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.
Highlights
Changes in the spatial distribution of species over time reflect variations in the suitability of both biotic and abiotic environmental conditions regarding their survival, reproduction or dispersion [1]
The gain function was consistently high for all the years between 1989 and 2001 suggesting that the depletion of the North Sea adult cod was a gradual process spanning over a decade
From the mid-1980s to the late 1990s, cod recruitment was highly variable (Fig. 3b), with the core distribution of the juveniles mainly located around the Skagerrak, Flamborough Head and the Central North Sea
Summary
Changes in the spatial distribution of species over time reflect variations in the suitability of both biotic and abiotic environmental conditions regarding their survival, reproduction or dispersion [1]. The fluctuations of a stock with a reduced age composition largely depend on recruitment success, which in turn may be strongly affected by different environmental factors [8,9,10,11]. Both overfishing and climate change are considered to be the main drivers of the drastic decrease of North Sea cod. Disentangling their relative impacts remains challenging [6]
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