Predation on Prerecruits Can Delay Rebuilding of Depleted Cod Stocks

Predation by clupeoid species (e.g., Atlantic herring, Clupea harengus Linnaeus, 1758) on the early life stages of gadoids is hypothesized to be an important source of mortality, especially for Atlantic cod, Gadus morhua Linnaeus, 1758. Adult cod also prey on herring, creating a positive feedback process ("trophic triangle") that may result in alternative dominance patterns of cod or herring depending on the relative levels of mortality. We estimated the effect of herring on cod recruitment with a predator-dependent Ricker stock-recruitment function, fit to time-series data. In this formulation, herring gradually reduce the per capita recruitment rate of cod, but this predation does not result in depensatory dynamics. We incorporated this predator-dependent recruitment function into a multispecies length-based model (LeMans) of Georges Bank, northeast US shelf, to investigate the consequences of predation mortality for the recovery of cod from overfishing. In LeMans, all fished species undergo the same size-dependent fishing mortality, following a logistic selection curve. We investigated two fishing patterns: a selective (L ₅₀ = 65 cm) and an unselective fishery (L ₅₀ = 15 cm). The effect of predation on prerecruits was more pronounced under selective fishing because herring were spared as a result of their small size. In rebuilding scenarios with selective fishing, cod started at low abundance and herring at high abundance. Without fishing, cod could rebuild in 10 yrs even with predation mortality before recruitment. In contrast, with low levels of fishing mortality, rebuilding took 25 yrs and even longer with such predation. These results suggest that predation on prerecruits and fishing can combine to delay rebuilding of depleted cod stocks.