35 



water fish-culture should be made several times larger 

 than the sectional area of the stream they intercept, 

 in order to allow for loss through the clogging of 

 some of the meshes and still be able to pass the full 

 quantity of water without difficulty. 



The reed component in the tressa wall is the one 

 essential to the proper performance of this function ; the 

 other parts are designed to give necessary support. The 

 reeds are first tied into handy-sized bundles and then 

 built up around the mouth of the covola or passage into 

 a very tighty compacted palisade six inches in thick- 

 ness. The bundles are made sufficiently long to admit 

 of being firmly imbedded at the lower ends in the mud, 

 while the upper project 2 to 2\ feet above the normal 

 level of the lagoon. The method of supporting these 

 bundles by a frame work of upright stakes and hori- 

 zontal bars may readily be understood by examination of 

 plates V and VI, figures 7 to 10. As seen in these photo- 

 graphs these reed palisades vary in thickness and in the 

 strength of their supporting frames according to the 

 duties they have to perform ; in the majority the reed 

 hedge is staked on both sides (fig. 10, pi. VI), in others 

 a single lateral row of supports suffices. 



When first built a tressa by its peculiar form offers a 

 very large percolation surface and the sea-fiood welling 

 through the feeder canals passes rapidly through the 

 reed hedge into the lagoon. Unfortunately the spores 

 of algae and the swimming larvae of various zoophytes 

 and other sedentary marine animals are likewise borne 

 up by the tide and settle upon the palisade ; here they 

 find conditions extremely favourable for their growth 

 and in a short time they fill the interstices so completely 

 that the walls of the tressa become all but impermeable ; 

 the volume of water passing through becomes so reduced 

 that it may be said to have no appreciable influence 

 upon the salinity of the lagoon thereby contributing 

 largely to the gravest danger to which Comacchio valli- 

 culture is exposed as we shall presently — (see p. 44). 



When once the tresse have been built and the gaps 

 in the embankments repaired, work becomes restricted 

 to surveillance and the cutting and preparation of the 

 reeds and other materials that will be necessary when 

 the fishing season begins in autumn. 

 3-a 



