バスの年齢による成長の関係【何年で何センチ?】

How fast does a bass grow? That’s a question a number of readers asked after seeing the photo of inch-long young-of-the-year largemouths netted by state biologists from Alabama’s Lake Guntersville in The Fishing Wire a few weeks back. The answer, according to James McHugh, former District IV Fisheries Supervisor for the Alabama Department of Conservation and Natural Resources, varies by lake and by weather conditions. “Fish can be aged by examining scales or various bones. Hard body parts grow as the fish grows, adding annual rings similar to the rings in trees," reports McHugh. “Because the growth in the diameter of the hard body parts is proportional to the growth in length of the fish, examination of these structures reveals not only the age of the fish but also its length at each birthday." McHugh says that growth can vary greatly between individuals, even within a single lake, depending on the abundance and availability of baitfish and other edibles. Here’s a general table he developed showing some of the averages for fish across the state:

growth in length is greatest in the first year and decreases each year thereafter. Old fish increase in length very little from year to year, but do continue to add weight. McHugh’s counterparts at Texas Parks & Wildlife have published a graph showing the average weight of bass in their state at a given length, and though conditions are not exactly the same there, the averages are likely to be similar–they say a 15-inch fish is typically around 1.8 to 2 pounds, 17-inchers are typically in the 3-pound class, 19-inchers are around 4 pounds, and a 5-pound fish is typically 20 inches plus. Fish of 23 inches are usually in the 7-pound class, and to weigh 10 pounds a bass needs to be at least 25 inches long. In other words, those 4 to 5 pound tournament winners everybody is always hoping for take well over five years to produce in most southern lakes. (Fish in northern lakes grow much slower due to slowed metabolism for half the year, and Florida strain largemouths stocked in some California lakes grow much faster, the result of stuffing themselves with stocked trout.)

McHugh says that bass in most habitats need to be about three years old before they are large enough to interest most anglers. The fish many anglers call “yearlings" are actually two-year-old fish. True yearling bass are too small to be caught with most bass baits, he writes. McHugh also writes that fish of the same age, in the same population, can have quite different growth rates. For bass, there is usually a range of about four inches between the largest and smallest individuals of the same age, he says. That is, although the average three-year-old largemouth bass is 13.3 inches long, some will be as small as 11 inches and others as large as 15 inches. That’s why we can’t tell how old a fish is without examining the scales or other hard parts. A 15-inch bass might be an average four-year-old, a fast growing three-year-old or a slow growing five-year-old. A typical northern-strain five-pounder might be anywhere between six and 16 years old, McHugh reports.

Bottom line is that it takes time and good habitat to produce trophy bass, and anglers who release their catch alive are contributing towards more big fish in the future, whether fishing Alabama, Texas or somewhere in between.