Hydrocarbon concentrations in the northern North Sea and effects on fish larvae

Abstract

Two surveys were carried out in 1993 and 1994 to measure the concentrations of oil in water and the associated effects on fish larvae along two north-south sections (between 57.00°N and 62.00°N at longitude 0.0°E and 1.5°E) through the northern North Sea. At each station occupied hydrocarbon concentrations were logged continuously from a towed in situ fluorimeter attached to a MIKT net used to sample larvae and deployed on an oblique tow from surface to bottom. The results show a distinct north-south gradient in hydrocarbon concentration peaking around installations associated with oil and gas exploration and production. Depth averaged data show that the mean concentration in the top 50 m was between 3 and 4 μg l−1 at latitudes between 56.00 and 59.00°N falling to values as low as 0.2–0.5 μg l−1 in the more pristine waters to the north and west. The hydrocarbon levels in the top 50 m between 56.00 and 59.00°N were elevated compared to those in deeper water. 7-Ethoxyresorufin O-deethylase (EROD) activity was measured in pooled samples of both sandeel and gadoid larvae. For both species the range of EROD activities was large and variable and this is probably due to damage to larvae associated with capture and subsequent denaturation of the enzyme. Despite this the results show elevated EROD activity in samples taken on the southern end of the transect where hydrocarbon fluorescence in the water was highest and there is a significant regression of EROD activity on hydrocarbon concentration. The results therefore suggest a significant elevation of hydrocarbons in a large area of the northern North Sea (5–10 times the true background) and that these concentrations are capable of inducing biological responses associated with deleterious effects in fish larvae.