Why and How to Measure Fish: Everything You Need to Know to Preserve the Sport and Stay Legal

Many species are governed by laws that stipulate precise lengths. Knowing how to measure your bass, pike, walleye, red drum, specks, snook, wahoo, triple tail, and other popular game species is essential for the future of recreational fishing.


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For many inshore species, regulations demand that keepers exceed certain lengths, fall within a specified range called a “slot limit” or don’t exceed a particular length.

For instance, Florida’s Fish and Wildlife Conservation Commission requires that red drums caught and kept in either the Atlantic or Gulf measure between 14 and 24 inches

One of the five redfish you can keep per day can be over 24 inches.

In Louisiana, by contrast, while the daily red drum possession limit is still five, only one of those fish can measure more than 27 inches. Speck possession is limited to 15 daily per person with no more than two of those fish measuring over 20 inches in total length.

Proper measurement doesn’t mean you can’t keep great fish.

For largemouth bass, freshwater regulations typically prescribe maximum lengths for keepers (there are exceptions to this), while for pike, muskie, walleye, and catfish, regulations stipulate minimum lengths.

Note that this is not how tournaments typically work for bass, where weights - not lengths - are assessed!

In Florida, largemouth bass shorter than 16 inches are legal to take, while only one fish over 16 inches is allowed per person per day.

Moving north to Michigan, the rules change substantially. There, largemouth bass must exceed 14 inches to be kept, with 5 fish per person per day being allowed. 

Pike must be no less than 24 inches, walleye no less than 15 inches, and muskie no less than 42 inches, with daily limits being 2, 5, and 1, respectively.

But before you bust out your reliable Stanley tape measure, it’s important to answer two questions:

Why is it important to follow these length regulations?

How will wildlife officers measure your fish to ensure that you’re complying with state law?


Why It’s Important to Measure Fish

Length regulations vary from location to location, even within states at times, to preserve game species and ensure a healthy future for recreational angling.

Preservation of the species and the sport

Every state with a coastline has regulations on the size and number of popular game fish that can be kept.

And while you may think these are merely attempts to curtail your fun, they’re necessary to ensure that sufficient numbers of breeding-age fish thrive to create the next generation.

As Lee Schoech, an intern with the Texas Parks and Wildlife Department (TPWD) Coastal Fisheries Division, explains, “it is often perceived through the eyes of anglers that size and bag limits are meant to be restrictive attempts aimed at anglers. However, the factors used to determine size and bag limits are meant to help preserve and maintain current and future sport fish populations.”

Zeke Vanas, Brian DeSanti, and Kyle Wald picked up a stingray with the Florida Fish and Wildlife Conservation Commission.

Wildlife biologists working for the state constantly assess the health of game fish populations, conducting field research to reveal the average size, weight, and health of these species. They watch the populations of prey items, predators, and water conditions, and they keep a close eye on the impacts of commercial and recreational anglers.

Their goal is to maintain a healthy breeding stock of each species, which is why regulations may change from year to year. 

“For example - the minimum size limit for spotted seatrout (trout) is 15 inches; the reasoning behind this size limit is to ensure that all members of the population spawn at least once before they become part of the harvestable portion of the population,” Schoech says. “Trout maturation is a size/age relationship so this limit is very important for the preservation of the fishery. If the minimum size limit was lower, a portion of the trout population would not have a chance to reproduce and replace itself before being eligible to be harvested.”

So the first reason that it’s important to measure your catch carefully is to ensure the future of the sport you love. 

Harvesting fish that are too large - or too small - impacts the next generation of that species. And in combination with other factors, overharvesting or incorrect harvesting can put a dent in species numbers, creating long-term impacts.

For a good example of careful regulation, take a close look at the snook regulations in Florida.

Each region of the coast is broken down in fine detail, reflecting the differences in seasonal water temperature, available prey items, fishing pressure, and other circumstances that impact snook growth and reproduction.

For instance, in the Southeast Region, bounded by the Martin-Palm Beach county line and the Miami-Dade/Monroe county line, the season is closed from December 15 – January 31, and June 1 – August 31. The slot limit is not less than 28 inches or more than 32 inches in total length, and you can keep only one snook per person per day.

By contrast, in the Panhandle Region, extending from the Florida-Alabama border to the eastern coastal boundary at 84°20.800ꞌ West Longitude in Franklin County near Alligator Point, the season is closed from December 1­­ – end of February and May 1 – August 31. The slot limit is not less than 28 inches or more than 33 inches in total length, while the bag limit remains one fish per person per day.

