Modeled larval fish prey fields and growth rates help predict recruitment success of cod and anchovy in the North Sea

ABSTRACT: We introduce a new, coupled modeling approach for simulating ecosystem-wide patterns in larval fish foraging and growth. An application of the method reveals how interplay between temperature and plankton dynamics during 1970-2009 impacted a cold-water species (Atlantic cod Gadus morhua) and a warm-water species (European anchovy Engraulis encrasicolus) in the North Sea. Larval fish growth rates were estimated by coupling models depicting trait-based foraging and bioenergetics of individuals, spatiotemporal changes in their prey field, and the biogeochemistry and hydrodynamics of the region. The biomass composition of modeled prey fields varied from 89% nano-, 10% micro-, and 1% mesoplankton to 15% nano-, 20% micro-, and 65% mesoplankton. The mean slope of the normalized biomass size spectrum was near -1.2, consistent with theoretical and empirical estimates. Median larval fish growth rates peaked in June for cod (24% d-1) and in July for anchovy (17% d-1). Insufficient prey resources played a substantial role in limiting the growth rates of cod larvae. Anchovy were consistently limited by cold temperatures. Faster median larval growth during specific months was significantly (p < 0.05) positively associated with detrended (i.e. higher than expected) juvenile recruitment indices in cod (rank correlation Kendall’s tau = 22%) and anchovy (tau = 42%). For cod, the most predictive month was February, which was also when food limitation was most prevalent. The continued development of modeling tools based on first principles can help further a mechanistic understanding of how changes in the environment affect the productivity of living marine resources.