With a tape measure and some math, you can easily determine the weight of the fish you just released. How big was it?” “What’d you catch it on?” It’s a toss-up which of these questions is asked more frequently of successful anglers. The answers to both sometimes bring out the creative, and maybe cunning, side of people. Anglers often answer the former with a generous weight estimate and spread their arms wide, especially when describing fish that were released.
What’s Wrong With Scales? Obviously, one way to determine the weight of a fish that you’ll immediately release is to weigh it on the spot with a top-quality portable scale. Live-weighing, however, can be harmful to big fish because they are often out of the water for a long time and held in a position that doesn’t support their internal organs. For this reason, very large fish should not be hung by the mouth or head. When weighed, lively fish will often thrash and squirm in such a way that they can be injured, either from falling or by the angler’s gripping them tighter to maintain control.
Thus, for large fish that are to be released, weighing is often harmful and impractical. Many fisheries agencies specifically recommend not weighing fish that will be released.
Tale of the Tape: Anglers and fisheries biologists have developed ways to calculate the weight of released fish based upon measurements of their length and girth. Owing to vast differences in body shapes, no single table or formula applies to all species, and no method is 100 percent accurate all of the time. But there are methods that come close to the live weight for most fish.
To estimate weight with these methods, you must first measure the length and usually also the girth of the fish. A soft measuring tape or a piece of marked cord or thin-diameter rope that will not shrink and is marked at regular intervals will work best. You can also remove line from your reel and cut it to the exact length and girth of the fish, then measure the two strands later with a ruler. In a pinch, the length of small fish can be measured by spreading your fingers and using the distance from pinkie to thumb. Another possible length measure can be your fishing rod, its butt aligned with the tip of the fish’s jaw. Obviously you have to lay the fish down to make these measurements, so find a fish-friendly surface, like a cooler top, a wet board, or a wide bench seat. Placing a wet cloth or wet towel over the head of the fish often calms it for quick measuring.
Measure the girth by wrapping the tape around the fattest part of the belly. Length is measured jaw tip to tail.
Tables: The easiest way to estimate fish weight using length and girth measurements is to consult a table specially made for that purpose. At some lakes in northern Canada, where lake trout must be released unharmed and people catch huge specimens, waterproof length-girth-weight tables are affixed to the seats or gunwales of guide boats, so anglers can instantly reference them as soon as a tape measure (also supplied) is wrapped around and along the trout. Some state freshwater fisheries agencies publish weight-estimation tables (usually in their regulations booklets) for the most popular and common species in their jurisdictions; these are correlated to total length only and are based upon the average weight for fish of that length in that jurisdiction.
One of the best of these tables, offered by Pennsylvania, is a waterproof, pocket-size booklet covering 16 freshwater fish; it also has instructions for photographing and releasing the catch.
Do the Math: If you don’t have a table, you can still estimate weight as long as you can do some simple multiplication and division. Weight-estimation formulas using length and girth measurements have been evolving for years; once, just a single formula — the one listed for trout below — was applied to all fish. Unfortunately, this formula is intended for fish with a cylindrical body shape and is not especially suited to elongate or round fish.
The Minnesota Department of Natural Resources publishes the following formulas, which take body shape into account, for popular freshwater species:
Walleye: length3 ÷ 3,500, (length x length x length) ÷ 3,500]
Pike: length3 ÷ 2,700
Sunfish: length3 ÷ 1,200
Bass: (length2 x girth) ÷ 1,200
Trout: (length x girth2) ÷ 800
The formula for pike is not accurate when applied to large, heavy muskies (it gives a result far too high); some people use the trout formula for those fish, which produces a weight that is slightly higher than the actual amount. It’s often tough to estimate the weight of unusually heavy fish with a formula; bellies become distended, and fish of equal lengths can have much different thicknesses.
Formulas and tables are based on averages, using standard fish up to large, but seldom extraordinary, sizes (because there are so few extraordinary fish). These formulas are guidelines, not absolutes.
Saltwater Guessing: Determining the weight of released billfish species is mostly a guess. The same is true for tarpon, since almost no tarpon are actually killed and weighed, and many people fishing for them have no real experience in weighing tarpon of any size. Eyeball estimates are usually well off the mark, generally being too high (as determined when tagged fish are recovered and actual weights are compared to estimated weights).
The old formula of (length x girth2) ÷ 800 is still used by some people for billfish and other saltwater species, but this is well off the mark as often as it is close; some people alter this formula by dividing by 900 for species that are very long and thin-bodied, like wahoo, king mackerel, and barracuda.
Some private parties, research groups, and fisheries agencies have created preliminary tables that convert length to weight, or have developed formulas, but these have been calculated for fish in specific oceans and regions (you can’t compare Pacific sailfish to Atlantic sailfish, for example, since the former are much larger on average than the latter).
Be Practical: It’s important to recognize that the difference between an estimated 39-pound fish and an estimated 42-pound fish is really minor in the overall picture, unless a record is involved, which likely means killing the fish anyway. Remember that the main purpose of using weight-estimation formulas or tables is to release the fish unharmed. With catch-and-release fishing being more prominent, it’s likely that existing tables and formulas will be refined and that ways to estimate weights for other species will be developed. However, spreading your hands apart no longer suffices.