That small difference in the season and slot limit has big effects, allowing snooks to thrive and reproduce as they should to maintain - and even increase - their numbers.

Conscientious anglers who value the sport have good reasons to obey local regulations. 

There are also penalties to consider for the less than conscientious.

Legal Consequences

Wildlife agents actively check keepers to ensure that anglers obey the specified limits.

In Florida, for instance, keeping fish when the season is closed or that measure outside the allowable bounds comes with significant penalties.

A first conviction can be punished by imprisonment for a period of not more than 60 days or by a fine of not less than $100 nor more than $500, or by both such fine and imprisonment.

A second or subsequent conviction within 12 months, can be punished by imprisonment for not more than 6 months or by a fine of not less than $250 nor more than $1,000, or by both such fine and imprisonment.

So if conservation doesn’t motivate you to measure your keepers, the law will.

How to Measure Species like Bass, Pike, Walleye, Red Drum, Snook, and Specks

Wildlife biologists and enforcement agents don’t measure by eye: they use a standardized tool and technique to measure each fish.

To make sure that you’re providing for the future of the sport and staying on the right side of the law, you need to use the same tools and techniques.





And while you can use a flexible tape measure, provided it’s placed on a flat surface, that’s not how wildlife biologists or enforcement agents get the job done.

As Schoech warns, “Throughout the summer while conducting surveys for the TPWD Coastal Fisheries harvest-monitoring program I observed some discrepancies between measurements obtained by anglers and the TPWD staff. 

Most of these discrepancies could have been avoided if the proper equipment and measuring technique were used. Most anglers typically have some sort of measuring device, such as cooler lids or horizontal measuring tapes. 

These devices may be to the correct scale, but lack a ninety-degree backing making it hard to obtain precise measurements.”

Cooler lids and tape measures are not as accurate as a measuring board, and wildlife agents aren’t going to cut you any slack for mis-measured fish.

Precision matters, and since it’s vital for the preservation of the sport and avoiding legal consequences, we recommend that you always use a rigid ruler with a 90-degree backing.

These are called “bump boards,” and you need to get one long enough to measure the species you’re chasing to its minimum or maximum allowable lengths.

This Hurricane Bump Board can measure fish as long as 40 inches.


Technique matters: measure properly

This is an incorrect technique. Can you spot what she’s doing wrong?

Biologists and agents measure fish carefully; you need to do the same thing.

Proper technique isn’t complicated, but it is precise.

Start by laying the fish on its side and pressing the fish’s closed mouth against the end of the bump board.

For species with a forked tail like tuna, amberjack, mackerel, conia, permit, and pompano, the measurement runs from the tip of the closed mouth to the center of the forked tail, not its tips.

Melissa Crouch with the Florida Fish and Wildlife Conservation Commission demonstrated this technique.

For species like red drum, sea trout, grouper, snapper, largemouth bass, pike, and walleye, the measurement runs from the tip of the closed mouth to the tip of the compressed tail.

Notice that the tail is compressed, extending its length.

Does icing shrink fish?

Plenty of anglers want to obey local regulations and try to measure their fish properly.

But they worry that icing their keepers can shrink them below their fresh-measured lengths, resulting in game violations that carry serious consequences.

Is it true that icing fish causes them to shrink?

The short answer is yes, but not very much.

Schoech explains that “According to a published study conducted by Coastal Fisheries staff in 1989 an average of three millimeters, which equates to about 3/25 inches, in length was lost due to shrinkage. Over half of the estimated shrinkage occurred in the first hour of the fish being on ice, and no fish experienced a decrease in size greater than 1/5 of an inch.”

What does that mean for you?

To be extra safe, make sure your fish is well over or under the required mark so that ice-related shrinkage doesn’t put you in harm’s way.

So if the minimum length is 16 inches, make sure you’re well over 16. Conversely, if the maximum length is 24, nothing more than 24 inches dead on will do.

Final Thoughts

Learning how to measure fish properly is part of the sport, and anglers who want to preserve the excitement of fishing for future generations will want to ensure that they’re doing their part.

We hope that you learned something from this article today, and we’d love to hear from you if you did - or if you have any questions.

Please leave a comment below!

About The Author
John Baltes
If it has fins, John has probably tried to catch it from a kayak. A native of Louisiana, he now lives in Sarajevo, where he's adjusting to life in the mountains. From the rivers of Bosnia to the coast of Croatia, you can find him fishing when he's not camping, hiking, or hunting.