Ocean Newsletter

No.555 September 20, 2023

  • The Scientific Contribution to Promoting Climate Change Countermeasures KIMOTO Masahide (President, National Institute for Environmental Studies, Japan / Recipient, 15th National Maritime Award)
  • Rising Sea Water Temperatures and Changes in Marine Ecosystems KIMURA Shingo (Professor, Graduate School of Frontier Sciences / Atmosphere and Ocean Research Institute, The University of Tokyo)
  • The Ecosystems of the Rocky Reef Created by Boring Sea Urchins YAMAMORI Luna (Assistant Professor, Seto Marine Biological Laboratory, Field Science Education and Research Center, Kyoto University)

Rising Sea Water Temperatures and Changes in Marine Ecosystems

KEYWORDS Change in Distribution Area of Marine Life/Complex ecosystems/Distribution economics
KIMURA Shingo (Professor, Graduate School of Frontier Sciences / Atmosphere and Ocean Research Institute, The University of Tokyo)
The impact of rising sea water temperatures due to global warming is already manifesting itself through the migratory and spawning behaviors of marine life, as well as their survival and growth. Assessing these impacts is vital to fisheries management and economics, and a higher prediction accuracy is called for. Marine ecosystems are complex and intertwined in space and time; and although it is difficult to assess the effects of global warming through simple mechanisms, it is crucial at this stage to clarify each cause-and-effect relationship.
The Significance of Natural Science Research for Distribution Economics
There are different optimum water temperatures for marine life. The rise in sea water temperatures due to global warming undoubtedly affects the current migratory and spawning behaviors of marine life, as well as their survival and growth, and some of these impacts are believed to be already coming about. The catch of Japanese amberjack (yellowtail)in Hokkaido surged after 2011; by 2020, it accounted for about 15% of the total catch, making it the largest catch in Japan. Farming of young yellowtail has become popular, producing nearly 100,000 tons annually, almost equal to the number of fish caught. Although the catch amount is not the price leadership (main factor that affects price), changes in landing ports significantly impact the fisheries economy through differences of logistics infrastructure such as markets, refrigeration facilities, and logistics processes. In other words, even if catches increase, the catch will not be distributed if logistics infrastructure is not in place. On the other hand, capital expenditure without corresponding catch amounts could lead to overinvestment. Therefore, the results obtained from natural science research to determine whether the changes in the migratory routes of Japanese amberjack are transient or a constant phenomenon will have direct implications for economic activities, including the distribution process.
Rising Sea Temperatures in the Waters Around Japan and Predictions of the Impact on Fish Species
Bluefin tuna, which like the Japanese amberjack has high market value, is increasingly caught in fixed nets along the coast of Hokkaido after passing through the Soya Strait rather than the Tsugaru Straits. This northward shift in the species is often attributed to the impact of global warming. In the case of bluefin tuna, reports also exist of mass catches in Kushiro city during the late 1920s and early 1930s when warming was not yet noticeable. Therefore, it cannot be said definitively that the recent northward shift in migratory routes is solely due to global warming. However, it is undeniable that the number of fish coming to Hokkaido have been increasing in recent years. According to a report from the Japan Meteorological Agency, the rate of increase in the average sea surface temperature (annual average) in the central part of the Sea of Japan and off the coast of Kushiro is exceptionally high, at 1.87℃ and 1.55℃ per century, respectively (Figure 1). As indicated by the Intergovernmental Panel on Climate Change (IPCC), the rate of temperature rise over land is higher than that over the ocean, thus the rise in water temperature in the Sea of Japan, which is surrounded by the continent and the Japanese archipelago, will be significant. This trend is particularly strong during the winter when these fish species migrate, reaching 2.54℃ per century in the central part of the Sea of Japan, which is thought to be accelerating their northward movement. 
The optimal spawning temperature for bluefin tuna is 26±2℃, and if this is exceeded, either the parent fish does not spawn, or, even if it does, the growth and survival of the fry are likely to be poor. In other words, even if the fry survive, they may not achieve sufficient growth during their transformation into juvenile fish. This leads to an increased likelihood of predation due to their small size and poor swimming ability. This species spawns in waters from southern Okinawa to east of Taiwan around May, but predictions indicate that the surface water temperature in this region will exceed 28℃ by the year 2100, making it unsuitable for spawning. In that case, it is conceivable that reproduction may be attempted by shifting the spawning season earlier to around February, when the water temperature is optimal for spawning or by shifting the spawning area to the Sea of Japan, where the water temperature is cooler and where some spawning already occurs. Either scenario depends on whether the parent bluefin tuna can adapt by changing the timing and location of their spawning. In fact, considering the increasing catch in the Sea of Japan in response to rising water temperatures, they may already be shifting their spawning grounds to the Sea of Japan. However, if they do not adapt, the water temperature in the spawning grounds 100 years from now will be more than 2℃ higher than it is today, making the area unsuitable for spawning in terms of subsequent survival. Some estimates suggest that survival rates could drop to one-third of current levels due to this change. 

