Preliminary study of growth and mortality of black scallop (Chlamys varia Linnaeus, 1758) in Mali Ston Bay (southern Adriatic)

Transcription

1 ORIGINAL SCIENTIFIC PAPER Preliminary study of growth and mortality of black scallop (Chlamys varia Linnaeus, 1758) in Mali Ston Bay (southern Adriatic) Mara Rathman, Valter Kožul, Jakša Bolotin, Nikša Glavić, Nenad Antolović University of Dubrovnik, Institute of Marine and Coastal Research, Kneza Damjana Jude 12, Dubrovnik, Croatia Abstract An experimental monitoring of growth intensity and mortality of black scallop (Chlamys varia) was carried out in Mali Ston Bay, from September to September. The aim was to investigate the possibilities for commercial rearing of this bivalve mollusc. The samples of 270 juvenile individuals were distributed equally in three experimental cages placed on depths of 1, 3 and 5 metres. The growth rate is not significant between individuals from cages on different depths, while mortality rate is highly significant in cage on 5 m depth. Key words: scallop, Chlamys varia, growth, mortality, Mali Ston Bay Introduction The black scallop (Chlamys varia) is a bivalve, widely distributed along the Atlantic coast of France, being less abundant in the English Channel, and known from a few localities in the Mediterranean Sea (Letaconnoux & Audouin, 1956). It is distributed along the Croatian coastline of the Adriatic Sea (Marguš et al. 2005). The most common size in shell length is mm, but individuals of up to 65 mm have been reported (Lucas 1965). In the European fishery, contribution of black scallop is minor, except in some parts of France. Despite this, much research has been done on its biology. The most detailed studies on black scallop have been carried out in Lavéneoc, Bay of Brest, France (Shafee 1979, 1980, 1981, 1982; Shafee and Lucas 1980, 1982), comprising metabolism, growth and reproduction. Growth and reproduction has been studied on black scallop from the west coast of Ireland (Rodhouse and Burnell 1979, Burnell 1991, Burnell 1995). Numerous studies describe growth in bivalves as an increase in some dimension of the shell, usually length, and the maximal distance between the anterior and posterior margins (Seed 1980). Growth rate may be expressed as an increase in size per unit time (usually years), i.e., the absolute growth rate, or as a proportional increase per unit time, i.e., the relative growth rate (Seed 1976). Although the population of black scallop in the Adriatic Sea has an economic importance, the obtained data on growth intensity and mortality in Mali Ston Bay can facilitate further diversification in Croatian aquaculture. Materials and methods Experimental fieldwork was carried out at locality Bistrina, situated in the Mali Ston Bay (southern Adriatic), which is marine protected area in Croatia. Strong currents and freshwater springs affect the sea temperature and salinity in the bay. Juvenile black scallops were collected by scuba diving from their natural environment in the Mali Ston Bay, from depths between 4 and 7 m. The sample of 270 individuals was distributed by 90 individuals in each of three cages, placed on commercial shellfish farm on depth of 1, 3 and 5 meters. Black scallops were kept in 4 mm mesh net cages (40 cm x 40 cm). Growth rate was measured monthly, from September to September. Measurements included maximal dorsal to ventral shell height, and maximal anterior to posterior shell length to nearest 0.1 mm, using vernier calipers. Sea temperature was th Croatian & 8 th International Symposium on Agriculture Dubrovnik Croatia

2 Ribarstvo, lovstvo i pčelarstvo monitored daily at depth of 1, 3 and 5 meters by using Tidbit temperature loggers (Onset Corp., USA). Salinity was monitored monthly by using handheld conductometer 315i (WTW, Germany). Mortality rate has been assessed monthly. Experimental values were compared using a one-way analysis of variance (ANOVA), and the significance of mean differences was tested using a Tukey s multiple range test. The significance level was set at level p<0.05 Results and discussion Sea temperature (Graph 1, top) varied seasonally with maximum of 25.5 C recorded in July (1 m and 3 m), and August (5 m), and minimum of 8 C in February. The salinity (Graph 1, bottom) showed fluctuation. In July (1 m, 3 m) and May (5 m) reached minimum of 29.4, with maximum of 38.1 in November temp ( C) 1m temp ( C) 3m temp ( C) 5m 22 Temperature ( C) sep 08 nov 08 feb 09 apr 09 jun 09 aug 09 oct 08 dec 08 mar 09 may 09 jul 09 sep 09 sal ( ) 1m sal ( ) 3m sal ( ) 5m 36 Salinity ( ) sep 08 nov 08 feb 09 apr 09 jun 09 aug 09 oct 08 dec 08 mar 09 may 09 jul 09 sep 09 Graph 1. Monthly values of sea temperature (top) and salinity (bottom) recorded in the Mali Ston Bay on 1, 3 and 5 m depth (September September ). 48. hrvatski i 8. međunarodni simpozij agronoma Dubrovnik Hrvatska 595

3 At the start of experimental rearing of black scallops in cages, the average shell length of collected individuals was mm. The smallest had 9 mm, and largest had 22 mm in length. During the experimental period of 12 months, the individuals increased their mean length for 50 %. At the end, in September, mean shell length was mm in cage on 1 m depth; mm on 3 m depth, and mm on 5 m depth (Table 1). Table 1. Monthly mean values of shell length, shell height and mortality rate of black scallop (Chlamys varia) in experimental cages on depth of 1, 3 and 5 meters. cage on mean shell length mean shell height mortality rate (%) month depth (m) (mm) (mm) September October November December February March April May June July August September / / / / / / / / / / / / / / / / / / / / / / / / / / / / / Seasonal growth in bivalves is influenced by the interaction of several environmental variables, particularly water temperature and food supply (Bayne and Newell 1983). Scallops are generally very vulnerable to the effects of low salinities, as they are unable to maintain prolonged valve closure. In bivalve species Argopecten purpuratus positive th Croatian & 8 th International Symposium on Agriculture Dubrovnik Croatia

