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Dealing with Gassed Up Fish

Sep 19,2006 SACN

From time to time SACN gets requests from anglers asking about the possibility of returning fish bought up from depth where the swim bladder appears to be 'blown'.

Usually such fish cannot get back below the surface and are taken by gulls.

Sometimes the stomach is protruding from the mouth, often mistaken for the swim-bladder, but if the stomach is pierced, the fish will die.

If a fish needs to be vented, this article shows how it should be done:

See http://isurus.mote.org/research/cfe/fish-bio/how-to-vent-a-fish.htm

(Includes both illustrations and a video)


The following is correspondance that we have received from CEFAS: 

The Centre for Environment,Fisheries & Aquaculture Science
The Centre for Environment, Fisheries & Aquaculture Science Lowestoft
Laboratory, Pakefield Road,
Lowestoft, Suffolk NR33 ORT UK
Tel: 44 (0) 1502 562244
Direct: 44 (0) 1502 524359
Fax: 44 (0) 1502 513865
Email: d.righton@cefas.co.uk

Thank you for your enquiry regarding our cod tagging programme.

Our electronic tagging programme began in March 1999, and we have released 380 cod in a number of locations in the North Sea since then, with plans to release 250 to 300 more over the next couple of years.

To date we have had 108 tags returned to the lab.

Our tagging procedures are very strict.

All of our tagging staff ('taggers') are trained and qualified to tag fish, and do so under a personal licence from the UK Home Office.

We do not tag cod unless they are in excellent condition after capture, and all fish are handled extremely carefully during their time on the tagging vessel.

We ensure that contact between the taggers and the fish is minimal, that taggers wear latex gloves for when the fish needs to be handled during tag surgery, and that fish are brought aboard and released from the tagging vessel with great care.

Fish are returned to the location of capture within 2 hours of being caught, and are never out of the water for more than a couple of minutes.

The biggest problem with cod caught in water deeper than ten metres is the expansion of the swimbladder, a gas filled organ responsible for buoyancy control in cod.

Cod control the volume of gas in the swimbladder by either secreting gas into it through a special gland, or absorbing gas into the blood stream.

Absorbtion occurs relatively quickly.

The size of this organ is pressure dependent, and as an example, a fish brought to the surface (1 atmosphere of pressure) very quickly from ten metres (2 atmospheres of pressure) will experience a reduction in pressure of 50% and the swimbladder will expand.

Each 10 metres depth of water exerts a pressure of 1 atmosphere, so a fish at 40 metres will be under a pressure of 5 atmospheres, at 6 atmospheres at 50 metres and so on.

With respect to the seriousness of the expansion of the swimbladder, any water deeper than 30 metres can be considered 'deep'.

Two things can happen to the swimbladder when it expands.

First, it can remain intact and exert outward pressure on the internal organs and the body wall.

These organs are compressible to some extent, and the body wall can expand outwards, but if the increase in size of the swimbladder is very large the stomach can be forced out through the mouth.

I'm sure you have seen this phenomenon.

Alternatively, small holes can form in the swimbladder wall (these repair within a few hours or days), allowing gas to escape into the body cavity.

This gas can often escape from the body cavity, but in some cases will remain inside and prevent the fish from exhibiting normal posture and buoyancy.

This gas can be vented to the atmosphere with minor surgery.

Rod and line caught cod can usually be brought to the surface in good condition, unless the fish has been foul hooked in which case the chances of survival are very slim, and can therefore be returned to the water with a good chance of survival.

We bring cod to the surface at as slow a rate as possible (several minutes in water> 40 metres) to minimise the effort expended by the fish when it fights against the line and to maximise the amount of swimbladder gas that the fish can absorb as its depth is decreasing.

This therefore minimises the increase in size of the swimbladder.

On arrival at the surface, all fish are checked for evidence of buoyancy failure and swimbladder expansion by placing them immediately into a large holding tank.

Those requiring' deflation' are operated on and returned to the water, as they are unsuitable for tagging.

Of the remaining fish, those in the best condition are tagged and returned carefully to the sea.

The longer the cod spends on the tagging vessel prior to
tagging, the more closely it has adjusted its buoyancy to atmospheric pressure because it will absorbs excess gas from the swimbladder into its bloodstream all the time.

On release therefore, the cod will find itself much less buoyant than when it was caught.

Cod therefore often show a characteristic post-tagging behaviour related to re-inflation of their swimbladders (see chart).

This is characterised by a declining average daily depth and occasional rapid ascents or descents that may be interpreted as the fish 'exploring' the limits of its buoyancy.

After a few days, cod have usually regained the correct buoyancy for their capture depth, and then behave normally once more.

CEF AS have had great success in tagging cod from water as deep as 90 metres, but we collaborate with fisheries institutes in Norway and Iceland where cod captured at more than 200 metres have been successfully tagged and subsequently returned to scientists many months later.

