But the population is declining and it's declined fairly substantially, about 21 percent since 1987, when I started these surveys, in terms of number of active nests.
因为企鹅数量一直在减少， 而且减少幅度相当大，大概有21%。 从1987年起，当我开始调查的时候， 这是根据企鹅巢穴得出的数据。