DISTRIBUTION 



and shallow water plankton different from that over deep water. A buoy 

 marking a channel, if placed in the spring of the year, will provide a settle- 

 ment area for the spring larvae of mussels, and have a luxuriant crop of small 

 mussels a year later, and quite big ones in two years' time. If it is not placed 

 until the end of June it will be too late and get no mussels until the following 

 spring and after two years will only have the equivalent mussel crop of the 

 earlier buoy after one year. 



It is, too, this dispersal of free living planktonic larvae in the sea that is 

 responsible for ships' fouling, an expensive nuisance in time and money 

 (Plate XXXIII). Increased drag means slower speeds and increased fuel costs, 

 and delays are necessary for dry docking and scraping. Special anti-fouling 

 paints have to be used — a double expense as these paints tend to cause corro- 

 sion if applied directly to the hull of a ship and a coat of normal paint has to 

 be applied first. Anti-fouling paints usually contain copper, as red cuprous 

 oxide, and this is why most ships' hulls are painted red below the water line. 

 But whatever treatment is used, new methods of cathodic protection, 

 paint or scraping, it is expensive and the cause of the expense lies in the 

 plankton. 



The severity of fouling depends on a number of factors including the 

 ships' routine movements. Sudden changes of salinity on passing between 

 fresh water and the sea kill off some species; others arc strongly resistant to a 

 quick change but cannot feed in fresh water and so die if the ship stays in 

 river water for considerable periods. Scouring action in rivers where there is 

 a strong tide carrying sand has a sandblasting effect if sufficiently prolonged. 

 Ships normally travelling at a good speed at sea between river berths are less 

 severely affected than those which have periods of immobility in salt water 

 harbours. Ships' routine movements make a distinct difference to the type of 

 fouling as well as its severity. For example, ships that spend months in warm 

 waters get a different fauna from those that stay in the higher latitudes. It 

 depends on where the ship happens to be when the larvae of the fouling 

 organisms are in the planktonic stage and whether her movements take her to 

 places where the conditions are suitable for their growth and survival. 



To illustrate this point the hulls of two ships being scraped in a dry dock 

 at Liverpool can be compared. Both ships spent their whole time in the Liver- 

 pool area; one was a tug which spent much of its time in and out of the docks 

 and in the Mersey estuary, the other was a hopper that spent most of its time 

 within the extensive Liverpool dock area, rarely being taken into the river. 

 The hull of the hopper w^as luxuriantly coated with masses of large sea 

 squirts, animals which feed by filtering small planktonic organisms from the 

 water, but the tug had no sea squirts at all. In the quiet waters ot the docks 

 the worst of the mud and silt tends to settle, and the sea squirts lived on the 



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