# aquaculture fish counting

in fish farming the fish are fed at a percentage of their weight. i measured the weight of the fish that i put into the tank, and i can remove the fish once in a while to reweigh them but that is very stressful to them. i would like to insert a laser into my fish tank, that can count the amount of time that the beam is interrupted by a fish- kind of like the lasers that are at the entrances to stores. i think there should be a strong correlation between the amount of time that the laser beam is covered and the weight of the fish in the tank. can someone help me out of what kind of laser to use? i read in another post (Underwater distance measurement/Sensing) that a 405 nm laser is good for underwater, but i don't need to know distance, just time the beam is interupted. i plan to run the whole thing with arduino thanks, tom.

• This isn't exactly a trivial problem. For this to be effective, you're going to have to find some mechanical means to make sure the fish swim through an area single file, block the laser with their body the entire time, and swim at a constant velocity every time. Further, do you have any data that shows their is strong correlation between length and weight? Commented Mar 21, 2015 at 17:57
• @MattYoung From what I'm reading, Tom is building an experiment to collect such data. Commented Mar 21, 2015 at 20:25
• hi, i weigh the fish periodically, once a month. the heavier the fish the bigger the fish- they're all the same type, so the body shape to weight ratio stays the same, if i measure long enough i assume i'll get a pretty linear relationship. the tank is 1m3, they won't have to be in line.
– tom
Commented Mar 22, 2015 at 18:31
• Could you estimate this through displacement? You know how much water you start with. Add fish and measure. Add water with a flow meter so you know how much new water you added. A control tank could give you an idea of your evaporation rate. Just a passing thought Commented Mar 24, 2015 at 0:24
• The water displacement is measured but is a measure of the evapotranspriration from the plants growing in the system
– tom
Commented Mar 25, 2015 at 5:25