Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance

Abstract

The decline of the North Sea cod Gadus morhua has been attributed to both overfishing and ocean warming. However, another hypothesis is that overfishing of piscivorous cod has caused an increased abundance of formerly predatory-controlled pelagic fish including herring Clupea harengus, which in turn has suppressed the recruitment of cod through predation on the early life stages of cod. Here we analyze 40 yr of trawl survey data in order to explore how the abundance of young herring affects cod recruitment, and how cod biomass affects the abundance of herring. In both cases we also take into account the effects of spawner biomass (of cod and herring, respectively) and sea temperature. We take a novel, explicitly spatial, approach by analyzing these effects on a local (185 × 167 km) scale. Our results indicate large spatial variability in ecological mechanisms. In the German Bight, high cod recruitment is associated with low herring abundance, low temperatures and high overall cod spawner biomass. This area used to contain a large portion of the cod recruits, and there is a strong correlation between the fraction of cod recruits found in this area and overall recruitment. In this area, herring recruitment is also negatively associated with the abundance of large cod. Thus, for this part of the North Sea, our findings are consistent with a reversal of dominance between cod and herring; however, herring may affect the cod by competition rather than by predation.

Population growth across heterogeneous environments: effects of harvesting and age structure

“…NEA cod spawns along the Norwegian coast. The North Sea (NS) cod is another cod population whose abundance has declined steadily since the 1960s (ICES 2011b, Hjermann et al 2013, this Theme Section). Age-structured information on NEA cod is available from 1946 (ICES 2011a) and for NS cod from 1963 (ICES 2011b).…”

“…The temperature variables used have been reported to explain the dynamics for NA, SA and BI hake (Hidalgo et al 2012), NEA (Hjermann et al 2004) and NS cod (Hjermann et al 2013), and GOA (Ciannelli et al 2004) and EBS pollock (Mueter et al 2011). Sea surface temperature (SST) for hake were obtained from the Climate Diagnostics Center (NCEP/NCAR) reanalysis fields (Kalnay et al 1996) on a 1 × 1 degree grid and averaged over the area of each studied stock.…”

“…Data are available from 1960 to 2007 with 1 × 1 degree grid resolution for the cod from 1880 to 2009 and with 2 × 2 degree grid resolution for the pollock (based on the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) Sea Surface Temperature data). The data were averaged as follows: North Sea, Long 51.5° to 55.5°/Lat 0.5° to 4.5° for March to May (SST spring ;Hjermann et al 2013); Bering Sea, Long −174° to −166°/Lat 56° to 60° for April to July. For the Gulf of Alaska, we extracted summer temperature (SST sum1 , Long −164° to −162°/Lat 50° to 54°f or June− July− August) and winter temperature (SST wint2 , Long −164° to −160°/Lat 56° to 60° for December−January− February).…”

The importance of within-system spatial variation in drivers of marine ecosystem regime shifts

“…Recognizing the pronounced differences in the North Sea environmental conditions at multiple spatial scales and spatial autocorrelation among adjacent sites [46], Hjermann et al [47] re-evaluated the generality of the herring role reversal hypothesis at smaller spatial scales and spatial variation in the relative importance of determinants of cod biomass, ranging from predominantly sea surface temperature (SST) in the southwest to combined influence of temperature and herring biomass in the east (figure 3a). Similar spatial heterogeneity in the key determinants of herring biomass was reported [47]. Taken together, these analyses illustrate the need to simultaneously evaluate the potential influences of cod parental biomass, temperature and predator biomass in a spatial context (see also [48])-factors that while acting locally may sum to determine the sensitivity of the entire ecosystem to regime shifts.…”

“…B 370: 20130271 regions and among pelagic species [45]. Recognizing the pronounced differences in the North Sea environmental conditions at multiple spatial scales and spatial autocorrelation among adjacent sites [46], Hjermann et al [47] re-evaluated the generality of the herring role reversal hypothesis at smaller spatial scales and spatial variation in the relative importance of determinants of cod biomass, ranging from predominantly sea surface temperature (SST) in the southwest to combined influence of temperature and herring biomass in the east (figure 3a). Similar spatial heterogeneity in the key determinants of herring biomass was reported [47].…”