http://www.ices.dk/reports/FTC/2005/WKSAD05.pdf
Random vs Systematic Sampling:
...
The international bottom trawl survey used stratification based on depth. There is evidence that in some areas, bottom sediment type makes a significant difference in fish populations, so characteristics of the seabed will be incorporated into future stratification designs. Stratification can become a confounding factor in samples of several different species where the species have different spatial distribution characteristics. Preliminary analysis is need to know how to stratify when surveying several species.
(to be continued)