In the late 1980s, annual carbon isotope cycles in the baleen plates of bowhead whales formed the basis of the first effective means of estimating bowhead age. However, the baleen aging method could only be used for aging subadult whales less than about 20 years old, due to wearing of the plates. Since then, seven additional methods of estimating the age of bowheads have been developed. Each age estimation method has strengths and weaknesses depending on the life stage of the whale. No single method is fully applicable through the entire 200-year lifespan of a bowhead, although a combination of techniques can be quite effective and precise.