Multispecies Mortality Patterns of Commercial Bivalves in Relation to Estuarine Salinity Fluctuation

Abstract

Fluctuations in salinity may cause huge economic losses in estuaries with exploited commercial bivalves owing to their effect on mortality of these species. However, the same decrease in salinity does not affect all species in the same way, so it is interesting to study the effect of salinity from a multispecies standpoint. In the management of exploited bivalve beds, it is important to know the tolerance thresholds of the species, not only in cases of extremely low salinities but also over prolonged periods when salinities are low but not extreme. An analysis of mortality episodes of commercial bivalves in the Ulla River estuary (Galicia, NW Spain) from 1977 to 2009 revealed two mortality patterns related to how greatly the different species were affected. A mathematical model was designed to estimate salinity in the estuary based on weather conditions and tidal amplitude. By applying this model, it was possible to deduce the intensity and duration of the salinity decrease in the days prior to each mortality episode with the goal of relating these factors to mortality patterns. The two parameters found to be sufficient to explain the mortality observed were the minimum salinity at high tide and the number of consecutive days below a specific salinity threshold.

Cockle Cerastoderma edule fishery collapse in the Ría de Arousa (Galicia, NW Spain) associated with the protistan parasite Marteilia cochillia

“…The shellfish bed located in Lombos do Ulla, in the Ría de Arousa, used to be among those with the highest cockle catch in Galicia. High cockle mortality in this bed occurred occasionally due to very low salinity associated with heavy rains because of its location in the mouth of the Ulla River, the main tributary of the Ría de Arousa (Parada et al 2012). Two pathological conditions have been associated with periodic episodes of cockle mortality in this bed: one is disseminated neoplasia and the other is characterised by large foci of heavy haemocytic infiltration of connective tissue of various organs (Carballal et al 2001, Villalba et al 2001.…”

“…Ruditapes philippinarum (75 t), R. decussatus (40 t) and Venerupis corrugata (8 t), all of which are sympatric with cockles (CIMA unpublished). The estuarine character of the area involves wide salinity fluctuation; occasionally, sharp salinity decreases caused by heavy rain leads to high shellfish mortality (Parada et al 2012). …”

“…The lack of earlier salinity records necessitated an indirect way to estimate salinity values. Thus, a model designed by Parada et al (2012), integrating other environmental variables with long-term time records (net rainfall in the river basin, wind direction and force, and tidal amplitude), provided the daily mean salinity of the bottom water in the centre of the bed at high tide. The mean monthly temperature and the monthly minimum salinity in the years 2011 and 2012 were compared with those of previous years to determine whether temperature or salinity values recorded in 2011 and 2012 were abnormal and could have caused cockle mass mortality.…”

Impact of Hydroelectric Projects on Bivalve Clams in the Sharavathi Estuary of Indian West Coast

“…Not much work, however exists on the salinity decrease in the estuary due to execution of hydroelectric projects in the upstream areas causing almost constant release of freshwater into the estuary following power generation. In a rather isolated study related to the second scenario Parada et al (2012), reported the mortality of estuarine molluscan community on account of salinity decline in the estuary of Ulla River of Spain due to an upstream hydroelectric project. Salinity and substratum composition (for instance, sand-silt-clay mix) are the main factors influencing the estuarine molluscs (Harkantra 1975a, Kanaya andKikuchi, 2008;Mohan and Velayudhan 1998, Parker 1959, Sundaram and Shafee 1989.…”

“…Salinity and substratum composition (for instance, sand-silt-clay mix) are the main factors influencing the estuarine molluscs (Harkantra 1975a, Kanaya andKikuchi, 2008;Mohan and Velayudhan 1998, Parker 1959, Sundaram and Shafee 1989. In the case of Ulla estuary, salinity reduction directly *Address correspondence to this author at the Energy and Wetlands Research Group, Centre for Ecological Sciences, Indian Institute of Science, Bangalore -560012, India;Tel: 91-80-22933099/22933503;Ext: 107;Fax: 91-80-23601428/23600085/23600683;E-mail: cestvr@ces.iisc.ernet.in or in synergy with other factors, caused the bivalve mortality when its level declined below the optimum or lasted for long periods (Parada et al 2012). …”