Sublethal toxic effects

These are the effects which reduce the capacity of a population to retain an internal balance within its community.

This loss of balance can take the form of reduced growth rates or fertility (alteration of gametes), or increased mortality in larvae and juvenile stages. It can disturb communication between individuals or between them and the environment, causing, for example, an alteration in their migratory behaviour. It can also lead to stunted growth, either through a loss of appetite or a reduction in their capability of transforming food into energy. Finally, it can produce various physiological or behavioural changes. These changes can generate a reduction in resistance to stress and the capacity to find or consume food. Furthermore, they can also lead to late egg-laying and brooding.

Laboratory-based studies have shown a decrease in the activity of planktonic algae when in contact with non lethal concentrations of oil. Others have demonstrated behavioural troubles in crustaceans, in particular as concerns feeding. Others have brought to light a reduction in the adherence capacity of shellfish to rocks by their foot or their byssus. These effects are however not always important, nor necessarily negative: cases of acceleration of metabolic activity have been observed in certain algae and high tolerance to the ingestion of oil has been demonstrated in certain seabirds.

Feeding in a clinic




In order to compare the results of laboratory-based experiments and the reality of the field, simplified natural environments (mesocosms) were constructed in several thousands to several tens of thousands of litres of seawater. Experiments, carried out in a mesocosm, provided interesting scientific results, which were however difficult to relate to real pollution in the natural, open environment. This is due, in particular, to the difficulty in reproducing, in these volumes of seawater, all the effects of dilution and agitation which occur in the natural environment, as well as the processes of recolonisation.

These sublethal effects of oil spills therefore remain a vast and controversial subject for heated debates, which can be all the more intense when it comes to discussions about the compensation of medium and long term damages. In any one botanical or zoological group, these effects differ considerably between taxonomic groups and from one species to another, in relation to the physiology and the behaviour of the organisms. For example, in birds, a distinction can be made between species that are highly vulnerable to oil spills, being completely dependant on the marine environment (puffins, common guillemots, cormorants…) and species with a high adaptation capacity, which are therefore mildly vulnerable (gulls, terns, albatrosses…).

Cleaning an oiled seal