Abstract
Stranding combined with mass-mortality events of sandy-beach organisms is a frequent but little-understood phenomenon, which is generally studied based on discrete episodes. The frequency, magnitude, and possible causes of stranding and mass-mortality events of the trigonal clam Tivela mactroides were assessed based on censuses of stranded individuals, every four days from September 2007 through December 2008, in Caraguatatuba Bay, southeastern Brazil. Stranded clams were classified as dying (closed valves did not open when forced) or dead (closed valves were easily opened). Information on wave parameters and the living intertidal clam population was used to assess possible causes of stranding. This fine-scale monitoring showed that stranding occurred widely along the shore and year-round, with peaks interspersed with periods of low or no mortality. Dead clams showed higher mean density than dying individuals, but a lower mean shell length, attributed to a higher tolerance to desiccation of larger individuals. Wave height had a significant negative relationship to the density of dying individuals, presumed to be due to the accretive nature of low-energy waves: when digging out, clams would be more prone to be carried upward and unable to return; while larger waves, breaking farther from the beach and with a stronger backwash, would prevent stranding in the uppermost areas. This ecological finding highlights the need for refined temporal studies on mortality events, in order to understand them more clearly. Last, the similar size structure of stranded clams and the living population indicated that the stranded individuals are from the intertidal or shallow subtidal zone, and reinforces the ecological and behavioral components of this process, which have important ecological and socioeconomic implications for the management of this population.
Highlights
Stranding and mass mortality on the shore are common events for many invertebrate groups, and their frequency, extent, and duration have been increasingly reported in recent decades [1,2,3]. These events are attributed to several factors, such as storms, positive sea-temperature anomalies, parasitism, harmful algal blooms, and hypoxia [1,2,4,5,6,7,8,9,10,11,12]
The trigonal clam Tivela mactroides is an important sandy-beach species, both economically and ecologically [20,27,28,29,30,31,32,33,34]. It has a wide distribution from Venezuela to Brazil (Pará to Santa Catarina states) and in the West Indies [35]
The trigonal clam frequently reaches high abundance in Venezuelan beaches, where it is an important resource of artisanal fisheries [28,30,31]
Summary
Stranding and mass mortality on the shore are common events for many invertebrate groups, and their frequency, extent, and duration have been increasingly reported in recent decades [1,2,3]. Large numbers of stranded clams were observed in the intertidal zone of these beaches, and these mass strandings were presumed to be related to population density-dependent processes such as individual growth and mortality rates [32]. For this population, an assessment of harvesting intensity showed that under ordinary conditions, the amount of clams harvested yearly is about 25 tons. Some physical changes related to global climate change can increase the frequency and severity of mass-mortality episodes on sandy beaches [9,11] In view of these considerations, assessment of mortality events of T. mactroides is a crucial step in managing populations of this species
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