their potential pathogenicity and their contribution to fish poisoning cases. Conclusions : These results indicate that the tuna liver is an unexplored microbial niche whose role in the health of both the host and consumers remains to be elucidated.