As anglers, we believe that when we hook into a large halibut, it has to be an old fish. Why wouldn’t it? It sounds logical, yet biology of Pacific halibut doesn’t match our belief. Most of biggest fish you will catch aren’t the old ones we all envision. They follow a different story for size and age in there lives.
Knowing this change how you see science of fisheries management. It also changes how you see ecosystem and even catch itself. Most experienced angler are surprised at growth pattern (see breakdown of growth below in graphic).
The Surprising Truth About Halibut Size and Age
Once mature, females grows rapidly and often double their size over several years. They continue this growth until they reach giant proportions. Males, however, reaches maturity quickly and level off. A 20-year-old female will be three-to-four times larger then her male peer. This isn’t just for show, but rather evidence of a biological plan for females: large mass for large eggs.
If you’re landing a fifty pound halibut, odds are high it’s a juvenile when compared to its ten inch male companion swimming next to it. That said, length isn’t always an indicator of age. To find that out, biologists uses a tool called an otolith, tiny ear bones within the head of fish that contain annual growth rings just like those found in trees. The best way to get a precise read on age is to count the rings using a microscope.
While scales were once thought to be the answer, they often fall short when accounting for older fish whose inner rings begins to fuse over time. Because of this type of compression, otoliths aren’t subject to the same error and are considered gold standard for understanding population dynamics. It is a painstaking task that takes skilled eyes, but without it there would of been no sustainable quotas.
These long-lived females has great reproductive value. During one spawning run, a young female can deposit up to half a million eggs. When she’s three or four decades old, it become in the millions. “The bigger females are really driving the population,” they are the engine. “And you see that more with conservation measures as well; we’re protecting those large females more and more because when you take out lots of smaller individual, it doesn’t matter as much, whereas if you remove a few older females, it makes a huge difference in the number of fish coming back.” Their fecundity guarantees that species will survive.
Seasonal movement patterns adds another problem to managing halibut within an area. Halibut migrate dramatically, moving from thousands of feet deep during winter months to within just several hundred feet of the coast during summer months when they spawn. What this means is a fish you catch in July nearshore could very well be exact same fish you catch offshore in January. Because of their movement, they connects different geographical areas stretching from Japan to California. Without considering migratory pathways, localized protections will be made ineffective.
With that change in size comes a change of diet. As larvae, they float around eating plankton. Juvenile fish feeds on shrimp. Adult fish becomes ambush predators feeding on octopus, cod and, yes, even salmon! That adaptability of diet helps them survive so many types of environments; ranging from the continental shelf down to nearshore sandy bottom areas. But it is also what causes commercial fishing to compete with them since they feed on similar food sources.
If you’ve ever been out on the deck with a halibut in your hands, take a moment to think about what it weighs. If it’s a big one, chances are it’s a seasoned mother. She is a survivor of several decades of fishing pressure and predation who’s lived through multiple spawning cycles. It’s safe to say her large size doesn’t reflect an old age so much as successful growth.
Understanding this can help anglers better understand the environmental contribution halibut make in addition to their value at the dinner table. So when you’re measuring your next catch, keep in mind that a length measurement reflects growth history, but only the otolith will tell full story. This slight variation guides our management of one of the most highly sought-after fisheries along the Pacific coast.