The model of ideal free distribution (IFD, Fretwell and Lucas 1970, Fretwell 1972) can predict the optimal distribution of individuals of a given animal population between patches of different resource profitability. The two most important assumptions of the IFD model are that (1) each individual is able to assess the level of resources and chose the most proficient location which offers the highest possible fitness gain, and (2) each individual is free to move from one patch to another with no costs involved in its relocation. The IFD model predicts that the density of an animal population reflects resource profitability: the higher the resource levels (resource productivity), the higher the population density (Figs. 1 and 2a). While early examples demonstrated the great predictive value of the IFD model, more recent studies have shown that the model is too robust to accurately reflect animal distribution. Considerable empirical data have shown that there are consistent deviations from the IFD, and that densities in proficient sites are often lower than expected from the outcome of this model, and vice versa. The original imperfect IFD model (Fig. 2a) has therefore been gradually replaced by various modifications, including those that consider the inability to correctly assess the resource profitability level (Fig. 2b), the effect of competitive interference (Fig. 3), differences in superiority in resource competition (Fig. 4), and other factors such as travel costs. However, the aforementioned modifications are not as important for individual fitness as the separate components of individual growth P, either by reduced assimilation A or elevated respiration R (P = A - R).
The current veterinary legislature on aquaculture was developed 20 years ago when the European Union consisted of only 12 Member States. It was created in order to protect the-then basic field of activity of this sector, which was the culturing of salmonids (trout and salmon) and oysters. Currently, this law requires updating so as to reflect the wide scope of activity within the aquaculture sector and species now found in the enlarged Union. It should also include significant developments in this branch and the experience gained through the last 15 years of implementing the existing law and scientific progress in this discipline. EU regulations should also be updated to conform to international agreements and norms (e.g.: WTO/SPS and OIE). The proposed new regulations submitted to the Council will abolish the present laws (Council directives 91/67/EEC, 93/53/EEC and 95/70/EC) and replace them with a new directive on health requirements for aquatic animals and products thereof, and on the prevention and control of certain diseases in these animals. This will facilitate higher flexibility and give operational responsibility to the Member States, thus enabling effective disease prevention through initiating decision-making at a local or regional level.