Measuring vs weighing your catch

Why measure?  Why not weigh?
There are a number of good reasons for the general move towards measuring rather than weighing fish.  In the good ol’ days we didn’t think that our fishing had any significant impact on fish stocks.  The couple of fish that died waiting to be weighed was regarded as acceptable losses.  This was especially true for non-edible species such as sharks, skates and rays.  With a general increase in the awareness of the vulnerability of our resources came the realization that we should do everything in our power to return as many fish alive as possible.  Measuring fish allows just that - the ability to quickly measure and return your catch - none the worse for wear.
 
The argument of just how much of an impact shore anglers can and have had on dwindling fishing resources will forever be hotly debated.  But that’s not the point.  The point is that we need to have an awareness and appreciation for our fishing stocks.  Dead fish on the beach means bad press, a bad image for the sport, fewer memberships for organized angling, etc. etc.
 
And then there’s the shear practicality of it.  No need to break your back carrying the 200kg+ scale needed to weigh your record-breaking raggie!.  Not to mention all the nets and rigging required and the gang of fellow anglers needed to lift it all!  All you now need is a measuring tape!  And if you forgot your measuring tape, just measure your catch with a length of line and measure the line once you're back home!
 
The past excuse of not having sufficient data being available for all species is now out the window.  There now is conversion data available for more than 270 species of South African fish.  And the list is constantly expanding and refined.
 
Calculation of weights
The calculation of weights based on fish lengths is the result of years of research by many dedicated scientists and research institutions such as the Oceanographic Research Institute, the Natal Sharks Board, and others.  This research was summarized into a format that can be easily accessed and used by all anglers. 
 
How do I measure my catch?
Fish should be measured in a straight line from the tip of the snout to either the fork of the tail (known as fork length or FL), the end of the tail (known as total length or TL), or in the case of sharks to the notch just before the start of the tail fin (known as pre-caudal length or PCL).  Skates and rays are measured in a straight line across their wingspan.  This is known as disc width or DW.  Billfish such as marlin and sailfish are measured from the tip of the lower bill to the fork in the tail, known as lower bill fork length or LBFL.
 
There are a number of general rules that make knowing which length to measure quite easy.  For all edible fish measure either fork length (FL) or total length (TL).  For edible species that has a forked tail such as shad or garrick, measure fork length.  For all species that does not have a forked tail, such as kob or rockcods, measure total length. 
 
As a general rule-of-thumb, sharks with cutting teeth are measured to the notch before the tail fin (pre-caudal length) while those without cutting teeth (such as smooth-hounds and spotted gully sharks), are measured to the tip of the tail (total length).  If you're in doubt as to what measurement to measure, measure both!
 
For all flatfish (skates and rays) measure the wingspan in a straight line (disc width).   One easy tip when measuring skates and rays, is to place two sticks in the sand at the edge of the fish's wingspan or to draw parallel lines in the sand.  Then, after returning your catch to the water, just measure the straight line distance between the sticks or lines. 
 
How accurate are the weights?
Accurate measurement is the first step in ensuring that the estimated weight of your catch is accurate.  Always measure in a straight line, not over the back of sharks, rays or skates!   As mentioned, the weights are based on the most current published scientific research.  There are, however, a couple of factors to keep in mind.
 
Firstly, fish, like humans, can have different weights for the same length (yip – you get fat fish and thin fish too!).  Typically, scientists doing research on fish would measure and weigh large numbers of fish and plot the results on a graph.   Based on such a graph, an equation is calculated, which is the basis for the length-to-weight conversion for the species.  Obviously, the more data (individual fish) are available, the more accurate the conversion will be.  The process of refining conversions for the less common species is an ongoing process.  The more data we can gather on actual weight and lengths for these species, the more accurate the conversions will be.  It’s noteworthy that through the efforts of the South African Shore Angling Association, South Africa is far ahead of other countries in getting length measurements established as the basis for points allocation at the competitive level.  A world first developed under the auspices of SASAA!
Other factors that have a more obvious influence on the accuracy of the weights are the condition of the fish (sick vs healthy), and whether the fish just had a big meal (for example during the sardine run when fish gorge themselves with easily available prey!).
The last word on accuracy is that it really doesn’t matter all that much.  We need to make a mind-shift change from thinking in terms of weight to thinking in terms of length.  But it’s nice to know the estimated weight of your prized catch.  You probably won’t make the same impression on your non-angling friends around the braaivleis fire when you boast about your 209cm pre-caudal length raggie!  But you’re sure to impress when you quote the weight at 176kg !!
 
What to do if your fish’s actual weight differs significantly from the weight returned by the conversion?
If you do catch a fish that you are able to both measure and weigh, and the returned weight differs substantially from the actual weight, let us know!  The more data we have available, the more accurate the returned weights can be.  Tell us about it in an e-mail to This e-mail address is being protected from spambots. You need JavaScript enabled to view it