The base-line study consisted of three visits to Sullom Voe before the oil terminal became operational. General stress indices were used to measure the health and condition of four native and two transplanted populations of mussels (Mytilus edulis). A mobile laboratory was used to measure the physiological responses under ambient field conditions. These were then integrated into the stress indices, scope for growth and oxygen :nitrogen ratio. The general biochemical and cytological stress indices, namely taurine: glycine ratio, gametogenic development and latency of lysosomal β-hexosaminidase were also measured for each population. All these general stress indices indicated that the mussel populations were in a relatively healthy physiological condition. However, mussels from the Houb of Scatsta showed a transient stress response, probably caused by freshwater run-off from the surrounding peat beds.
In addition to the general stress indices, specific cellular responses known to be induced by petroleum hydrocarbons were monitored. The activity of enzymes forming components of the mixed-function oxidase system (NADPH neotetrazolium reductase, G6PDH and NADP-ICDH, and aldrin epoxidation) were measured.
The rationale for measuring these general and specific stress indices as part of a Sullom Voe environmental monitoring programme is discussed in the light of experiments with petroleum hydrocarbons in the laboratory and with reference to the future operation of the oil terminal.