Definitional approach to categorization is the approach that is used making judgement about any object whether or not it belong to the category based on the definition of category.If category's definition go along with object then it is considered as member of category.
According to the question, fish category can have gill, scale or fins which creates issue in classification of them into fish category as per definition of fish.Thus, problem in using definitional approach for categorization occurs in this case.
The Black Death is estimated to have killed 30–60% of Europe's total population. In total, the plague may have reduced the world population from an estimated 450 million down to 350–375 million in the 14th century.
