Introduction
Bivalve mollusks play a vital role in coastal ecosystems and global aquaculture, contributing significantly to food security, nutrient cycling, and seafood-related economies. Among them, two clam species hold particular ecological and commercial importance: Ruditapes decussatus (the grooved carpet shell), native to the European Atlantic coasts and the Mediterranean, and Ruditapes philippinarum (the Manila clam), native to the Indo-Pacific region now naturalized in the Mediterranean Sea. Over recent decades, and increasingly in recent years, the clams’ populations have undergone significant changes and are threatened by the growing frequency and intensity of extreme events (e.g., heatwaves and flash flood) in lagoon ecosystems coupled with the expansion of invasive species, such as the blue crab Callinectes sapidus. Indeed, a recent study reported that the blue crab shows a feeding preference for R. decussatus under mesocosm condition. According to FAO data, bivalves are among the most produced mollusks in aquaculture, with R. philippinarum being the second most cultivated bivalve species after oysters. Historically, Italy has been the leading European producer of clams and one of the top global producers, accounting for over 90% of European clam production in 2010. On the other hand, the species R. decussatus is traditionally harvested using artisanal fishery methods in the Mediterranean Sea, particularly in brackish coastal lagoons.
The objective of this study is to develop a pre-fattening system for clams that ensures an optimal cost-benefit ratio to strengthen clam supply chains. Specifically, we explored the effect of 1) different pre-fattening systems (lanterns and poches), 2) the effect of the presence of sandy substrate in lanterns (S vs B), 3) the effect of the installation depth in poches (SP vs BP). We considered clam growth and survival as response variables. The trials were tested in a suitable production area within the Santa Gilla lagoon, in collaboration with the fishery cooperative Consorzio Ittico Santa Gilla.
Material and methods
For the trials we considered a homogeneous starting density of 0.2g/cm2 of clams (0.13g/ind) in both lanterns and poches. The trials started in March 2024 (t0) and lasted four months. Bottom poches (BP) were placed at a depth of around 1.5m, while lanterns (L) and suspended poches (SP) were positioned within the top 20 cm of the water column. Each system was arranged along a long-line setup, spaced approximately 50 cm apart from each another.
Every month environmental parameters (temperature °C, Salinity and dissolved oxygen) were recorded using a multiparametric probe (In-Situ SmarTROLL Multiparameter Handheld). Additionally, a HOBO pendant data logger for temperature was placed in situ. At the same time, specimens were sampled and measured to estimate growth. Water samples were also collected to analyze the concentration of some inorganic nutrients (nitrate, nitrite, ammonia, total phosphorus). At the end of the trials all specimens were collected to estimate the survival rates under different conditions. Differences in clams’ growth were assessed with univariate permutational analyses of variance (PERMANOVA), considering treatment and time as sources of variation.
Results and Discussion
Biometric data (weight, length, width, thickness) of R. philippinarum were monitored over four periods (t0–t3) and showed significant differences in treatments and time, but not in their interaction. Weight increased progressively in all systems, with the highest gains observed in the Lantern Sand treatment. Systems using Poches, particularly the BP type, showed lower weight gain. Pairwise tests confirmed significant differences between t0 and t1 across all treatments, and among most system combinations at later time points. Length and width followed similar trends, with steady increases over time, particularly in lantern systems. Significant differences were detected across most group comparisons, except between some Poches treatments at later stages. Thickness also increased steadily, with the highest gains again seen in Lantern Sand. In summary, Lantern systems, especially those with sand substrate, consistently outperformed Poches systems across all biometric parameters. Moreover, survival was higher in the Lantern system (75–78%) compared to the Poches (47–52%).
This pilot study evaluated the effectiveness of different pre-fattening systems for clams in the Santa Gilla lagoon, comparing two cages: lantern and poches, with and without sandy substrate. Results showed that lantern systems with sand significantly improved clam growth and survival, supporting previous findings. However, higher stocking densities negatively impacted survival rates, highlighting the need for thinning practices and regular biofouling removal. Lantern systems were more reliable in offering protection and stability to the juveniles. The lagoon’s nutrient-rich but eutrophic environment showed oxygen and nutrient levels suitable for clam farming, although continuous monitoring is essential, especially during periods of environmental stress. The observed growth rates aligned with previous studies from other Italian lagoons and confirmed the suitability of Santa Gilla for clam aquaculture. Nevertheless, improved management strategies are needed, such as increasing cleaning frequency and optimizing stocking densities. These findings also emphasize the need for adaptive practices to face challenges such as extreme weather events and variable water quality.
Acknowledgement
We acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.5 - Call for tender No.3277 published on December 30, 2021 by the Italian Ministry of University and Research (MUR) funded by the European Union – NextGenerationEU. Project Code ECS0000038 – Project Title eINS Ecosystem of Innovation for Next Generation Sardinia – CUP F53C22000430001- Grant Assignment Decree No. 1056 adopted on June 23, 2022 by the Italian Ministry of Ministry of University and Research (MUR).