Oyster farms are the main spawning grounds of the black sea bream Acanthopagrus schlegelii in Hiroshima Bay, Japan

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.

Spawning time of black sea bream Acanthopagrus schlegelii, related to underwater photoperiodism in oyster farms

“…In addition, planktonic A. schlegelii eggs started to sink after the initiation of the heartbeat (Kitajima et al ., 1994), and some eggs in this study sunk to the bottom of the incubation tank although they exhibited normal development. Although A. schlegelii in Hiroshima Bay prefer to spawn in the oyster farming areas located close to the sea surface (Kawai et al ., 2021), during their spawning season, the Bay registers a reduction in seawater temperatures based on depth (Hiroshima City Agriculture, Forestry and Fisheries Promotion Center: http://www.haff.city.hiroshima.jp/suisansc/suion.html). Considering that the recent increase in sea surface temperature expands the vertical gap in water temperature in Hiroshima Bay (Takatsuji, 2003), A. schlegelii eggs could easily be surrounded by low temperatures owing to their tendency to sink as they develop, resulting in potentially high mortality rates of egg stages.…”

“…In contrast, the authors could not observe a relationship between the reproductive rhythm of A. schlegelii and the tidal rhythm in Hiroshima Bay. Nonetheless, A. schlegelii might not be sensitive to the timing of the tide in Hiroshima Bay because the species mainly spawns around oyster farms and not in the estuary in the bay (Kawai et al ., 2021). The maximum speed of the tidal current is approximately 1.11 km h −1 in the north of Ohkurokamishima Island (Japan Coast Guard, 2003), but generally only 0.04 km h −1 around the underwater oyster raft structures (Kawanisi et al ., 1999).…”

“…Eggs were collected from mid‐May to early June in 2018 (three times) and from early May to mid‐June in 2019 (five times) in the oyster farming area to the north of Ohkurokamishima Island, which has the highest density of A. schlegelii eggs in Hiroshima Bay (Kawai et al ., 2021). Horizontal trawling of a NORPAC net (diameter: 450 mm, length: 1800 mm, mesh: 0.335 mm) was performed with a flowmeter four times a night (18.00, 21.00, 0.00 and 3.00 hours) in 2018 and five times a night (17.00, 19.00, 21.00, 0.00 and 3.00 hours) in 2019 at depths of 5 and 10 m (trawling was not conducted at 0.00 or 3.00 hours on 17 June 2019 because of severe sea conditions).…”

“…(2020) reported that since the 1980s, the spawning seasons have shifted and occurred earlier in the season. This shift may be correlated to increased seawater temperatures (Kawai et al ., 2020); oyster farming areas are the preferred spawning sites for this species (Kawai et al ., 2021). Nonetheless, the reproductive rhythms and external factors related to spawning timing in the natural environment have not yet been uncovered.…”

The complete mitochondrial genome of Acanthopagrus pacificus (Perciformes, Sparidae) from Iriomotejima Island, Okinawa, Japan

“… 2020 ; Kawai et al. 2021 ) but also in genetic population structure (Song et al. 2021 ; Yamashita et al.…”