Biomarkers are routinely used to screen animals for exposure to the polycyclic aromatic hydrocarbons (PAH) found in crude oil and oil products. For example, measurement of EROD (Ethoxyresorufin-O-deethylase) activity detects enzyme levels in liver tissue, involved both in the metabolism and elimination of toxins and also in the development of cancerous tumours. This technique is sufficiently sensitive to indicate exposure to PAH without detectable body burden and so can provide an early warning of potential damage. However, changes in the levels of activity of this enzyme are also indicative of other causes of stress, such as the presence of other similar toxic materials unrelated to oil. Activity levels also reflect the age and reproductive status of the animal, as well as changes in temperature. It is important, therefore, that such studies take account of these potentially confusing factors
Studies may be prioritised according to a number of factors. Firstly, the baseline against which effects are to be established: whether by reference to pre-spill data, where this exists; by comparison with equivalent species, communities or ecosystems at reference sites outside the affected area; or by monitoring the recovery of a feature of conspicuous damage, such as the mortality of seabirds or shell fish. Plankton provides a poor subject for investigation. Although both laboratory and field studies have demonstrated mortality and sub-lethal effects upon exposure to oil, variability of the plankton is so high that comparisons between pre- and post-spill situations are likely to be unreliable. Other factors to be considered include the geographical extent of the affected area, the degree of contamination and related levels of exposure (concentration and duration) and the importance of the affected resource, i.e. its rarity or ecological function. Finally, the practical feasibility of conducting the studies should be considered. Feasibility may relate to financial support or simply the practicality of accessing study sites or the risk of disruption to the site during the period of study. Further guidance on designing and conducting post-spill studies can be found in the separate paper on Sampling and Monitoring of Marine Oil Spills