The International Pacific Halibut Commission has surveyed and aged halibut for many decades, which gives an unusually long record of length and weight against age. That record shows a substantial decline in size-at-age from roughly the 1980s onward: fish of a given age are markedly lighter than fish of the same age in earlier decades.
The consequence for management is direct. Halibut quotas are set in weight, but the fishery is constrained by a minimum size, so if fish grow more slowly the number of fish that must be caught to fill a given tonnage changes, and the yield expected from a cohort changes with it. Assessment models have had to accommodate a moving growth curve rather than a fixed one.
The explanation is not agreed. Density dependence, ocean regime shifts affecting prey, and size-selective fishing removing the fastest-growing individuals have all been proposed, and they are difficult to separate because they operated over the same period. Growth has shown some signs of stabilising in recent years, which itself is a clue nobody has fully accounted for.