Tunas, which are known for their large-scale migration around the world, are a commercially important international fishery resource. Thus, it is essential to establish appropriate management policy to maintain stock biomass at an appropriate level. For this purpose, detailed knowledge on migratory ecology is indispensable, but it is often difficult to investigate the ecology of tunas that migrate widely in the open ocean. This paper reviews ecological research of tunas using trace element and stable isotopic compositions of otoliths as a natural tag, which serve as a technique to complement conventional migratory ecological research. This paper outlines how otoliths as natural tags can play an important role for ecological research of tunas in the future.

Massive occurrences of juvenile clams Ruditapes philippinarum have been appearing at Rokujo tidal flat in the Toyo River mouth at the inner part of Mikawa Bay every year. These have been collected by permitted fishermen and have been released to replenish about 3,000 tons of resources into the coastal area of Ise and Mikawa Bay since 2003. However, the mechanism of the massive occurrences has not been fully elucidated. Therefore, we investigated temporal and spatial changes in the shell length composition and number of juvenile clam individuals from May 2015 to March 2017 in Rokujo tidal flat (Case1) and from March 2017 to June 2018 in Rokujo tidal flat and river mouth part (Case2). As a result, a rapid increase in the number of settled individuals in the spring was observed during the spring in both Case 1 and Case 2. The number of individuals that increased rapidly in spring was larger than those that settled in Rokujo tidal flat in the fall and survived. The main resource for the release was considered to be individuals that increased rapidly in spring. Although the source of this massive occurrence was not clear, they might be born in the tidal area of the Toyo River. The stock born in the tidal area of the Toyo River might have an important role for the massive occurrence of juvenile clams in the Rokujo tidal flat in spring.

It has been pointed out that the decline of western sand lance (Ammodytes japonicus) catch in Harima-Nada is mainly caused by food shortage. In this study, the stomach content index (SCI), condition factor, and prey composition of juvenile western sand lance collected in Harima-Nada in March were investigated from 1987 to 2018. The SCI decreased significantly from 1987 to 1997 and remained less than 1% after 2001. The condition factor also decreased significantly from 2001 to 2018. Main prey items of the juvenile western sand lance were the copepods Calanus sinicus and Paracalanus sp. throughout the investigation period, and the frequency of their occurrences did not change significantly before or after 2001. On the other hand, the dissolved inorganic nitrogen concentration in January and the chlorophyll a concentration from February to March showed a significant positive correlation, and the chlorophyll a concentration and SCI also showed a positive relationship. These results suggest that poor nutrient levels were responsible for low primary production, which was in turn a potential cause of low zooplankton production. As a result, the juvenile western sand lance possibly suffered from chronic food shortage in recent years.