Among small- to medium-sized marine mammals such as seals and dolphins, the longer an animal can stay submerged during a dive, the greater the depth the animal can reach. Dolphins can dive to greater depths than northern fur seals can, and elephant seals can stay submerged longer than Weddell seals can.
Northern fur seals can stay submerged longer than elephant seals can, but Weddell seals can dive to greater depths than dolphins can is not true if the information above is accurate.