Posts Tagged ‘Fish buoyancy’

Thoughts on Hugh’s Fish Fight

February 1, 2011

I have just signed Hugh Fearnley-Whittingstall’s petition to stop dead by-catch being thrown back into the sea. But pausing to think about this part of the catch, I realize that there are actually many problems associated with removing it from the marine environment.

Of course the real answer is not to take by-catch; nor to over fill the nets when bringing them to the surface. Having been out on a university research trawler in the 1990s, I am fully aware that the sonar scans and computer information from other boats  mean that the species of fish in a shoal, and its precise size, is known as the trawler approaches the region; and thus well in advance of the net being released in the first place.  Bottom trawls are a very different ‘kettle of fish’.

The number of fish that will die depends on several factors, amongst which, the way that they control buoyancy is probably the most important (see “Buoyancy Matters” end of this note).  Fish that cannot, or are slow to dive are the ones that become prey to the wake hugging gulls.  Those that have lost the ability to control their buoyancy, gradually sink through the water column, becoming dinner for scavengers on their journey to the bottom.  Other fish, krill, copepods and indeed bacteria and algae would normally expect to benefit from the extra nutrients that are thus released into the marine ecosystem – after all most marine organisms live and die in the sea.  If the by-catch came from the sea floor, then providing that they are still alive and don’t get caught by a gull, they don’t usually have swim bladders and will slowly drift down, be reunited with their food source and will survive the experience; though even for them the sudden pressure change may be a problem.  But normally, the ecosystem in which trawled fish live, is sustained by a rich mixed food source that contains some photosynthetic organisms and so moves up and down within the surface layers, stimulated by light and dark at the surface.  For these pelagic fish, it is the speed with which the trawl comes up to the surface and is hauled aboard that is the problem.  If it happened slowly enough most swimbladders could adjust, but too fast and they burst.  They can also become disconcerted when released to the surface and this may help to explain their slowness in swimming back down to their home level.  From the gulls’ point of view, the slower the better; as this allows them to attack the fish in the bag.  From the fishermen’s point of view, the faster the better; as this allows them to get the whole catch on board in an undamaged state – the customers tend to dislike battered fish with speared chunks taken out of their sides!  Arguably the seabirds do not rely on an abundance of fish deposited on the surface by a boat.  This happens naturally when prey species are hunted by faster hunters e.g. tuna who drive the shoals to the surface and coincidentally into the beaks of the gulls.  The only real problem here is that, without the trawler, these fish would probably have lived to see another day and even spawn, before becoming dinner for other marine inhabitants.

Hugh Fearnley-Whittingstall is correct in that it is daft to throw dead fish/sea creatures back into the sea, in terms of human food.  However, if one brings the by-catch ashore, then another very real problem springs to mind, concerning what it is right to do with it i.e.  If the fishermen land this extra catch and sell it, then they are benefitting from the fish that they should not have caught in the first place!  With our huge population, it is difficult enough as it is to maintain fish stocks with enough adult fish, of mature size, to optimise breeding.  I fear that if the fishermen find that they can actually earn an income from the extra catch, then they will do so – after all the human population is now so huge that all that they catch will be eaten.  Unfortunately, the likely result is that even more species of fish and marine organisms will end up on the critical list.

Perhaps the answer is to ensure that any extra catch is taken into custody on the dock and distributed to local fishmongers for the cost of the distribution exercise only.  That would actually have the benefit of providing more jobs for humans, but then of course they would have to be paid out of the sale of the by-catch, with the result that they too would be hoping that the by-catch was as large as possible.  If the fishermen are fined, then they will simply dump the catch at sea as before!

It’s a difficult question.

Buoyancy Matters: Fish in general have a specific gravity of 1.07 and so will tend to sink in water, unless they keep swimming.  This is no problem for predator fish such as mackerel, sharks and indeed rays; but for those that feed in the plankton, neutral buoyancy is an advantage meaning they can stay at the same level as the plankton without having to expend extra energy on maintaining their level in the water column – plankton move up and down according to the light levels.  This neutral buoyancy is achieved by the presence of a swimbladder (air filled bladder) whose pressure can be adjusted to suit the feeding requirements.  Most young fish can adjust the volume of their swim bladders by actually swallowing air from the surface, and in some species this ability is retained in adulthood.  However, in many species of adult fish this duct closes and the volume is adjusted by the removal of gases from the blood.  Since most fish can survive out of water for a short while, provided that they are kept wet and cool; most are returned to the sea alive.  However, whether they will survive or not, depends on the factors mentioned above; plus whether the benthic species can survive the sudden decompression from several atmospheres up to the surface at an air pressure of only 1 atmosphere.  Some fish with swimbladders have been found at depths of over 4000m depth.  For each 10 metres of depth, the pressure increases by 1 atmosphere; so at a feeding depth of 100 metres the water exerts a pressure of 10 atmospheres.  This means that when these pelagic fish surface, the gas in a closed swimbladder expands 10 times and will definitely burst the organ before it reaches the surface.  Even a swimbladder that is connected to the mouth relies on the fish being able to burp out gas fast enough to avoid bursting the swimbladder.  Non-ducted swimbladders are filled with gas from the blood, which implies that their adjustment is normally slight/slow; and big changes in the volume of gas required would take a long time to achieve.

Once on the surface, another problem arises as the fish has to refill its swim bladder adjusting the fill to the increasing pressure as it moves back down.  If only the surface air were available, then obviously as the fish swam down this would be compressed leaving a very deflated swimbladder.  In turn this could cause displacement and abnormal compaction of the internal organs.

Apart from these problems, the swim bladder of some species contains the normal gas mixture of nitrogen, oxygen and carbon dioxide in ratios that differ not only from the air, but also from those in the water at their living depth.  Thus the ratio of nitrogen in a cod’s swimbladder may vary from 7.6% to 56% with associated percentages of oxygen of 76.6% and 30.1%; and carbon dioxide of 15.8% and 13.9%.  All this is managed by a specialized rete mirabile type structure of the supplying blood capillaries – a similar structure is used to cool the blood going to the feet of penguins and to warm the returning blood so that the core temperature is not affected unduly.  Nature is very good at re-using good ideas.

 

Advertisements