A research team led by Professor Shin-ichi Ito and Associate Professor Kotaro Shirai of the Atmosphere and Ocean Research Institute, the University of Tokyo, and Senior Scientist Motomitsu Takahashi of the Fisheries Resources Institute, Japan Fisheries Research and Education Agency, investigated the migration of Japanese jack mackerel to close to the bottom of the ocean by measuring the oxygen stable isotope ratio in the otoliths of the fish. They subsequently hypothesized that the fish that cannot obtain sufficient energy to support their growth from surface food alone migrate to the near-bottom layer of the ocean in search of more nutritious food.
Although Japanese jack mackerel are known as a small buoyant fish, past research has shown that as they grow, they deepen their habitat layer, and by the time they reach 5 cm in length, they shift their habitat to the near-bottom layer around 70 to 140 meters depth. However, it was not clear whether the transition was short-lived or gradual, and it remained a mystery as to what triggered the transition to the near-bottom layer. According to Ito, "The lack of detailed ecological information on Japanese jack mackerel has been an obstacle to proper resource management."
The research team investigated the near-bottom migration of Japanese jack mackerel by analyzing the oxygen stable isotope of otoliths (calcium carbonate-based crystals that form in the inner ears of fish, commonly known as ear stones). Since a diurnal ring is formed on the otoliths of Japanese jack mackerel each day, the daily age and hatching date of collected fish can be determined by analyzing these rings. The team cut otoliths from 10 days of diurnal rings with a precision drill and analyzed the oxygen stable isotope ratios in the ground otolith powder.
In the ocean, the surface layer is generally warmer and less saline, while the water temperature decreases and salinity increases as the water layers deepen. This also means the oxygen stable isotope ratio is higher. Taking advantage of this feature, the team tried to detect the migration of Japanese jack mackerel to the near-bottom layer from the increase in stable oxygen isotope ratios in otoliths during growth of the fish.
Based on the increase in oxygen stable isotope ratios, they estimated that the fish began their near-bottom migration at about 23 days after hatching, taking about 31 days to migrate to the near-bottom layer. Since they estimated that the metamorphosis from larvae to juvenile of the Japanese jack mackerel used in the analysis occurred at about 28 days after hatching, they found that the fish had already begun their near-bottom transition before metamorphosing to juveniles.
The growth rate can be estimated from the width of the diurnal rings of the otoliths. Using this method, they determined the period of maximum growth rate and found that the growth rate reached its maximum at approximately 42 days after hatching during near-bottom migration. As a result of examining the factors related to the start time, peak time, and end time of migration to the near-bottom layer, the team found that factors such as body length, growth rate and age of juvenile metamorphosis are related, and that Japanese jack mackerel, which have a low growth rate, begin to migrate to the near-bottom layer at an early stage.
Based on these results, they inferred that the less well-nourished the fish are, the earlier they move into the near-bottom layer, suggesting a theory that fish that could not obtain sufficient energy to support their growth from surface food alone are moving into the near-bottom layer earlier in search of more nutritious food. "We will measure the ratio of otolith oxygen stable isotope of Japanese jack mackerels collected in an environment of rising water temperature and compare them with the results from 2005 to 2012 to investigate changes in habitat and to evaluate how the fish move three-dimensionally, not only in the depth direction, but also as they are swept downstream by the Kuroshio and other currents. We also want to evaluate the effects of global warming on Japanese jack mackerels in the future," explained Ito.
Publication: Progress in Oceanography
Title: Vertical habitat shifts of juvenile jack mackerel estimated using otolith oxygen stable isotope
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