returning fish

[jack]

hi iv been told not all fish can be returned from deep marks why is this and what is classed as a safe depth to return them

[greysfanboy]

It's because they have adapted their bodies to the pressure deep down, so when they are caught and exposed to atmospheric pressure it's unnatural. You will often notice that their eyes have slightly swollen, but the main problem is in the swim bladder. Some fish like pout are hard to return alive, regardless if it's deep water or not. If you think it might die, then keep it for bait or to eat.

It affects species like pollack in particular but any fish brought up can suffer. Some anglers have claimed success at releasing the air from the swim bladder with a hypodermic needle before releasing them.

[vifferrider]

I had wondered this myself.

greysfanboy has given a logical answer , as it sounds a bit like the bends but in reverse. If the fish is use to the high pressures exerted by the sea

Even though we do not feel it, 14.7 pounds per square inch (psi), or 1kg per square cm, of pressure are pushing down on our bodies as we rest at sea level. Our body compensates for this weight by pushing out with the same force.

Since water is much heavier than air, this pressure increases as we venture into the water. For every 33 feet down we travel, one more atmosphere (14.7 psi) pushes down on us. For example, at 66 feet, the pressure equals 44.1 psi, and at 99 feet, the pressure equals 58.8 psi.

To travel into this high-pressure environment we have to make some adjustments. Humans can travel three or four atmospheres and be OK. To go farther, submarines are needed.

Animals that live in this watery environment undergo large pressure changes in short amounts of time. Sperm whales make hour-long dives 7,380 feet (2,250 meters) down. This is a pressure change of more than 223 atmospheres!

I personally would not be using a hypo on fish, as there is no telling if this does really work, whats to say that if they get to depth the bladder does not rupture due to the hole and extreme pressure?

[jack]

thanks all

[squidlips]

if you immediately gut a fish with the supposed swim bladder hanging from its mouth you will see that it is actually the stomach that has turned inside out,that is what you are seeing hanging from their mouth.
you will see the swim bladder is still in place attatched to the gut cavity near the back bone.
so if you use a needle to puncture the thing hanging from the mouth you are putting a hole in its stomach.
also if you reel fish up from deep water slowly sometimes they are ok to go back,but whether they survive i dont know
hope this helps
karl