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there were increase in numbers of all the three sea mammals over the period of 12 years even though (however,) the numbers of seals and whales only had a slight improvement.
Secondly, no marked variation in numbers of whale seen since, initially they were only half the number of seals in 2006 (20 numbers),- not clear; Try not to loose the essence of sentence in order to add complexity
Band : 2/3