“Unreliable Fish” or Unreliable Hypotheses: What Happens to Nekton in Ocean Waters off the Kuril Islands?

• Published in: Russian journal of marine biology 2021-12, Vol.47 (7), p.519-533
• Main Authors: Shuntov, V. P., Ivanov, O. A.
• Format: Article
• Language: English
• Subjects: Biomedical and Life Sciences; Freshwater & Marine Ecology; Life Sciences
• ISSN: 1063-0740; 1608-3377
• DOI: 10.1134/S1063074021070075

The phenomenon of population waves of nekton in the Kuroshio Current system is discussed using the example of the Japanese sardine Sardinops melanostictus , one of the most common species that show the greatest amplitude of abundance fluctuations. So-called “sardine epochs” are distinguished on the basis of the pattern of abundance fluctuations that occur in this species. The last such epoch ended in the first half of the 1990s. Since 2014, the nekton communities in the Pacific waters off the Kuril Islands have again undergone structural changes with the expansion of southern fish and squid species. The rearrangement of the nekton species structure was mainly due to a noticeable increase in abundances of the Japanese sardine and the chub mackerel Scomber japonicus and also a noticeable decrease in the abundances of the Japanese anchovy Engraulis japonicus and the Pacific saury Cololabis saira . The magnitude of these changes allowed Russian fishermen to resume fishing for Japanese sardine and chub mackerel in the Russian exclusive economic zone since 2016. The Japanese sardine catch was increasing steadily from 6700 (in 2016) to 315 500 t (in 2020). Over 5 years, Russian fishermen landed a total of more than half a million tons (531 700 t) of this fish; the chub mackerel landing was 167 900 t. Even in this situation, many Russian forecasters, apparently trusting the formal climatic and hydrological indices too much and without investigating the mechanisms that underlie population waves carefully, suggest this was only a forerunner of the upcoming sardine epoch. The authors of the present publication assume with great confidence that, even with different interpretations, certain epochs (sardine one or other ones) cannot be predicted as being very similar to the previous ones. As well, the authors critically consider some of the previous hypotheses about the causes of the onset and end of Japanese sardine abundance outbreaks and make conclusions about the difficulties of solving the problems of such predictions. Certain mechanisms responsible for the formation of year-class strength and, especially, population waves in Japanese sardine and other fluctuating nekton species, remain poorly understood.