The most likely effect of reduction in the size of an Antarctic fish population is, the sperm whale population will decrease. This is because of the given food web, the sperm whale population which is independent of Antarctic fish population. The excess consumption of these fish by humans reduces the sperm whale availability. This results that the population of sperm whale will decrease.
A reason could be the bodies ability to process the water based vitamin as well as the person's diet. many things have b12 in it but choosing to cut something out, like meat, could cause a deficiency
