Spatial and Long-term Variability in Demersal Fish Species of the Western North Sea

Abstract

This thesis explores historical and contemporary patterns of spatial and temporal change in demersal fish species of the western North Sea. Comparing trawl data from over a century ago with those from contemporary trawl surveys are challenged by biases associated with differences in gear types and methods of operation. To compare contemporary data with rare 1892-1913 data from Northumberland Sea Fisheries Committee surveys on the Northumberland coast (UK), an attempt was made to replicate the original trawl gear and methods used by drawing on the literature, historical photographs and expertise of the trawl industry. The replica gear, comprising a 6.7 m beam connecting two Brixham-style wroughtiron trawl heads with a triangular-shaped trawl net and rounded ground-rope, was trialled in August 2018 and March 2019, and catches were compared with a modern otter trawl. In the first trial period, the otter and replica beam trawl had similar efficiencies in catching flatfish, whereas catches made by the replica gear in March were either very low or zero. As a result, the otter trawl was employed in place of the replica gear in all subsequent resurveys and catches were standardised for comparison with historical surveys. Catches revealed substantial declines in the abundance of the overall inshore fish assemblage and among individual species between 1899-1913 and 2018-2019. Elasmobranchs and historically dominant species such as grey gurnard Eutrigla gurnardus were either completely absent or rare in the 2018-2019 surveys. Abundance-size spectra also exhibited significant differences between periods, declining more steeply in contemporary trawls. Analysis of demersal fish stomach contents data revealed large shifts in diet composition over the period spanning 1896-2015. Bivalves dominated plaice Pleuronectes platessa, dab Limanda limanda and haddock Melanogrammus aeglefinus diets in the early and mid-20th century but declined ii substantially in subsequent decades. Conversely, polychaetes increased in importance by number and mass overtime and were the main prey resource for plaice in the 1970s and 2000s. These diet shifts point to likely changes in the benthic prey base of the North Sea, linked to increased beam trawling in the 1960s-1970s, eutrophication, and climatic processes. Finally, contemporary stomach contents and bulk stable isotope data were used as complementary techniques to assess temporal and spatial variation in the diet within and between two sympatric flatfishes, plaice and dab, in four coastal bays off the Northumberland coast. Stomach data indicated similar diets and significant dietary overlap at short temporal scales, yet dorsal muscle and liver tissue δ 13C, δ15N and δ 34S data revealed substantial interand intra-specific variation in resource use among sites and lower levels of niche overlap at longer time scales. Sandeels Ammodytes tobianus were highly abundant in flatfish stomachs, whereas Bayesian isotope mixing models indicated that squid were the important contributors to both predators. Dorsal and liver isotope data also indicated that bivalves and ophiuroids were major contributors to plaice and dab diet, respectively, across sites. These findings provide evidence of profound change in demersal fish species and their trophic ecology, further underpinning the value of using historical and contemporary data to elucidate broad-scale temporal and spatial patterns of change