The Problem of Returning Deep Sea Fish

Fish Swim Bladder

The location of the swim bladder in a round fish.

In these days of immense pressure on fish stocks many shore anglers practice catch-and-release, only keeping a fish or two for the table and releasing other fish that they have caught to live and fight another day. In the case of bigger, rarer fish – such as skate, ray and small shark species such as smooth-hound and spurdog – many anglers are happy to take a picture or two of their catch and then release the fish to preserve (and hopefully increase) future stocks. Shore caught fish that have been lip hooked are easy to release, especially if the right sized hooks have been used. Even fish which swim off with a hook in their mouths still have a high chance of survival if they are released quickly. However, fish that have been dragged up from deeper water (25m/80ft or more) have a much lower chance of survival.

Swim Bladder

Rudd swim bladder

The swim bladder of a rudd, a species of freshwater fish.

The reason for this is that most fish have a swim bladder. This is an internal gas filled organ that allows the fish to control its depth. The fish can adjust the pressure in the swim bladder to ascend or descend to whichever depth it chooses. The development of the swim bladder is an evolutionary development which allows the fish to change depth with minimal effort. This saves the fish energy compared to having to constantly swim to the depth they want to be at. The problem comes when fish are caught at depth (either in a commercial bottom trawl net or by rod and line) and rapidly pulled up through the water levels. The fish has no chance to adjust to its new depth and the swim bladder ruptures or swells up massively. Fish with a blown swim bladder sometimes look fine, at other times they look horrific with the swim bladder coming out of their mouth and eyes literally bulging out of their sockets. In either case a fish in this condition will be unable to swim or dive and will be stuck at the surface of the water until it dies through starvation or is picked off by gulls or other predators.

‘Saving’ Fish with Blown Swim Bladders

Some anglers believe that fish with a blown swim bladder can be ‘saved’ by piercing the swim bladder with a needle or narrow bladed knife. This will shrink the swim bladder and allow the fish to dive again, although there is plenty of evidence that fish which have had this done to them soon die through injury or infection. Sometimes charter boats look like they have taken an excessive amount of fish, and people ask why they did not return some of them. The reason for this is that all of the fish they caught would have had blown swim bladders by the time they were on the boat and it is better to keep the fish than return them to the sea to die in a misguided attempt at conservation.

Trawl net

The vast majority of fish which have been dragged up from the depths by a commercial trawler will die, even if they are returned to the sea, as their swim bladder will be ruptured.

A few species of fish (notably sharks, skates, rays, mackerel and the humble sea scorpion) do not have swim bladders and therefore sink when they stop swimming. These fish can therefore only alter their depth by pure swimming effort. For this reason sharks and mackerel are always on the move looking for prey fish, while skates and ray are happy sink to the seabed and live and feed there. These species do not have the same problems with swim bladders becoming blown or ruptured and can therefore survive being returned to the water, even if they have been brought up from a great depth. However, other species – notably wrasse – have swim bladders which are extremely sensitive to depth changes, and can be killed by being brought up from relatively low depths.


As stated in the article on commercial fishing the EU’s insane rules mean that commercial vessels fishing in European waters have strict quotas of the amount of each species they can catch and any fish caught over quota have to be legally returned to the sea. The swim bladder issue means that these fish will inevitably die as their swim bladder will be ruptured, leading to a pointless and grotesque waste of perfectly good fish. Thankfully, this issues has received media attention in recent years (a lot of this down to the campaign of celebrity chef Hugh Fearnley-Whittingstall) and European countries have put forward initial plans to phase out discards over the forthcoming years.

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