Target Species Profiles
California Halibut (Paralichthys californicus)
Written by: Dr. Larry Allen
(originally printed in Western Outdoors News 1992)
When you catch a halibut do you wonder how old it is? I know you wonder how much it weighs, that is, after you have determined if it is of legal size. For that matter, have you ever wondered why there is a 22-inch minimum size limit, anyway? If you have ever pondered such questions, then this article is for you.
In preparation for the most recent Marina del Rey Halibut Derby this October, I had both the opportunity and motivation to do something about answering these often pondered questions accurately.
First, I gathered the available data on halibut age and growth and calculated the growth equations for male and female halibut. This rather straight forward exercise has been long overdue with even the recent, extensive Cal Fish and Game Bulletin on halibut falling short on this account. Age was determined by Fish and Game biologists by reading annual growth rings in the ear bones (otoliths) of the fish. I simply took the data presented in this bulletin and feed it into my computer and calculated the most commonly used growth model in fisheries science, namely the von Bertalanffy growth model, for both sexes. Two growth models are necessary since male and female halibut grow a different rates throughout their lives.
The von Bertalanffy model, as with all growth models, calculates the predicted growth trajectories of an average length of a fish of a particular age. These predictions can then be plotted.
Secondly, I was able to plot lengths and respective weights of halibut measured by Fish and Game personnel over the last four decades based on the published equation.
Together, these two graphs allow any California halibut angler to determine, on average, both how old their fish is likely to be and how much the fish should weigh based solely on an accurate measurement of total length (snout to tip of tail).
For instance, from the graphs I can estimate that a 22-inch halibut is 4 or 5 years old (depending on whether it is a male or female) and should weigh about 4.5 lb. A 36-inch female halibut should weigh 20 lb. and be 10 years old. The 45 inch, 33.5 lb., female halibut caught by Nancy Hawkins that won this year's Marina del Rey halibut derby was probably about 23 years old. Likewise, the 30 inch, 10.2 lb. fish caught by the youth division winner Samantha Medina can be estimated to be 7 years old assuming it was a female fish. By the way, how does one tell males and females apart? The only sure way is to open them up and look for ovaries or testes.
What is so "magical" about 22 inches? Halibut with a total length of 22 inches are 4 or 5 years old. It has been known for years that male halibut are mature at 3 years of age and most females are mature by year 4-5. Originally, the minimum size limit was set by Fish and Game to allow halibut a chance to spawn at least once before they were subject to sport and commercial take.
Is satisfying personal curiosity the main reason for gathering such scientific information? Of course not, such basic information has a higher, longer term purpose. It is just this kind of information which drives effective fishery management decisions. Knowledge of length-weight and, particularly, age-length relationships help us to determine all of the following: 1) the length of time to sexual maturity, 2) the length of time for fish to first enter the fishery, 3) the length of time to attain certain large sizes and weights to maximize yields, and 4) how quickly the populations can recover from over-fishing. Each of these represent critical fishery management considerations.
In brief, short-lived, fast growing species tend to mature early and have the potential to recover quickly from heavy exploitation. On the other hand, long lived, slow growing, late maturing species are slow to recover and we must be careful and patient in our management practices.
The take home message? California halibut and many other prized nearshore sport fishes are relatively long-lived, slow growing, and late maturing fishes.