Population distribution of amphioxus (Branchiostoma belcheri tsingtauense) was investigated in the Luan River Estuary from 1999 to 2011, to describe its trends and discover the factors that influence its decline. Amphioxus are distributed on the seabed at 5-10 m depth, between the Xinkai Estuary and the Dapu River Estuary, where the primary sand components of the sediment are medium and fine, with particle size ranging from 0.001 to 1 mm. In recent years, the population density and biomass of amphioxus have sharply decreased. Changes in sediment granularity composition may significantly influence amphioxus distribution. Our data showed that the highest density region of amphioxus occurred where >90% of the sediment particles were between 0.063 and 0.5 mm in size. A reduction in sediment discharge from Luan River and the expansion of raft-breeding mariculture may be causing the decline in amphioxus through habitat destruction.
Keywords: Biomass; Branchiostoma belcheri tsingtauense; grain size; population density; population dynamics.