Not all aquatic species require screening to prevent their escape down the bottom center drain. Tanks (4.9–9.1 m diameter tanks) have been used to culture salmonid species with no fish exclusion screening over their center drain (S. Wilton, P.R.A. Manufacturing, Nanaimo, BC, personal communication). This design links the unscreened bottom drain to an external standpipe chamber, which contains a weir to control water depth, a screen to capture dead fish, and a drain (Fig. 6). According to Wilton (personal communication) fish escapement down the uncovered center drain was not reported as a problem, because salmonids do not tend to swim down that kind of current. Depending upon the regulatory authority, more stringent escape preventing methods may be required.