Simulation of copepod biomass by a prey–predator model in Hiuchi-nada, central part of the Seto Inland Sea: does copepod biomass affect the recruitment to the shirasu (Japanese larval anchovy Engraulis japonicus) fishery?

Abstract

We have modeled the prey-predator dynamics between nutrients, phytoplankton, and copepods in Hiuchinada, central part of the Seto Inland Sea. The model parameters were estimated by stepwise regression using data sampled from 2001 to 2005. We re-created the fluctuations in copepod biomass in the spring-summer of 2001-2004 by model simulation and investigated the relationship between the re-created copepod biomass and anchovy Engraulis japonicus reproductive success rate in Hiuchi-nada. The anchovy reproductive success rate was proportional to the copepod biomass during the last 10 days of May, a period that immediately preceded anchovy recruitment. This relationship indicates that a possible key factor in the regulation of anchovy population levels is the fluctuation in abundance of the copepod assemblage and that the crucial period for anchovy recruitment in Hiuchinada would be the period just before anchovy recruitment to the shirasu (body length: approx. 20-35 mm) fishery. These results provide a potential framework for forecasting the anchovy recruitment level that is based on both larval abundance and survival rate as estimated from the biomass of copepods in the pre-recruitment period of anchovy.

Interannual decrease in condition factor of the western sand lance Ammodytes japonicus in Japan in the last decade: Evidence for food‐limited decline of the catch

“…In conclusion, the present results indicate that decline of western sand lance catch after 1990 was caused by food shortage. Such a bottom-up effect has already been indicated for a predator-prey relationship between the larval Japanese anchovy Engraulis japonicus fishery and copepod biomass in the present study area (Zenitani, Kono, & Tsukamoto, 2011).…”

Temporal variations in hatch date and early survival of Japanese anchovy (Engraulis japonicus) in response to environmental factors in the central Seto Inland Sea, Japan

“…Moreover, Zenitani et al, (2009) demonstrate that size‐specific growth rates in larvae and juveniles increase with increasing prey availability, while temperature has no significant effect. Consequently, anchovy reproductive success rates are proportional to copepod biomass during the last 10 days of May, which immediately precedes anchovy recruitment (Zenitani et al, 2011). Although potential predators and competitors can affect the early survival of anchovy in these waters (Shoji et al, 1997; Zenitani et al, 2017), prey availability during early‐life stages may be one of the most important environmental factors that regulates anchovy population dynamics.…”

Daily growth rate model of Japanese anchovy larvae Engraulis japonicus in Hiuchi-nada Sea, central Seto Inland Sea

“…To evaluate the survival or food condition of anchovy larvae during the first-feeding stage, we recommend that the 0 % growth criterion for RNA:DNA ratio be calculated against temperature [10]. [14] that ''periods of the last 10 days of May to first 10 days of June was the crucial period for recruitment of anchovy in Hiuchi-nada Sea.'' Although the observed copepod density does not represent sufficient food availability for anchovy in some cases, the newly developed growth model is considered as optimal at present and useful to link environmental conditions and larval growth.…”

“…A possible key factor in the regulation of anchovy population levels is the fluctuations in abundance of the copepod assemblage, and the crucial period for recruitment of anchovy in Hiuchi-nada Sea would be the period just before the anchovy recruitment to the shirasu fishery [14]. To investigate when food shortage for larval anchovy and recruitment failure occur in Hiuchi-nada Sea, anchovy food requirements were estimated by using the growth model, and we compared the food requirement with anchovy food availability in Hiuchinada Sea.…”