Causes of oyster (Ostrea edulis L.) mass mortality Limski kanal, Istria 1960

Abstract

       During the decades that have elapsed since the rearing of shellfish has begun in Limski Kanal, the annual mortality percentage observed in oyster reached from 10 to 20%. This mortality percentage can be considered normal under following circumstances:

       a) careless handling by forcible treatment of plants (especially by cementing),

       b) waves buffeting some parts of the installations,

       c) periodical attacks by fishes of the species Sparus auratus L.,

       d) periodical attacks by the harmful tuberlarian Stylochus pilidium Lang. Significant attacks by this enemy were registered in 1934. The registered mortality at that time was about 87% in oysters from 8 to 35 mm in length.

       In October 1960 a mass mortality occurred unexpectedly in oysters of all sizes.

       Different opinions as to the causes were given. For this reason a number of investigations were carried out:

1. First of all, 10 positions were chosen at which data were taken on oyster mortality during a period of several months.
2. On these positions hydrographical data were measured.
3. Meteorogical information for the same area were registered.
4. The oysters (shells and edible portions) were examined.
5. The sea water, muddy sediments, employed fertilizers (superphosphate and sulfuric acid), live oysters from the rearing places and natural banks, and dead oysters from the rearing places were analysed.
6. Different laboratory investigations of the influence exerted by low water salinity on oysters were carried out.
7. The mortality of mussels was compared with that of oysters. Here are the conclusions drawn on the basis of the above results:

       1. The following mortality percentages were registered in 1960:

                                                                                                  mortality

                  oysters                               year                           percentage

           cemented in fours                 1958                                 76,66

            cemented in fours                1959                                 61,81

            attached to sticks                  1959                                53,46

            attached to sticks                  1960                                36,66

        2.   Dead oysters were not damaged. Parasites were not found. Dying oysters were fat.

        3. All chemical analyses (water, oysters, fertiliser, etc.) excluded poisoning by chemical substances.

        4. 1960 was very rainy, particularly through most of February, March, July, September, October, November, and December. According to Vatova (1950), the influence of salinity changes is noticed a month or more after the end of the main rainfalls. Therefore we consider the whole of 1960 as a very rainy year.

       5. The surface water salinity in 1960 was therefore often between 4‰ and 19‰.

       6. The laboratory investigations have shown that salinity values under 20‰ are harmful to oysters.

       7. Together with freshwater, large harmful quantities of mud enter Limski Kanal settling on shells and edible portions of the animals.

       Similar results covering other areas and other species were obtained by Loosanoff 1952, 1961; Lambert 1950; Ranson 1951; Vatova 1950; Bytinski-Salz 1935 and Korringa 1950.