Life history of Pseudogobius masago in the Manko Wetland, Okinawa-jima Island, southern Japan

Abstract

Pseudogobius masago, an endangered mud-dwelling goby inhabiting mudflats in Japan, is threatened by habitat loss due to human activities, such as land-fills and coastal development. However, basic information for this species is scarce, especially on Okinawajima Island, the southernmost known population. Life history traits of the species, including longevity and maturity, were investigated by a monthly sampling program conducted at Manko Wetland, Okinawa-jima Island, from March 2010 to November 2012. The spawning season of P. masago was determined from histological observations of ovaries and appearance of recruits over a year long period. The estimated batch fecundity was 202–542 eggs, fewer than that determined for individuals on the Japanese mainland (264–961). Daily increments of sagittal otoliths were counted for age and growth determinations, validation being attempted using alizarin complexone (ALC) fluorescent stain. The maximum life span of P. masago was found to be ca. four months, individual maturation and survival differing seasonally, being dependent upon birth month. P. masago was smaller and shorter-lived on Okinawa Island compared with populations in Mie or Fukuoka Prefectures, the overall life history strategy of the species apparently differing with latitude. Populations inhabiting temperate mainland Japan grew larger and produced more eggs over a shorter spawning season, whereas those inhabiting subtropical Okinawa Island grew to a smaller maximum standard length and produced fewer eggs, but had an extended spawning season with overlapping generations. Future conservation of the species is dependent upon reducing human activity on soft mudflats as much as possible.