5:15 PM - 7:15 PM
[HCG24-P04] Combined effects of high water temperature and hypoxia on cultured bivalves
Keywords:climate change, coastal area of Japan Sea, Nanao Bay, Fulvia mutica
Increase of seawater temperatures due to climate change may have a major impact on the survival of non-swimming organisms such as bivalves. The rate of increase in water temperature in Nanao Bay, which is an enclosed sea on the Sea of Japan side, has been 0.027℃/year over the past 30 years. In addition to the direct impact of high water temperatures on the survival rate of bivalves, there is concern that the effects of a decrease in dissolved oxygen concentration (hypoxia) will increase due to the acceleration of metabolism associated with high water temperatures. Although previous studies have investigated the effects of strong oxygen depletion, with dissolved oxygen concentrations (DO) below 2 mg L-1, on bivalves, there is little knowledge regarding the effects of weak oxygen depletion, with DO levels of 2-4 mg L-1, which are frequently observed in coastal waters. In this study, we aimed to clarify the combined effects of high water temperature and hypoxia on the survival rate of cockle (Fulvia mutica), which has become an important aquaculture target species in recent years, using rearing experiments and hazard models.
In 2022-2023, we carried out a 4-day experiment in rearing cockles using 180 individuals (large individuals) with an average shell length of 43.2 mm and 240 individuals (small individuals) with an average shell length of 14.6 mm under a total of 17 conditions combining water temperature (25, 28, 29, 30, 31, 32℃) and DO (2 mgL-1, 4 mgL-1, saturated). The effects of shell length, water temperature, and DO on the survival rate of young cockle were estimated using a hazard model.
As a result of variable selection in the hazard model, a model including shell length, water temperature, and DO was estimated, and the coefficient values suggested that the smaller the shell length, the lower the survival rate. Under hypoxic conditions of approximately DO 4 mgL-1, the survival rate changed significantly for each 1°C change in water temperature. Furthermore, regardless of the difference in DO conditions, the survival rate was almost zero at water temperatures of 31°C or higher. In the bay, there are years in which high water temperatures exceeding 30°C are observed from the upper to middle layers in July and August, so adjusting the culture depth by monitoring water temperature and dissolved oxygen concentration would become increasingly important as one of climate change countermeasures in the near future.
In 2022-2023, we carried out a 4-day experiment in rearing cockles using 180 individuals (large individuals) with an average shell length of 43.2 mm and 240 individuals (small individuals) with an average shell length of 14.6 mm under a total of 17 conditions combining water temperature (25, 28, 29, 30, 31, 32℃) and DO (2 mgL-1, 4 mgL-1, saturated). The effects of shell length, water temperature, and DO on the survival rate of young cockle were estimated using a hazard model.
As a result of variable selection in the hazard model, a model including shell length, water temperature, and DO was estimated, and the coefficient values suggested that the smaller the shell length, the lower the survival rate. Under hypoxic conditions of approximately DO 4 mgL-1, the survival rate changed significantly for each 1°C change in water temperature. Furthermore, regardless of the difference in DO conditions, the survival rate was almost zero at water temperatures of 31°C or higher. In the bay, there are years in which high water temperatures exceeding 30°C are observed from the upper to middle layers in July and August, so adjusting the culture depth by monitoring water temperature and dissolved oxygen concentration would become increasingly important as one of climate change countermeasures in the near future.