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Fisheries Research Board photo by D. B. Quayle
This X-ray photo of wood infested with shipworm shows the shell material lining the burrows. Pairs of shells appear as dark crescents.
NEW HOPE IN OLD STRUGGLE
Board Tests Treatment To Protect Wood" Against Marine Borers
• This article by M. L. H. Thomas of the Fisheries Research Board's Ellerslie, PEl Biological Sub-Station was published last August by the St. Andrews, N.B., Biological Station as its general series circular No. 53, from which it is condensed.
By M. L. H. THOMAS
EVER SINCE man has been sailing the seas of the world in wooden vessels, shipworms and other borers have created problems.
D'Anghiera, writing in 1516 of the return of Christopher Columbus to Jamaica during his fourth voyage, stated:
"Their return to Jamaica, which is the island lying south and near to Cuba and His-paniola. was accomplished with great difficulty, for their ships had been so eaten by worms that they were like sieves and almost went to pieces during the voyage. The men saved themselves by working incessantly bailing out the water that rushed in through the great fissures in the ship's side and finally, exhausted by fatigue, they sue-
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THE FISHERMAN — JANUARY 17, 1969
ceeded in reaching Jamaica. Their ships sank ..."
This story has been repeated many times over and for hundreds of years men have been seeking methods to protect wood from marine borers. At the same time man has compounded the problem by distributing borers virtually throughout the world in the wooden bottoms of his ships.
Of course the problem is not confined to ships. Borers attack almost any wooden structure in the sea, including fishing equipment, piers, wharves.
Perhaps one of the most famous and serious situations to be caused by borers occurred in Holland about 1730. Much coastal land in Holland had been reclaimed from the sea and farms and even towns were below sea level and protected by dykes faced with wood.
Prior to 1730 no trouble with borers had been encountered, but from that time borer attack was so severe that dykes were destroyed and towns threatened   .. .
*    * *
ALTHOUGH  THEY RESEM-
ble "worms" in their elongated appearance, shipworms (Teredo navalis) are really a type of clam or technically a bivalve mollusk.
If a shipworm is carefully removed from its burrow in the wood, its two shells, which show that it is a clam, can be seen as small objects at the larger end. These altered shells have toothed edges and are used by the shipworm as jaws for boring through the wood.
In Malpeque Bay, PEI, where shipworms have been studied, they breed from June to September. Breeding occurs between water temperatures of 60 and 75 degrees and consequently in other locations the breeding season may be longer or shorter according to tempera^ ture.
Young shipworms, released into the water as tiny larvae, swim for a short period before settling on wooden surfaces; there they bore in using their shells. Growth following settlement may be very rapid. In Malpeque Bay they are sexually mature in about six weeks and produce at least two generations per year.
As the animal grows, it elongates the burrow into the wood and may attain a length
of over one foot. However, it always remains attached at the surface of the wood where it first entered. The burrow is lined with shell material and can be closed at the opening by two pallets.
Food is obtained both by filtering the water drawn in through the opening of the burrow and also from the excavated wood, which is eaten.
Shipworms can live in brackish water and can even survive
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in completely fresh water for a period by closing off their burrow.
They can also survive long periods of near freezing water temperatures and can also survive air exposure for many days —in cool places where the wood does not dry out . . .
* * *
THE RAVAGES BY THESE borers and the cost and inconvenience produced by the damage has spurred the search for effective methods of wood treatment. This search has employed countless people and encompassed several centuries.
The oldest method of protection was simply to dry wood in the hot sun at frequent intervals. Charring the surface of the wood was also an early method but it was not very effective. A coating of tar was also used very early. Some woods were found which resisted attack but none in ready supply.
The only really effective method of protecting wooden ships from attack was to sheath the hull with a hard material such as copper plates or to cover the entire bottom with broad headed nails.
In more recent times several protective coatings and paints have been developed.
Creosoting under pressure is perhaps the best of these because it penetrates deeply into the wood but it is generally only used on wood for fixed structures, such as wharves. Additionally, creosoting is fairly expensive and adds greatly to weight. Some marine borers in other areas attack creosoted wood.
Ordinary paints and other coatings do give effective protection as long as they remain intact.
Special anti-fouling paints rely for their protective action on poisons, usually compounds of the metals copper or mercury, which are incorporated into them. These poisons gradually dissolve into the water and the paint has to be replaced at intervals.
Copper paint is almost universally used on boats but has disadvantages. It must be still wet when put in the water, it is fairly soft and has a short life, often necessitating recoating in mid-summer.
All these methods, except pressure creosoting, have the disadvantage that damage to the coating will allow shipworms to enter and spread beneath the surface.
* * *
SEVERAL YEARS AGO WE
learned of a new and promising treatment for wood which was claimed to stop marine borers. The treatment uses a new poison based on the metal tin, which is quite deadly or repellent to marine borers. The   active   ingredient   is a
chemical whose technical name is bis itri-n-butyltin) oxide but which is usually referred to as TBTO. TBTO is a clear liquid which will not mix with water but mixes readily with mineral spirits or kerosene.
The resultant mixture is thin and will readily penetrate deeply into dry wood. This chemical has the additional important advantage that it is strongly attracted to wood fibres and becomes almost part of the wood and will only dissolve very slowly back into the water.
Experiments carried out at the Ellerslie Station over a three year period tested varying concentrations of the chemical in several solvents on blocks of various woods as well as on boats, stakes, oyster trays and lobster traps. We have also checked for possible harmful effects on oysters.
Our tests have shown that a two percent solution of TBTO in mineral spirits or kerosene, brushed or applied by dip to dry or almost dry wood will give excellent protection against borers for at least two years. Penetration is sufficient to give moderate protection even where mechanical damage such as scuffing or scraping occurs.
Since success of this treatment depends on the TBTO combining with the wood, it is ineffective if applied over paint or on wet wood.
Its important advantages are ease of application, length of protection, ready penetration into wood and relatively low cost. Although TBTO is relatively expensive, the fact that it is used at low concentration, covers a large area, and lasts longer than former treatments makes it quite inexpensive to use.
Tests using TBTO on wood in close contact with ovsters showed that the chemical does alter their normal growth pattern. It should, therefore, not be used on oyster trays or on wood that may be in contact with other edible shellfish.
New anti-fouling paints incorporating TBTO or related chemicals are now commercially available. Our test results with these paints show great promise and some important advantages over regular copper paint, the most important advantage of which is the production of a hard, smooth, durable surface which does not require frequent repainting . . .
• This article concludes with recommendations for use of preservative solutions containing TBTO and a list of manufacturers of wood preservatives containing TBTO. The only Canadian manufacturer on the list is Osmose Wood Preserving Company of Canada, Pratt Avenue, Montreal 8, Quebec, whose product is sold under the brand name Pentox X2205.
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