The global average temperature around 6,000 years ago, when sea levels rose during the Jomon period (c. 14,000 and 300 BC; the Holocene glacial retreat), was about 2℃ higher than today. According to IPCC predictions, global average temperatures will rise to similar levels by 2100. We could say that we are currently heading on a path back to the Holocene glacial retreat when seawater penetrated deep into the Kanto Plain. Considering the dramatic geographical differences from 6,000 years ago, it does not seem unnatural for species like Japanese amberjack and bluefin tuna to exhibit unique northward migratory trends due to global warming.
Figure 1: Rate of Increase in the Average Sea Surface Temperature (Annual Average) in the Waters Around Japan (℃/century) (Source: Japan Meteorological Agency Website)

Figure 1: Rate of Increase in the Average Sea Surface Temperature (Annual Average) in the Waters Around Japan (℃/century) (Source: Japan Meteorological Agency Website)

The Need for Comprehensive Marine Ecosystem Research
However, marine ecosystems are not so simple that temperature alone can have a unidirectional impact; we must understand the phenomenon in question in relation to the marine ecosystem as a whole. The causal relationship between coral bleaching and Zooxanthellae algae is often cited as an example, but the impact of warming on rocky-shore denudation is also significant. In other words, it is thought that the rising water temperatures cause southern fish species to move northward to feed on algae, and sea urchins that should have reduced activity or died during hibernation would increase, leading to rocky-shore denudation. Figure 2 shows a photograph illustrating rocky-shore denudation. Given that seaweed beds are often referred to as the "cradles of the sea," the impact of this denudation in a complex marine ecosystem, which includes not just inter-species but also inter-regional connections, is incalculable. In addition, the increase in carbon dioxide is believed to also produce ocean acidification as well as global warming, which could significantly impact the shells of mollusks made from calcium carbonate, raising concerns for important fishery species such as abalone. Generally speaking, the impact of these changes on coastal fishery resources is considerable and has already become apparent. 
Furthermore, marine organisms will change their distribution areas in response to changes in the marine environment. Growth and survival of marine organisms are determined by the so-called match-mismatch hypothesis, which states that high productivity occurs when the production of prey organisms coincides in time and space with the appearance of fry and juvenile fish. This concept can also be applied to stages of growth from juvenile to adult fish, requiring an understanding of marine ecosystems that include food chains. 

As we consider the impacts of global warming in this way, the situation becomes overwhelmingly complex, and, in general, only the negative aspects are focused on, which can lead to such discussions as the potential decrease in fish for sushi. However, while some fish species are decreasing, others are increasing; for instance, in the Seto Inland Sea, the catch of sand lance (Ammodytes japonicus) has drastically decreased, while the catch of red sea bream and Japanese Spanish mackerel has increased. It is thought that anthropogenic factors like global warming and the reduction in nutrient salt supply have impacted sand lance resources, but, as a result, the increase in catches of the other two species may be balancing out the environment’s carrying capacity. 
Evaluating the impact of warming within marine ecosystems is extremely difficult due to the complexity of the various processes involved, and research spanning generations and decades may be necessary. However, clarifying each fundamental cause-and-effect relationship could advance our comprehensive understanding of these phenomena.
Figure 2: Seafloor undergoing rocky-shore denudation due to the mass outbreak of sea urchins (Provided by Professor Mitsuo Yamamoto, The University of Tokyo)

Figure 2: Seafloor undergoing rocky-shore denudation due to the mass outbreak of sea urchins (Provided by Professor Mitsuo Yamamoto, The University of Tokyo)

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