Federal and state officials say the seal known as RK68 died a slow and painful death.
X-rays showed the Hawaiian monk seal swallowed a hook, leading to possibly months of suffering.
"The hook lodged in the esophagus near the stomach and it slowly migrated through the esophagus into his trachea," said Charles Littnan from the National Oceanic and Atmospheric Administration.
Littman said the seal could've been hooked as far back as August. Only recently did crews find it injured on the Big Island and flew it to Oahu for treatment.
But, it was too late -- the animal wound up suffocating.
"He was already a lost cuase by the time we stepped up to intervene," said Littnan.
Wildlife officials say if they knew about the hooking earlier, the seal could have survived. That's why the chairman of the Department of Land and Natural Resources is pleading with fishermen to report the accidents sooner.
"It's not like we are not going to find out. Obviously, there is a hook. That hook is used by shore casters, so we know that the shore casters are hooking seals. What we need them to do is take that extra responsibility by calling as quickly as possible, as accurate as possible," said William Aila, chairman of the DLNR.
DLNR officials say you can file an anonymous report and there are no penalties for accidental hooking.