How Do You Measure Fish Properly?
- New Wave Fishing Academy
- May 6
- 4 min read
Measuring Fish
When it comes to quantifying your progress or new personal best catch a length measurement is a great way to do it. A length measurement is non-invasive, very risk adverse, and can be done quickly. Depending on if you keep your fish a length measurement is also needed to ensure it is a legal fish to keep. If you care, it also allows you to compare with others if the size of fish being caught matches up. However, this only works if you measure your fish properly. However, there are a number of mistakes that can be made which will impact the accuracy of your catch! You may be wondering "how do you measure fish properly?" - keep reading to find out how to measure fish and the potential error you could get by measuring incorrectly.
How To Measure Fish
Depending your goal with a measurement close may be good enough - say it is not a trophy but you want to know anyways - but for tracking your personal best, submissions in tournaments, or when keeping fish the accuracy is paramount. Failing to measure fish properly can lead to keeping a fish of incorrect size (which can lead to fines) or even set a standard that can never be beaten.
In Ontario the Fisheries Regulations state that the "size" of a fish refers to the "total length which is a measure from the tip of the mouth with the jaws closed to the tip of the tail, with the tail fin lobes compressed to give the maximum possible length." Where this can be confusing or lead to problems for anglers is the method of measuring or device used. If you place a fish on top of a measuring stick or tape you will get one measurement, if you put the tape overtop of the fish you will get another, if you have a sticker measuring tape stuck to the side of your boat that has a slight curve in it you will get another measurement yet again. If you measure a fish while holding it vertically you'll probably end up with another new measurement.
So which is correct? How do you measure fish properly?
Alberta Fishing Regulations are much more clear on this matter with indication to lay the fish on a flat surface on top of the measuring device. Ontario regulations do not have this clearly depicted or even stated, believe us, we've looked high and low!
From conversations and experiences with Ontario MNRF conservation offices the above is still the case for Ontario and as such in order to measure fish properly a bump board is your best option. Fish measuring tapes are also an option when used properly but can easily be used incorrectly and put you in an uncomfortable position.
Bumpboard
Bump boards eliminate almost all the potential for measuring incorrectly and provide a rigid "0" point to start your measurement for. By having a fixed "0" point and printed measurement increments on a rigid board there is really no way to get an incorrect measurement while using one. This is why all tournaments that go by length will use some model of bump board to measure.
Fish Measuring Tape
When comparing to measuring fish with a bumpboard there are many potential sources of error when using a fish measuring tape ; not starting at "0", the tape is folded or scrunched up under the fish, or you're measuring at a weird angle or overtop of the fish which will incorporate some of its girth into the measurement skewing the results. How much can it skew the results?
A lot!
The bigger the fish is (length & girth) the more incorrect measuring can skew the results.
Something as simple as not measuring straight through the centerline of the fish can add 0,25 to 0,5" as shown in example 1 below.
The measurement given by the tape when placed overtop of the fish can be approximated by the arc length of an oval. Now, depending which fish you are measuring it will better or worse represent a real oval as shown in the photo below. There are two different approximations for the arc length of an oval given certain conditions which are shown in the table. Regardless of which one you use the measurement from a tape can be 0.3 to over 7" off from the actual measurement if you laid the fish flat on top of the tape! Given that fish are not perfectly oval shaped there is some error in these approximations and the actual "measurement error" is likely to be somewhere between the two.
When dealing with slot limits these measurement errors can easily turn a legal fish into an illegal one putting you in potentially costly situation! Don't make these mistakes - measure your fish properly and consistently.
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