Abstract
Understanding the anthropogenic impact of oyster farms is essential for the management and conservation of marine fishes. In Japan, Hiroshima Bay is the region with the most intense oyster farming and thus suitable to study the impact of these farms. Here, we surveyed spherical planktonic eggs of the black sea bream Acanthopagrus schlegelii, one of the most abundant fish in the Bay. Our survey was performed at fourteen stations which included places with oyster farms and historical spawning grounds. We found the highest egg densities in four stations, one with historical spawning aggregations and three with major oyster farms. Besides, surveys at the innermost part of Hiroshima Bay, where two major rivers discharge, showed a low density of eggs indicating that black sea bream avoids spawning in low salinity areas. Our study suggests that oyster farms benefit spawners of black sea bream by providing more food sources than historical spawning grounds for efficient spawning. Yet, whether oyster farms represent a full advantage for the species remains unclear, particularly because they are known to host jellyfishes that prey on eggs and limit water flow that can influence the survival of fish eggs.
Highlights
Hiroshima Bay is an enclosed bay in the western Seto Inland Sea of Japan with an average seabed depth of 25.6 m
Survey of fish eggs Fish eggs were surveyed at 14 stations (St. 1–14) in Hiroshima Bay from late April to early July of 2016 for seven days, and of 2017 for eight days
Mean egg densities and relative abundance of black sea bream eggs at St. 1 (2016: 0.63 eggs/m3, 7.14%; 2017: 1.20 eggs/m3, 19.61%) and St. 2 (2016: 1.06 eggs/m3, 15.38%; 2017: 1.09 eggs/m3, 14.29%) were the lowest compared to the other stations
Summary
Hiroshima Bay is an enclosed bay in the western Seto Inland Sea of Japan with an average seabed depth of 25.6 m. The many scattered islands across the Bay make it topographically complex (Kawamura & Shimizu, 1985; Blanco Gonzalez, Umino & Nagasawa, 2008). Water quality in the Bay is strongly influenced by freshwater input from classes A and B of Ohta River and Yahata River located at the most inner part (Hashimoto, Aono & Yamamoto, 2007). With over 40% of total production in Japan, the aquaculture of the Pacific oyster Crassostrea gigas is one of the major profitable industries in Hiroshima Bay (e-Stat, 2020). In the Bay, large-scale oyster farming occurs around
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