Finding the cause(s) of bivalve mortalities can be challenging if there is no single unusual event that provides compelling evidence. However, changes to the population dynamics of the affected species give important clues as to the nature of the mortalities. Analysing spatio-temporal patterns of the decline assists in making an informed judgement on the likelihood of potential causes. Determining the spatial extent of mortalities and whether they are episodic or continuous will narrow the number of potential causes. Analysing the population dynamics of other benthic species, especially those with similar biological traits, indicates whether environmental factors affect the entire ecosystem, including the bivalves, or whether the problem is species specific and more likely of biological origin. While this approach reduces the suite of potential causes, it is unlikely to pinpoint individual factors, and it can only inform further, more focused analyses. Should a suite of sublethal effects be responsible for unusually high mortalities, the causes and their individual contribution may be almost impossible to unravel. In the case of the Burry Inlet, the cockle mortalities have occurred for several years, which has allowed a more detailed analysis of their population dynamics. This is not necessarily the case for all cockle beds, and mortalities can be one-off events. Having reliable baseline data on a suitable spatial and temporal scale is necessary to ascertain unusual patterns in the population dynamics. If a species is of importance commercially or for conservation, it is advisable to design any monitoring programme accordingly. The number of anecdotal reports of unexplained bivalve mortalities (Elliott et al. 2012) suggests that the current programmes are not sufficient to provide the necessary information. It is also of concern that a current reduction in monitoring, due to the economic situation, may result in a lack of suitable spatial and temporal coverage (Borja & Elliott 2013). Despite this, it is emphasized that a robust scientific approach is required to verify anecdotal reports or counter perceptions held by commercial interest groups regarding such mass mortalities.