Effects of macroalgal expansion triggered by the 2011 earthquake and tsunami on recruitment density of juvenile abalone Haliotis discus hannai at Oshika Peninsula, northeastern Japan

Abstract

AbstractThe effects of the 2011 mega‐earthquake and massive tsunami on the recruitment process of juvenile abalone Haliotis discus hannai inhabiting the rocky shore ecosystem were investigated at Oshika Peninsula, northeastern Japan. Before the earthquake and tsunami, H. discus hannai of 2007–2010 yr classes were shown to undergo habitat shifts with growth from the crustose coralline algal area (CCA) to kelp beds (KB) via algal turfs (AT). These habitat shifts of abalone appear to occur because of its migration, associated with the ontogenetic changes in feeding behavior. Densities of 0‐yr‐old juvenile abalone (2010 yr class) and the sea urchin Mesocentrotus nudus inhabiting the CCA largely decreased just after the earthquake and tsunami, mainly affected by the catastrophic disturbance by this event. After that, previously unrecorded recruitments of juvenile macroalgae were observed in the CCA after the event probably because of the large‐scale removal of the primary herbivore, the sea urchin M. nudus. Subsequently, the KB and AT expanded into the previous CCA area. After the tsunami, the major factors causing the continuously low densities of the 0‐yr‐old juvenile abalone (2011–2013 yr classes) are inferred to be the limited nursery CCA habitat because of mud and silt sedimentation, the expansion of the KB and AT, and/or the decreased densities of reproductive adult abalone due to the direct disturbance of the tsunami. The results of this study imply that maintaining diverse algal communities which enable the habitat shifts of juvenile abalone H. discus hannai is one of the crucial factors to their successful recruitment.