Effect of environmental change after the 2011 tsunami on the population dynamics of Japanese tubesnout Aulichthys japonicus (Gasterosteiformes)
The Japanese tubesnout Aulichthys japonicus Brevoort, 1862 is a small fish species, which lives in shallow coastal waters, especially in seagrass (Zostera spp.) beds. The fish is known for their unusual spawning behaviour of concealing their eggs in ascidians (Halocynthia roretzi). By studying the larval and juvenile fish fauna in a seagrass bed in southern Iwate, Japan since 2007, we observed that A. japonicus was a dominant species. On 11 March 2011, the seagrass bed was washed away by a Tsunami. The Japanese tubesnout population decreased after the Tsunami along with the loss of the seagrass bed, which also played an important role as a nursery ground for various coastal fishes. When the seagrass bed recovered from 2012, the tubesnout number also increased from 0.08 individuals/m2 in 2012 to 0.21 individuals/m2 in 2013. However, this number decreased to 0.05 individuals/m2 in 2014. Assuming that the increase and decrease of ascidian individuals affected the tubesnout number, we counted the number of ascidians growing on the sea-wall adjacent to the seagrass bed. The ascidian number increased from 0 individuals/m2 just after the Tsunami (2011) to 10 individuals/m2 in 2013, then decreased to 8 individuals/m2 in spring 2014 due to human activities. In 2015, all ascidians on the sea-wall were scraped off due to reconstruction, and tubesnout number decreased down to 0.05 individuals/m2 in 2016 even though the seagrass bed has recovered. We observed some egg masses of tubesnouts in the ascidians removed from the sea-bottom. The results of the statistical analysis using model selection by AIC suggest that ascidian decrease led to tubesnout decrease. Therefore, the decrease of ascidians appears more effective than the decrease of seagrass area in reducing the Japanese tubesnout number. It is also suggested that human activities including restoration work disturbs the natural resilience after the disaster.