In recent years recurrent bivalve mass mortalities considerably increased around the world, causing the collapse of natural and farmed populations. Venice Lagoon has historically represented one of ...the major production areas of the Manila clam Ruditapes philippinarum in Europe. However, in the last 20 years a 75 % decrease in the annual production has been experienced. While climate change and anthropogenic interventions may have played a key role in natural and farmed stocks reductions, no studies investigated at multiple levels the environmental stressors affecting farmed Manila clam to date.
In this work we carried out a long-term monitoring campaign on Manila clam reared in four farming sites located at different distances from the southern Venice Lagoon inlet, integrating (meta)genomic approaches (i.e. RNA-seq; microbiota characterization), biometric measurements and chemical-physical parameters. Our study allowed to characterize the molecular mechanisms adopted by this species to cope with the different environmental conditions characterizing farming sites and to propose hypotheses to explain mortality events observed in recent years. Among the most important findings, the disruption of clam's immune response, the spread of Vibrio spp., and the up-regulation of molecular pathways involved in xenobiotic metabolism suggested major environmental stressors affecting clams farmed in sites placed close to Chioggia's inlet, where highest mortality was also observed.
Overall, our study provides knowledge-based tools for managing Manila clam farming on-growing areas. In addition, the collected data is a snapshot of the time immediately before the commissioning of MoSE, a system of mobile barriers aimed at protecting Venice from high tides, and will represent a baseline for future studies on the effects of MoSE on clams farming and more in general on the ecology of the Venice Lagoon.
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•In recent years recurrent bivalve mass mortalities considerably increased around the world.•Criticalities affecting clam stocks in different sites within the Venice lagoon have been characterized.•Microbiota and gene expression profiling allowed the identification of seasonal- and site-specific environmental stressors.•Identification of molecular mechanisms adopted by Manila clam to cope with stressors
The Mediterranean mussel Mytilus galloprovincialis represents one of the most important aquaculture species in the Mediterranean Sea. The Venice lagoon is an historically important mussels farming ...area, contributing 25% of the total mussel production of the Veneto region. This area represents a vulnerable ecosystem, subject to significant spatio-temporal variations due to climate change and anthropogenic interventions as for the recent commissioning of MoSE (Experimental Electromechanical Module), a system of mobile barriers aimed at protecting Venice from high tides. In this study, we monitored for two consecutive production cycles three mussels farming sites located in Chioggia at different distance from the southern Venice lagoon inlet. During each season of the two productive cycles, mussels were collected in the three farming sites and site- and seasonal-specific variations of growth, chemical composition and fatty acid profile were investigated. In addition, we also characterized whole gene expression profiles of mussels grown in two investigated farming sites. Biometric traits and gene expression results highlighted narrow differences between rearing sites, inconsistent between the two production cycles. Conversely, seasonality led to important changes in mussel transcriptional profiles, fatty acid composition and condition index due to mussel reproductive cycle and seasonal conditions of the Venice lagoon (e.g. temperature, food availability).
Fatty acid differences were primarily linked to dietary changes, rather than stress-related or physiological differences. Regardless of the rearing site, the lowest condition index was observed in October, probably due to intensified metabolic expenditure and depletion of energy reserves to cope with summer stressful environmental conditions. This hypothesis is corroborated by gene expression profiling and by the reduction in the mussel lipid content observed between July and October.
The differences between mussels farmed in different sites were mainly related to the hydrodynamics which influences water physico-chemical parameters and feed availability for mussels. Overall biometric traits (i.e. growth) were more favourable in mussels farmed in the most external site characterized by higher hydrodynamics and more favourable water conditions compared to the other two sites. The most favourable condition of mussels farmed close to Chioggia inlet was also measured by gene expression profiles suggesting stressful conditions in inner site at the end of the second production cycle. Despite comparisons between farming sites at transcriptional level showed different results between production cycles, mussels exhibited comparable growth patterns during corresponding months or seasons, suggesting limited and not yet noticeable effects of MoSE commissioning in mussel production.
•Venice lagoon mussels farming sites were characterized.•Mussels show site- and seasonal-specific variations of growth, fatty acid and transcriptional profiles.•Fatty acid differences between mussels were primarily linked to dietary changes.•Summer stressful environmental conditions lead to metabolic expenditure and depletion of energy reserves.•Stressful conditions were mainly observed in inner site.
Abstract
Fishery discard survival depends on multiple conditions; caution is essential when survival study outputs are employed to support management decisions. The study presents a stepwise ...procedure, devised to estimate discard survival, that accounts for the variability characterizing commercial fishing practices. The procedure was applied to the first survival study performed onboard rapido trawlers targeting Solea solea in the Mediterranean Sea. Undersized specimens collected during sorting were assessed for vitality; some were retained for captive observation. The main drivers affecting discard survival at the time of catch sorting (immediate survival) were identified and used to outline four different operational conditions set (scenarios). Immediate survival in each scenario was subsequently modified by applying a hazard coefficient of survival after 5 days of captive observation in relation to each vitality class, thus obtaining relative survival estimates following discarding. Temperature and air exposure duration were found to exert a major effect on survival, with catch weight and seabed type being additional important factors. The relative survival rate showed an aggregate value of 22.9% (10.5–33.4%). Scenario approach can enhance our understanding of the stressors influencing discard survival. The outcomes are discussed to explore the potential applications of the procedure to the identification of mitigation strategies.
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