Study on Sustainability of Oyster Production and Its Values in Hiroshima Bay, with Special References to Larval Settlement and Ecological Services

概要

Doctoral Thesis

Study on Sustainability of Oyster Production and Its Values in Hiroshima Bay,
with Special References to Larval Settlement and Ecological Services


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(Summary)

Wahyudin

Graduate School of Biosphere Science
Hiroshima University

September 2020

Hiroshima Bay is famous with its oyster culture, supplying for 60% of
total oyster production in Japan. However, recent declining success rate of oyster
spat collection in Hiroshima Bay is suspected to be basically the same root of
declining of oyster production for 30 years due to phosphorus load reduction
measure. Insufficient availability of phytoplankton as food sources, along with
feeding competition among various filter feeders might have generated these poor
culture conditions. The present study aimed to analyze the major causes of low
recruitment success of oyster larvae and to evaluate the ecosystem services
provided by oyster culture conducted in Hiroshima Bay. In chapter 1, general
introduction was made on the importance of nutrients as agents to grow
phytoplankton that are the exclusive food source for oyster. Particularly, abundance
of small-sized phytoplankton can be a critical factor for larval recruitment because
the larvae can feed only phytoplankton with the size of less than 5 µm. In this
chapter, it was also described the importance of oyster culture in the aspect of
economic and ecological benefit.
In chapter 2, the prey-predator numerical model with 25 compartments
consisting of nutrient (nitrogen and phosphorus), phytoplankton, zooplankton,
adult oyster and oyster larvae with different developmental stages, other filter
feeders, and fishes was developed to understand the material cycle in the norther
Hiroshima Bay (nHB) where the center of oyster culture. The field observations
demonstrated that low food availability, particularly small-sized phytoplankton
suitable for oyster larvae, was the primary cause for the failure of the oyster larvae
recruitment in nHB. Out of the filter feeders identified in this study, oysters had
the strongest grazing pressure (39%) on phytoplankton, followed by zooplankton
(19%). It was clarified by model analyses that the major cause of low recruitment
success in Hiroshima Bay was due to low concentration of small-sized
phytoplankton followed by feeding pressure by filter feeder animals, such as adult
oyster, mussel, and barnacle on planktonic oyster larvae. Here, the accidental
filtration of larvae by the adult of the same bivalve species is called “larviphagy”.
In chapter 3, sensitivity analyses were conducted to see if increase the
nutrient loads and decrease of cultured oyster biomass would lead the increase of
small-sized phytoplankton biomass and increase the oyster larvae settlement
success. The sensitivity analyses showed that phytoplankton biomass, irrespective
of size, increased with increasing nutrient load. Particularly, P was identified as a
main limiting factor of phytoplankton growth and also key to the correction of

declines in oyster larvae. Although the top-down forces (i.e., larviphagy) could be
another potential cause for failed oyster larval recruitment, it was not a significant
factor. Thus, we conclude that bottom-up forces, i.e., P in the bay was the main
factor, rather than top-down forces, e.g., larviphagy, in determining the successful
recruitment of oyster larvae. To maintain larval recruitment at the level required by
fishermen, apparently high P loading (ten times higher than the present level) is
needed. Relaxation of treated sewage discharge has just started as an
oligotrophication measure for the Seto Inland Sea by the local government.
In chapter 4, to evaluate the ecological benefit of oyster culture in term of
monetary value, ecosystem service provided by oyster culture was estimated.
Nitrogen (N) removal from the system through the process of grazing
phytoplankton and detritus by oysters was estimated to be 254 kg N km–2 year–1
($207 km–2 year–1.This value is higher than N removal though the denitrification
processes ($45 km–2 year–1) and the calculated oyster harvest ($10 km–2 year–1),
respectively. In addition, provisioning service for fish habitat by the oyster culture
was estimated to be $299 km–2 year–1, which was obtained by converting from the
catch amount of commercial fish (black seabream, black scraper, pufferfish,
Japanese seabass, and black rockfish) during the observation period.
Oyster culture in Hiroshima Bay is like an “artificial hanging reef.”
Oysters can filter suspended particles, improve water quality and provide a
nurturing environment for the small living organisms that serve as food sources for
the fish species with economic value. However, oyster production in Hiroshima
Bay has been decreasing since the 1980s, which could adversely affect the
ecosystem services for human well-being. Thus, understanding the value of
ecosystem services provided by the oyster cultures may lead to better ecosystem
management strategies in the coastal areas. Due to their numerous environmental
and economic benefits, oyster cultures would be of particular interest to threatened,
protected habitats. Their existence in Hiroshima Bay is important, not only for
harvesting oysters of course, but also for producing ecosystem service benefits.