You asked what were the chances of a cod caught in deep water surviving on release, so that anglers could judge the worth of releasing surplus deep water cod.

Cod are very robust fish and our data show that after the initial post-tagging behaviour, tagged cod have lived perfectly normal (and we hope happy!) lives, and have migrated to feeding and spawning grounds as they would be expected to.

This is borne out by the increase in weight and length offish (when we receive these details).

When we have the opportunity to talk to fishermen who have returned cod tags, we invariably hear that the fish were in good condition when captured.

I would therefore be optimistic that anglers could return cod to the water provided they were not foul hooked, they were brought to the surface relatively slowly, they were handled minimally and with care, and they were returned to the sea as quickly as possible.

I'm afraid I cannot comment on the chances of survival of other species.

David Righton

Fish Behaviour Team Leader
Fisheries Biology Section
CEFAS Lowestoft


Article that originally appeared at http://www.seafoodintelligence.com/

9 August 2005

Cod has an 'incredible ability' for fixing its swim bladder if it swells & cracks

A recent study by Norwegian scientists has shown that cod shows 'an incredible ability for restoration' when and if its swim bladder swells and cracks when the groundfish is brought back to surface quickly.

This has implications both in animal welfare terms and for the wild harvest of cod destined to be 'ranched' or farmed.

For the first time, tests have shown how the cod itself repairs the damages when its swim bladder cracks.

"The cod has an incredible ability for restoration", says Senior Scientist Kjell Midling at Fiskeriforskning, the Norwegian Institute of Fisheries and Aquaculture Research.

The difference between the pressure down in the sea and the surface causes the swim bladder to swell and crack before the cod is taken on board the fishing vessel.

A new study carried out by Veterinary Surgeon Christian Koren and Scientist Kjell Midling at Fiskeriforskning shows that the cod itself is capable of fixing the damages that occur.

On the inside of the gas bladder is a thin elastic membrane.

This membrane lies close to the bladder until this cracks.

When the gas bladder then collapses, this thin membrane slides such that the hole in the bladder is blocked.

The cod can then refill the bladder without the air seeping out.

"With that, the cod has a new bladder that functions immediately after the normal swim bladder has cracked, until the wound in the bladder wall has grown.

Almost like a self-repairing car tyre," says Midling.

He continues, "We have acquired more knowledge about the cod's biology and ability to cope with the handling during catching and live storage.

This knowledge is useful in connection with catch-based aquaculture and how this industry can be responsibly developed also with consideration to the welfare of the fish."


Article that originally appeared at:


Deflating fish improves survival, Fla. researchers say

By Susan Cocking
McClatchy Newspapers

MIAMI - A 15-year study by researchers at Mote Marine Laboratory in Sarasota, Fla., shows the practice of venting, or deflating a bloated fish, improves survival when the fish is released back to the water.

From 1990 through 2005, scientists and volunteer anglers, guides and commercial fishers tagged more than 36,000 reef fish - including red, vermilion, mutton, and mangrove snapper; red, gag, scamp and black grouper; amberjack; and cobia - on both the east and west coasts of Florida. Recapture percentages ranged from less than 1 percent for vermilions to more than 12 percent for cobia.

Some of the fish, when reeled up quickly from deep water, suffered from distended swim bladders, which pushed the stomach out the mouth and forced the intestines out the anus.

Without buoyancy control, these fish would be unable to swim back down to depth - a serious problem, given laws requiring undersized fish to be released alive.

But the researchers were able to vent the swim bladder by using a sharp-pointed tool with a hole in the shaft inserted behind the pectoral fin to release built-up gases.

Karen Burns of the Center for Fisheries Enhancement at Mote said venting helped many fish to return to the bottom to fight another day.

"Venting is useful for most species," Burns said.

"It doesn't appear to damage them physically, or with bacterial or viral infections.

Venting is really helpful below 70 feet.

Less than 70 feet, we don't need to vent.

After 100 feet, it is helpful to all fish."

Burns said some fish died after being vented because they were eaten by predators, and others because of temperature shock from being released into surface waters much warmer than the depths from where they came.

An alternative to venting, Burns said, is to use a deep-release rig favored by Jim Bohnsack of the NOAA Fisheries lab in Miami.

Tie the fishing line to the bend of a barbless hook, then tie a heavy sinker to the eye of the hook.

The barbless hook is then run through the fish's upper jaw.

Free-spool the fish down to the desired depth, then lift the rod sharply to dislodge the hook.

The rig, Burns said, should be prepared in advance to avoid keeping the fish out of water for too long.  


But this article suggests that venting can be counter-productive

 http://www.biol.ttu.edu/faculty/gwilde/Shared Documents/Reprints/WildeFisheries2009.pdf


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