Spatiotemporal dynamics of predators and survival of marine fish early life stages: Atlantic cod (Gadus morhua) in the North Sea

Abstract

Predation is one of the principle factors regulating the survival of early-life stages and recruitment success of marine fishes. Although challenging, it is important to understand how predation mortality varies in space and how this spatial variability affects the temporal dynamics of fish recruitment. We investigated the spatiotemporal variability in the magnitude of predation mortality and survival of Atlantic cod (Gadus morhua) in the North Sea by combining a Lagrangian individual-based model of cod early-life stages (eggs, larvae and age-0 juveniles) with an Eulerian representation of its fish predators based on field data collected quarterly from 1991 to 1997. The mean and variance (spatial and inter-annual) in survival of cod were different between simulations using homogeneous and heterogeneous spatial distributions of predators. Survival of cod through its early-life period was 1 order of magnitude higher in simulations using heterogeneous versus homogenous predator distributions, mainly due to reduced mortality of eggs and larvae in the former simulation. Our model predicted markedly different year-to-year changes in cod survival, when heterogeneous predator fields were implemented. In the heterogeneous simulation, Atlantic herring (Clupea harengus) and grey gurnard (Eutrigla gurnadus) emerged as the most important predators comprising, respectively, 68% and 13% of the predation mortality caused by 7 predators included in our study. A low (<0.1%) survival was predicted for cod progeny originating from known spawning grounds in the central and south-eastern North Sea while considerably higher survival (between 0.8 and 1%) was predicted for northern spawning areas and in the Southern Bight. Numerical sensitivity experiments indicated that seasonal and annual changes in the spatial distribution of predators were more important to the inter-annual variability in the modelled cod survival (52% of the mean survival) than annual changes in the biomass of predators (23%) and the variability of the North Sea hydrography (19%) with the relative importance of these factors depending on the life stage of cod. We argue that including spatial dynamics of predators will advance the predictive capacity of fish early-life stage models enabling more robust, science-based advice for spatially-explicit fishery management and marine spatial planning.