The collapse of clam fishery in the Gulf of Trieste is not surprising even if the fishery pressure declined year after year since the beginning of the 00’ties. The failure is referred mainly to baby clam Chamelea gallina, suffering extended depletion far to be justified, due to strong regulation imposed by the fishermen Consonsortium, supported partly by results of periodic surveys. In order to find some explanation an analysis was conducted on the hystorical data collected by our institution with special attention to the length-shell weight ratio, possibly a comprehensive index of the status of both environment and clam wellness. The data set contains clams processed in early 80’ies (from commercial fishery), in the 94-2000 interval and in 2009, the last two from surveys done with the same sampling gear. The results of the first data set are quite high (over 1.3) but as obtained from commercial clams must to be treated cautiously, the minimum (0.6) being in 1999-2000 (after a mass mortality occurred in 1996) and recovering (over 1) in 2009, but with very low clams density. The clam beds was moreover severely reduced by extending seagrass covering and molluscs vanishing beyond 5 meters depth, where once the most fishery pressure was exerted. The phenomena involved are complex and cannot be resolved simply by looking at exploitation even if in the 90’ies it was heavy and without control in spite of law severity and the buy-back project leaved “de facto” the operating boats equal in numbers . The hope of stock recovery is at the present quite low due to growth rate lower than previously supposed, but in the Gulf of Trieste this was clear since the mid of 90’ties to the fishermen and the scientific operators. Some encouraging signal may be found in Chlorophyll-a water content that, since 2006, seems progressively increasing and the recovery of length - shell weight ratio observed could be a positive response to food availability.
What went eventually wrong in the clam fishery in the Gulf of Trieste (Italy) (Prophecies, science and empiricism)
VALLI, GIORGIO;DEL PIERO, DONATELLA;
2012-01-01
Abstract
The collapse of clam fishery in the Gulf of Trieste is not surprising even if the fishery pressure declined year after year since the beginning of the 00’ties. The failure is referred mainly to baby clam Chamelea gallina, suffering extended depletion far to be justified, due to strong regulation imposed by the fishermen Consonsortium, supported partly by results of periodic surveys. In order to find some explanation an analysis was conducted on the hystorical data collected by our institution with special attention to the length-shell weight ratio, possibly a comprehensive index of the status of both environment and clam wellness. The data set contains clams processed in early 80’ies (from commercial fishery), in the 94-2000 interval and in 2009, the last two from surveys done with the same sampling gear. The results of the first data set are quite high (over 1.3) but as obtained from commercial clams must to be treated cautiously, the minimum (0.6) being in 1999-2000 (after a mass mortality occurred in 1996) and recovering (over 1) in 2009, but with very low clams density. The clam beds was moreover severely reduced by extending seagrass covering and molluscs vanishing beyond 5 meters depth, where once the most fishery pressure was exerted. The phenomena involved are complex and cannot be resolved simply by looking at exploitation even if in the 90’ies it was heavy and without control in spite of law severity and the buy-back project leaved “de facto” the operating boats equal in numbers . The hope of stock recovery is at the present quite low due to growth rate lower than previously supposed, but in the Gulf of Trieste this was clear since the mid of 90’ties to the fishermen and the scientific operators. Some encouraging signal may be found in Chlorophyll-a water content that, since 2006, seems progressively increasing and the recovery of length - shell weight ratio observed could be a positive response to food availability.Pubblicazioni consigliate
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