Although most streams in the park are very clear, cold and pollution free, they are not very productive in terms of growing big trout. Most trout in the park grow relatively fast, live only about 4 years, and die due to a lack of food resources. The diversity of aquatic insects in park streams is quite high, but the density of each species is fairly low making food resources for trout scarce. In fact, only 4% of brook trout and 30% of rainbow trout reach 7 inches. Less than 1% of brook trout and 17% of rainbow trout reach 8 inches. Only brown trout, who switch to a piscivorous (fish) diet at around 8 inches, have the ability to live beyond 4-5 years and reach sizes of nearly 30 inches!