The final osmoregulatory product of the gut is a rectal fluid composed of magnesium and other divalent ions having about the same total concentration as blood. Preliminary data from scale loss studies indicated that death occurred from toxic levels of magnesium in the blood. A possible cause of the high magnesium is that gut peristalsis stopped, leaving the rectal fluid to accumulate and the magnesium ions to be reabsorbed instead of being excreted.
Thus, digestion and osmoregulation are so inter-related that problems in one system could disrupt the functions of the other. Exactly how fish normally avoid such problems is largely unknown.