4 Ribarstvo, lovstvo i pčelarstvo growth potential (determined from physiological rate measures) was obtained at salinity 27 to 30 ppt. At lower salinities (18-24 ppt) growth potential was negative as a result of the combined effects of reduced clearance and ingestion rates, and high excretion and aerobic respiration rates (Navarro and González 1998). Mortality rate during the experimental period was 6.9% on 1 m depth; 3.33% on 3 m, and 39.71% on 5 m depth. Since mortality was lower at the minor depths, we can presume that higher mortality rate on 5 m depth was related to proximity of muddy sea bottom and its composition. For the comparison, juveniles of black scallops being reared in the Krka River estuary (Marguš, 2005), from initial size 16.4±2.1 to 17.6±2.2mm reached size 42.7±3.2 to 47.5±3.0 mm during one-year period. Total mortality ranged %. Comparison of growth intensity of black scallop between experimental cages on depth of 1, 3 and 5 meter showed that difference in growth rates is not significant, but difference in mortality rate in cage on 5 m depth is significant. Conclusion During the experimental period of 12 months, an increase of mean length by 50% was recorded in the individuals of black scallop. At the end of the experimental period, the largest mean shell length amounted to mm in cage on 1 m depth. Mortality rate is significantly higher in individuals reared in cage on 5 m depth. The results of the research can facilitate further diversification in Croatian aquaculture. Literature Bayne B.L., Newell R.C. (1983). Physiological energetic of marine molluscs. In: Saleuddin A. S. M., Wilbur K. M. (Eds.): The mollusca New York (USA): Academic Press. Burnell G.M. (1991). Annual variations in the spawning and settlement of the scallop Chlamys varia (L.) on the west coast of Ireland. In: Shumway S. E., Sandifer P. A. (Eds.): An International Compendium of Scallop Biology and Culture. World Aquaculture Workshops No Baton Rouge (USA): World Aquaculture Society Burnell G.M. (1995). Age-related protoandry in the scallop, Chlamys varia (L.) on the west coast of Ireland. ICES Marine Science Symposia. No. 199: Conan G., Shafee M.S. (1978). Growth and biannual recruitment of the blcak scallop Chlamys varia (L.) in Lanvéoc area, Bay of Brest. Journal of Experimental Marine Biology and Ecology. Vol. 35: Letaconnoux R., Audouin J. (1956). Contribution à l étude du pétoncle (Chlamys varia L.). Revue de Travaux de l Institute des Pêches Maritimes, Nantes. No. 20: Lucas A. (1965). Recherches sur la sexualité des Moluskues Bivalves. Bulletin Biologique Francaise et Belgique. No. 99: Marguš D., Teskeredžić E., Teskeredžić Z., Tomec M. (2005). Prihvat, preživljavanje i rast mlađi malih kapica (Chlamys varia L.). Ribarstvo. Vol. 63(3): Navarro J. M., Gonzáles C. M. (1998). Physiological responses of the Chilean scallop Argopecten purpuratus to decreasing salinities. Aquaculture. No. 167: Rodhouse P.G., Burnell G.M. (1979). In situ studies on the scallop Chlamys varia. In: Gamble J. C., George J.D. (Eds.): Progress in Underwater Science Plymouth (UK): Pentech Press. Shafee S.M. (1979). Underwater observation to estimate the density and spatial distribution of black scallop, Chlamys varia (L.) in Lanvéoc (Bay of Brest). Bulletin Office National Pêches (Tunisie). No. 3: hrvatski i 8. međunarodni simpozij agronoma Dubrovnik Hrvatska 597

5 Shafee S.M. (1980). Application of some growth models to the black scallop Chlamys varia (L.) from Lanvéoc, Bay of Brest. Journal of Experimental Marine Biology and Ecology. Vol. 43: Shafee S.M. (1981). Seasonal changes in the biochemical composition and calorific content of the black scallop Chlamys varia (L.) from Lanvéoc (Rade the Brest). Oceanologica Acta. No. 4: Shafee S.M. (1982). Variations saisonnièresde la consummation d oxygène chez le petoncle noir, Chlamys varia (L.) de Lanvéoc (Rade the Brest). Oceanologica Acta. No. 5: Shafee S.M., Lucas A. (1980). Quantitative studies on the reproduction of black scallop Chlamys varia (L.) from Lanveoc area. Journal of Experimental Marine Biology and Ecology. Vol. 42: Shafee S.M., Lucas A. (1982). Variations saisonnières du bilan énergétique chez les individus d une population de Chlamys varia (L.): Bivalvia, Pectinidae. Oceanologica Acta. No. 5: Seed R. (1976). Ecology. In: Bayne B. L. (Ed.): Marine Mussels: Their Ecology and Physiology Cambridge (UK): Cambridge University Press. Seed R. (1980). Shell growth and form in the bivalve. In: Rhoads, D. C., Lutz, R. A. (Eds.): Skeletal Growth of Aquatic Organisms New York (USA): Plenum Press th Croatian & 8 th International Symposium on Agriculture Dubrovnik Croatia