Spatio-Temporal Variability of the North Sea Cod Recruitment in Relation to Temperature and Zooplankton

Abstract

The North Sea cod (Gadus morhua, L.) stock has continuously declined over the past four decades linked with overfishing and climate change. Changes in stock structure due to overfishing have made the stock largely dependent on its recruitment success, which greatly relies on environmental conditions. Here we focus on the spatio-temporal variability of cod recruitment in an effort to detect changes during the critical early life stages. Using International Bottom Trawl Survey (IBTS) data from 1974 to 2011, a major spatio-temporal change in the distribution of cod recruits was identified in the late 1990s, characterized by a pronounced decrease in the central and southeastern North Sea stock. Other minor spatial changes were also recorded in the mid-1980s and early 1990s. We tested whether the observed changes in recruits distribution could be related with direct (i.e. temperature) and/or indirect (i.e. changes in the quantity and quality of zooplankton prey) effects of climate variability. The analyses were based on spatially-resolved time series, i.e. sea surface temperature (SST) from the Hadley Center and zooplankton records from the Continuous Plankton Recorder Survey. We showed that spring SST increase was the main driver for the most recent decrease in cod recruitment. The late 1990s were also characterized by relatively low total zooplankton biomass, particularly of energy-rich zooplankton such as the copepod Calanus finmarchicus, which have further contributed to the decline of North Sea cod recruitment. Long-term spatially-resolved observations were used to produce regional distribution models that could further be used to predict the abundance of North Sea cod recruits based on temperature and zooplankton food availability.

Spatially-Resolved Influence of Temperature and Salinity on Stock and Recruitment Variability of Commercially Important Fishes in the North Sea

“…Various hypothesis on underlying mechanism have been proposed: match-mismatch between first-feeding larvae and zooplankton production [54], temperature related changes in zooplankton community and its productivity [55], [34] and direct effects of temperature on growth and survival of cod eggs and larvae [56], [57]. Beside bottom-up controls, top-down processes have been proposed, e.g.…”

The predictive potential of early life stage individual-based models (IBMs): an example for Atlantic cod Gadus morhua in the North Sea

“…Since the 1980s, the abundance of Atlantic cod in the North Sea has markedly declined (Heath 2005, ICES 2005a, Nicolas et al 2014. This was potentially caused by extensive (over-) exploi tation of the stock, but several studies also indicate important changes in environmental factors potentially regulating the productivity (recruitment potential) of the North Sea cod stock.…”

Environmental Impacts—Marine Ecosystems

“…Production of a sufficient supply of the right sizes and quality of zooplankton prey, in particular the cool-temperate copepod Calanus finmarchicus, at the right time of year affects cod survival and recruitment Mieszkowska et al 2009). Climate-related changes in spring SST and copepod abundance have consequences for the spatial patterns of recruitment in the North Sea (Nicolas et al 2014) and probably for adult distribution as well.…”

“…Indeed, changes in plankton phenology linked to climate (Sect. 8.2) are seen as a factor contributing to the decline in the North Sea cod stock, although overfishing also plays an important role (Nicolas et al 2014). Copepod biomass, euphausiid abundance, and prey size have also been shown to influence survival of North Sea cod through early life stages .…”