Estimating the maximum sustainable yield of snow crab (Chionoecetes opilio) off Tohoku, Japan via a state‐space stock assessment model with time‐varying natural mortality

Abstract

A maximum sustainable yield (MSY) obtained by maintaining or restoring fish stock levels is a tangible benefit of ecosystem services. Snow crab (Chionoecetes opilio) off Tohoku, Japan has been managed by a total allowable catch since 1996, although the abundance has not increased. Surprisingly, there was no increase after 2011, when fishing pressure was greatly reduced because of the Great East Japan earthquake. This implies that some of the crab's biological characteristics, such as recruits, natural mortality coefficient (M) and terminal molting probabilities (p), might have changed. We developed “just another state‐space stock assessment model” to estimate the MSY of the snow crab off Tohoku considering interannual variations in M and p. The multimodel inference revealed that M increased from 0.2 in 1997 to 0.59 in 2018, although it did not vary according to instars, sex or terminal molt. The parameter p also increased by 1.34–2.46 times depending on the instar growth stages from 1997 to 2018. We estimated the MSYs in three scenarios, which changed drastically if M and p were set as they were in the past or at the current values estimated from this study. This result indicated that the MSY of snow crab would also vary with time based on their time‐varying biological characteristics.

Ecological responses to the direct and indirect disturbances by the Great East Japan Earthquake, 2011

“…The snow crab is an important commercial fishing resource off the coast of Tohoku. Surprisingly, Shibata et al (2020) found that the abundance of this crab was unchanged after the 2011 tsunamis, even though fishing was banned for several years after the GEJE. The study provided evidence that the snow crab populations off Tohoku may have suffered from other sources of mortality, such as warming and predation.…”

“…They also cautioned the possibility that, although the tsunamis' impact was a temporary event, the abundance of this porpoise species may continue to decline in the future due to human-induced threats such as fisheries bycatch. Shibata et al (2020) also examined offshore animals, but those living on the sea floor, that is, snow crab (Zuwai-gani in Japanese). The snow crab is an important commercial fishing resource off the coast of Tohoku.…”

Towards ecological, socioeconomic and cultural sustainability of marine ecosystem services

“…The next two papers (Nishijima et al, 2021; Shibata et al, 2021) focus on temporally variable biological parameters of specific fisheries stocks around Japan. Nishijima et al (2021) reveal regime shifts in the productivity of Japanese flying squid ( Todarodes pacificus ) stocks using a multi‐stock modeling approach.…”

“…Nishijima et al (2021) reveal regime shifts in the productivity of Japanese flying squid ( Todarodes pacificus ) stocks using a multi‐stock modeling approach. Shibata et al (2021) infer that a recent increase in natural mortality of snow crab ( Chionoecetes opilio ) has drastically changed maximum sustainable yield. Both studies use their tailor‐made state‐space models for stock assessment, with the advanced techniques able to capture the dynamic nature of fisheries resources in a changing environment.…”

“…In summary, this special feature introduces readers to current progress in marine ecosystem service studies. Population dynamic modeling is demonstrated to be useful for understanding fisheries productivity in a changing environment (Nishijima et al, 2021; Shibata et al, 2021). A large‐scale evaluation of biodiversity and ecosystem services will assist with the identification of hotspots and marine spatial planning (Sato et al, 2021).…”