I caught a halibut in northern BC that weighed exactly 50 pounds.

Anyone know about how old that size of fish would be?

Except for the last 15-20 years that would relate to about 15 years. The growth rates have dropped significantly for a bunch of reasons. Your fish could easily have been 20.

Interesting graphic that shows the changes over time.
https://f16a75a2-a-8ade7df0-s-sites.googlegroups.com/a/alaska.edu/halibut-size-at-age/home/age-15%20F.gif?attachauth=ANoY7crVYpfC3N7G5rKg5htTaOxz5OjKeZMe0Hsjj4LV127JXLkq1PcxfxMzcbt7uhRuwEFouwMj7pV7VHmfwqND-ywE50XLUCalHZApX1GJqHD-54JJ4QcCzKojWnso1TfHwjAtnbHoBIvGKhAA6hyd48hYHRK5ipDTYnL-G_cMrphBhkF9GkrRDcfJ_7BWNfw9Tr-x6ErBPdBDbpVoxAOrnjqipjLaj2TNygsAcsAHP-ghEM_9dD0%3D&attredirects=0

Thanks SD

Whats causing that?

They do not really know, but better guesses include competition from arrow tooth flounder which are now thought to be the largest biomass species in the Gulf of AK, water temp changes with decadal oscillation being the biggie, more fish of other species competing, and dozens more guesses.

Arrowtooth have an enzyme in the flesh that turns the meat to mush when they are cooked. After lots of dollars spent they figured out how to make Surimi from them, but they are not cost competitive with pollock even though pollock have had quota cuts right and left.

Halibut populations have been cyclical forever, but there are new findings on diet changes which include chalky flesh which they attribute to reduced feed. The fishermen see lower prices for chalky fish...