Many of the methods described so far require the collection of faeces. A great variety of devices have been designed to do this, most of them producing a place with a low velocity water flow so that finely particulate faecal matter is not swept away. That this can be a problem is illustrated by Table 1 of this paper which shows significant differences between faecal composition in the trough and in the rectum. Post (1965) designed a holding chamber to reduce this problem by having static water around the posterior half of the fish and collecting the faeces from this water. Even collecting faeces from the rectum of fish does not solve all problems of faecal analysis because it is difficult to determine by their location in the hindgut when water reabsorption is complete. Thus faecal collection always includes some degree of compromise which affects the subsequent results and the comparability with other experiments.