The effectiveness of the continuous underway fish egg sampler compared with standard vertical plankton hauls for sampling the eggs of demersal species in the southern North Sea

Abstract

AbstractTo date, the continuous underway fish egg sampler (CUFES) has been used primarily to map the spawning areas of pelagic fish in upwelling regions. To assess its utility in mapping the spawning grounds of demersal species, species composition and densities of eggs collected by CUFES were compared with results obtained from ring‐net samples (VET: vertical egg trawl) from the southern North Sea over three annual cruises conducted in January/February. CUFES collected slightly fewer eggs per unit volume of water filtered when compared with vertically integrated samples but maps of egg distributions from both devices showed broadly similar patterns. Geostatistical analyses revealed that small‐scale variability was greater with CUFES than with VET. CUFES may thus be able to resolve finer spatial structuring of egg distributions. At the sampling station scale, species compositions (in terms of proportion and abundance) from both approaches were similar, with the notable exception of flounder, which was much more abundant in CUFES samples; these results were similar when considering early and older stage eggs. Results from regression modelling showed that it was difficult to estimate total egg abundance in the water column from CUFES data with linear regression techniques even when additional factors which affect the vertical distribution of the eggs, such as depth, bedstress and wind‐induced mixing, were included. CUFES therefore provides a suitable method for mapping egg distributions of the main fish species producing planktonic eggs during the winter and early spring season in the southern North Sea but may not be used easily when absolute measures of egg production are required. However, CUFES can be added to standard trawl surveys with little disruption to the baseline ground‐fish survey.