When dolphins reach carrying capacity their population stops growing.
When dolphins immigrate into a population the population increases because dolphins live in large herds to help each other collect resources.
When dolphins emigrate out of their population their population decreases because the dolphins in the population have less help collecting resources.
When dolphins are born the population gains new dolphins so once they get older they will be able to help out the population.
When dolphins die the population looses a dolphin that helped collect resources so their population decreases.
Limited resources would lead dolphins to the carrying capacity because if the population got two big there would not be enough resources for all of the dolphins in the population.
Two density dependent limiting factors that could affect the dolphin population are diseases and drop in prey population because since the population is so big a disease would spread quickly through the population. Also if there is a drop in the number of prey (fish) then dolphins would not have enough food for the whole population.
Two density independent limiting factors that could affect dolphins would be a change in temperature and habitat interruptions like fishing nets. Temperature could affect dolphin populations because if it gets too cold and freezes the water surface dolphins wouldn't be able to go up and get air. Habitat interruptions would affect the population because fishing nets has caused many deaths in dolphin populations.