Effects of a marine‐protected area occurred incidentally after the Great East Japan Earthquake on the Pacific cod (Gadus macrocephalus) population off northeastern Honshu, Japan

Abstract

The population of Pacific cod inhabiting off northeastern Honshu Japan has remarkably increased in the 3 yr after the Great East Japan Earthquake in 2011. We examined the processes and factors leading to this increase based on the results of estimations of commercial catch and fishing activities, and trawl surveys. Pacific cod was highly exploited from 1 yr old before 2011. However, fishing pressure has markedly decreased after 2011 in the area around Fukushima waters. Fish abundance in 2013 and 2014 was estimated to be more than four‐fold of the maximum amount before 2011. The cohort structure of the population in 2013 and 2014 was primarily composed of 2–4‐yr‐old fish (2010–2011 yr classes) whereas the population in pre‐2011 was primarily of 1‐yr‐old fish. The 2010–2011 yr classes had almost the same population sizes until 1.3 yr old to the pre‐2009 yr classes, but became much higher more than 2.8 yr old. Pacific cod from 1.3 to 2.8 yr old were concentrated in the area off Fukushima Prefecture. These results suggest that Pacific cod increased post‐2011 not because of the occurrence of strong year classes followed by good recruitments but because of the lower mortality after recruitment owing to reduced fishing mortality. Waters off Fukushima, the primary nursery of Pacific cod, are effectively serving as a marine protected area after the tsunami events. A close coincidence between the nursery area of young fish and the protected area strongly altered the age composition of the population and enhanced fish abundance.

Impact of the great tsunami in 2011 on the quality of nursery grounds for juvenile Japanese flounder Paralichthys olivaceus in Sendai Bay, Japan

“…In contrast, Japanese anchovy, a main food item for the flounder (juveniles, young and adult) decreased. Potential competitors for forage fishes, e.g., Pacific cod Gadus macrocephalus (Narimatsu et al, 2017) increased. These changes should be caused by the change or variation of biotic and abiotic environments and a substantial decrease in fishing mortality because of the damage to fishery boats by the direct effects of the tsunami and temporal ban on fisheries owing to the radiocesium contamination Narimatsu et al, 2015).…”

Agricultural Implications of the Fukushima Nuclear Accident (III)

“…It is possible that radiocesium concentration in fish depends on the size and the age of fish. Year-class related difference in radiocesium concentrations in the body of fish was examined by Narimatsu et al (2015). Pacific cod (Gadus macrocephalus) captured from April 2011 to March 2014 off Fukushima Prefecture were analyzed and they found the radiocesium concentration of cod in the year-class of 2009 (i.e., cod born in the year 2009) and earlier was higher than that in the 2010 year-class.…”

“…Narimatsu et al (2015) further pointed out that the half-life was longer in old and larger individuals than in young and smaller individuals, probably a result of differences in metabolic rate and growth rates between age and body size classes as proposed in the study of Doi et al (2012). Radiocesium was rarely detected in the 2011 year-class, most likely because the fish were only exposed to very low levels of radiocesium when they started their life in the ocean bottom and after that radiocesium in the fish body was diluted by growth (Narimatsu et al 2015).…”

“…The population of key fish species in Fukushima waters showed a tangible increase after the 2011 disaster reflecting low fishing pressures from 2011 to 2014. Narimatsu et al (2017) reported that the population of Pacific cod (Gadus macrocephalus) inhabiting waters off northeastern Honshu, Japan, which includes all Fukushima waters, has increased in 2012, 2013, and 2014 after the Great East Japan Earthquake in 2011. Their research suggests that Pacific cod increased post-2011 not because of the occurrence of strong year classes followed by good recruitments but because of the lower mortality after recruitment owing to reduced fishing mortality in the Fukushima area.…”

Evaluating a harvest control rule to improve the sustainability of Japanese fisheries

“…Although it was recently estimated that radioactive contamination from the nuclear plant accident did not seriously harm marine fish (Okamura et al 2016), many fisheries still remained closed or restricted in Fukushima. During this period, some species of fish off the coast of Fukushima have become increasingly abundant, probably because of significantly reduced fishing pressures (Narimatsu et al 2017;Shibata et al 2017). Reducing fishing pressures in other areas has had similar positive effects on abundance (Neubauer et al 2013;Hilborn et al 2020); in other words, eliminating overfishing could rapidly lead to higher fish abundance, and more stable catches.…”