Decadal population dynamics of an intertidal bivalve (Austrovenus stutchburyi) bed: pre- and post- a mass mortality event

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ABSTRACT Three independent annual surveys with some spatial–temporal overlap were amalgamated to describe the population dynamics of the New Zealand intertidal cockle (Austrovenus stutchburyi) at Lews Bay, Whangateau Harbour from 1997–2018. After more than a decade of relative stability there was a dramatic decline in the biomass (from 7.4 to 1.8 kg wet weight m−2), density (1049 to 409 individuals m−2) and mean length (22.5 to 17.7 mm) of cockles following a mass mortality event in 2009. Large (≥30 mm) cockles were more severely impacted by the die-off, suffering mortality rates of more than 80%, compared with approximately 44–55% for cockles <30 mm. Since 2009 the density of A. stutchburyi at two survey sites has steadily increased, and at one of these sites has recovered close to pre-2009 densities. However, the density of large cockles has not increased since 2009, and the mean shell length has remained stable at 17.9 mm despite protection from harvesting. This hysteresis in the recovery of population size structure suggests exceptionally slow growth rates, high mortality of young adult cockles or a change in environmental conditions that no longer supports large individuals.