The riddle of the sands: how population dynamics explains causes of high bivalve mortality

Abstract

Summary1. Large-scale bivalve mortalities in estuarine and coastal habitats are a major environmental and economic concern. They may have obvious causes such as extremely cold winter temperatures, but in the absence of an apparent chain of cause and effect, a rigorous, objective approach is needed to define a probable set of factors responsible for such mortalities. This study interrogates recurrent mortalities of the cockle Cerastoderma edule L. in the Burry Inlet (South Wales, UK), where the causes of the observed die-offs were obscure. 2. Spatial and temporal patterns in cockle population dynamics were analysed to determine whether mortalities were episodic or continuous, and related to cockle density. Laboratory survival experiments complemented the field studies to indicate the fitness of the cockles. The population dynamics of other species were studied to test for environmental disturbances affecting the entire benthic community. 3. High cockle mortalities occurred throughout the estuary in a specific cohort, namely 1-year-old cockles after their first winter. The mortalities did not appear to be episodic, but happened over the spring to early summer months. Cockle densities played a minor role in the decline. 4. The laboratory experiments showed that cockles from the Burry Inlet had higher survival rates than those collected from other areas at early life stages (0-year class). However, by the time they reached the end of their first year (1-year class), their survival rates declined significantly. Results suggested a physiological weakness in the cohort after the first winter. 5. Other benthic species did not suffer unusual mortalities. 6. Synthesis and applications. We emphasize that (i) interrogating patterns in population dynamics allows us to limit the number of likely causes of high bivalve mortality and generate more specific hypotheses; (ii) extending studies to other benthic species indicates the likelihood of environmental problems that affect the entire ecosystem; (iii) joined-up field and laboratory studies suggested that probable causes of cockle mortality were primarily species-, or even cohort-specific, biological factors; and (iv) well-designed monitoring systems will indicate the population dynamics of the target species on spatio-temporal scales suitable for assessing shifts in the populations, such as unexpected extirpation.

Abundance distribution patterns of intertidal bivalvesMacoma balthicaandCerastoderma eduleat the Murman coast tidal flats (the Barents Sea)

“…Cerastoderma edule population dynamics has a very typical feature of periodically occurring mass mortality events, when adult densities sharply decline from thousands of individuals to single cockles per square metre due to a variety of factors. Slow population recovery takes place after that (Strasser et al ., 2001; Malham et al , 2012; Callaway et al , 2013). Cockle density in Wadden Sea could reach 54,000 ind.…”

Responses to salinity stress in bivalves: Evidence of ontogenetic changes in energetic physiology on Cerastoderma edule

“…This short-lived species is known to have high spatial and temporal variability 10 but during the last decades cockle stocks have shown a progressive declining trend 12 , 13 , 16 – 19 . Declines of stocks are mostly due to mass mortality episodes and recruitment failures; both provoked by climate-related events 8 , 9 , 12 , 16 , 17 , 19 – 24 . The literature suggests that mass mortality events in cockles are often linked to torrential rains 8 , 9 , 17 , 19 , 21 , 22 with the magnitude of a mortality event depending on the intensity and duration of the flooding episodes maintaining salinity levels below the lethal physiological threshold of the species (10–12.5) 4 , 5 , 7 , 8 , 25 , 26 .…”

Fine‐scale seascape genomics of an exploited marine species, the common cockle Cerastoderma edule, using a multimodelling approach

“…In the UK, recurrent mass mortalities have occurred at several long-established cockle fisheries, resulting in significant economic losses (Woolmer, 2013). These mortalities have not been attributed to any single environmental factor, and interactions between multiple stress factors are suspected (Callaway, Burdon, Deasey, Mazik, & Elliott, 2013;Malham et al, 2012). Sustainable management of the common cockle is hindered by poor understanding of their population connectivity.…”