Publication Abstract

The use of ecosystem indices to investigate the decline of recruitment and spawning stock biomass in North Sea cod (Gadus morhua L.)

The use of ecosystem indices to investigate the decline of recruitment and spawning stock biomass in North Sea cod (Gadus morhua L.)

P.J. Bromley

Ecosystem indices were used to investigate possible factors associated with the depression of the recruitment (the abundance of young fish) and spawning stock biomass of North Sea cod from 1977-1998, covering the period since the gadoid outburst of the 1970s. The ecosystem indices were derived from time-series of biotic (sandeel recruitment), abiotic (temperature) and anthropogenic (sandeel landings) data. Where appropriate, the original data were subjected to principal component analysis to generate fewer new variables for modelling purposes. The recruitment estimates for cod at various stages during the first year of life (the 0-group phase) were based on time-series of catch-per-unit-effort data from fishing surveys, and from the output of single species and multispecies virtual population analyses.

The ecosystem indices were related to the recruitment of 1-group (one year old) cod using a general linear model (GLM), which accounted for 90% of the variance in cod recruitment. Over time, there was a downward trend in the recruitment of 1-groups and subsequently of the cod spawning stock biomass (SSB). This was associated with climatic warming and with increasing levels of sandeel landings from the central North Sea. Conversely, cod recruitment was positively correlated with sandeel recruitment. A rise in temperature appeared to favour the survival of cod during the pelagic 0-group phase, but later switched to being detrimental to the growth and survival of the older 0-group cod when they became demersal in habit. Two aspects of environmental warming were implicated in the decline of cod recruitment, firstly, an overall rise in seawater temperature and secondly a rise in temperature stratification (the difference between the surface and bottom temperature) of the water column of the North Sea in the summer. The likelihood is that environmental factors in combination with heavy fishing moralities on adults have contributed to the long term decline in cod recruitment. Paradoxically, an environmentally stressed population is likely to be less resilient to fishing pressure than an unstressed population, so it becomes desirable to reduce catch rates in order to conserve the age structure of the spawning stock.

Reference:

P.J. Bromley, 2004. The use of ecosystem indices to investigate the decline of recruitment and spawning stock biomass in North Sea cod (Gadus morhua L.) ICES ASR Vigo, Spain 2004. ICES CM2004/FF:19.

P.J. Bromley*

ICES ASR Vigo, Spain 2004 ICES CM2004/FF:19