THE COLLAPSE OF Chamelea gallina IN THE NORTHERN ADRIATIC: INSIGHTS FROM A BIOENERGETIC MODEL

Introduction

Chamelea gallina represents an economically and ecologically important species in the Adriatic basin, accounting for over 98% of 'venus clams' of the Italian production. In the last decades, however, the harvest has been steadily decreasing: According to Romanelli, 2009, C. gallina landings dropped from about 100,000 to 21,000 tonnes/year in 2005. Since then, landings have been fluctuating, due to a combination of changing environmental factors, fishery regulations and management activities, carried out by Consortia for the Management of this resource (CO.Ge.Vo). Such activities include stock assessment, based on yearly sampling along transect in fishery area, and juvenile relocatiom: to same extent, C. gallina is therefore being managed in a culturt based fishery regime. According to (Grazioli&al., 2022), climate change related increase in summer water temperature, heat waves, extreme events leading to massive discharge of debris and sediment from the rivers might have contributed to the clams massive mortalities observed in several Northern Adratic areas. However, the role of the steadily decrease in the trophic level of Northern Adriatic may also be playing a role in determining the collapse of C. gallina population observed in 2024 and 2025 off the coast of Venice and Chioggia, which caused the complete stop of the fishery. In this paper, we used an individual bioenergetic model to investigate the potential role of energy limitation in reducing both growth rate and energy investment in reproduction of this species.

Materials and methods

The bioenergetic model, is based on the formulation used in (Bertolini&al., 2021) to simulate the individual growth of Ruditapes philippinarum. The two most important parameters, namely the Clearance Rate, CR, and the Respiration Rate, RR, of a standard animal (1g wet weight) at optimal temperature, were determined in a nine months long experiment, in which 300 specimens of C. gallina were kept in a land-based raceway, previously used for R. philippinarum prefattening, connected to a shallow water lagoon located in the Po River Delta. The trial started in October 2024 and ended in June 2025. The raceway bottom was covered with sediment. Salinity ranged from 20 to 24 psu and temperature from 3 °C to 28 °C. Mortality rate was low during the first 6 months but rapidly increased in May and June, probably due to higher water temperatures than those usually experienced by this species Specimens were sampled with approximately monthly frequency from October 2024 to May 2025. Besides the biometric parameters (total length, width, thickness, total weight, fresh and dry soft body weight, fresh and dry shell weight), the clearance rate and the respiration rate were determined through dedicated experiments in November 2024, February 2025 and May 2025. Respiration and clearance rates were normalized on correspondent dry weights. The effect of water temperature on clearance rate and respiration was taken into account based on the bell-shaped function used in (Bertolini&al., 2021). The clearance rate showed a higher variability, compared to respiration one. The median values of RR of the three data sets, 0.76 mgO2 gdw^-1 h^-1 and the median CR determined in February 2025, 1.39 L gdw^-1 h^-1, were in accordance with previous literature estimates. Considering that the clams were kept in a non-natural environment, the model application employed an estimate of CR = 2.28 L gdw^-1 h^-1 based on the third quartile value of the February distribution, assuming that clams would perform better in their natural environment, compared with the mesocosm trial.

Results and discussion

1a

1b

Figure 1: 1a) Time-series (September 2022- December 2023) of water temperature at sea-bed level [°C] and concentration of chlorophyll- a [mg L^-1] at a site monitored by Venice and Chioggia CoGeVo. The summer/early autumn 2023 was characterized by high temperatures and low chlorophyll-a concentration. 1b) simulated individual growth in terms of total fresh weight [g] of C. Gallina, initial length 5mm.

The bioenergetic model was used to simulate clam growth at a sampling site regularly monitored by Venice and Chioggia Co.Ge.Vo. The model was forced by water temperature and Chlorophyll a concentration, which is taken as a proxy of feeding particles. Time series of daily values of both inputs were downloaded from Copernicus Marine Service from September 2022 to December 2023: their evolution is shown in Fig. 1a. To test the potential effect of energy limitation, the growth of a juvenile individual was simulated. The results, shown in Fig. 1b shows that the growth is limited, in particular during the summer 2023 in which the concentration of chlorophyll-a was found to be, on average, around 0.001 mg L^-1. This preliminary finding suggests that the negative trend in primary productivity observed in the North Adriatic in the last decades (Giani&al. 2012) could be added to the list of causes of the decreasing trend and the slow recovery after mass mortalities of C. gallina abundances. However, microphytobenthos should also be considered as a potential energy source, which could be correlated to water transparency: this aspect could be crucial and should be investigated to get more insights on the collapse of the population and the recovery rate.

Acknowledgement

This research was funded by the Agenzia Veneta per l'Innovazione nel Settore primario.

References

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