DESCRIPTION OF THE PEARL-OYSTER BANKS OF THE GULF OF MANAAE. 121 



(2.) Next in importance come, we consider, the ravages of natural enemies the 

 most destructive of which are : 



(a.) Voracious fishes chiefly Rays (Trygon uarnak) and File-fishes (Batistes 



mitis and B. .stcllatus) ; 

 (b.) Boring Molluscs chiefly Sistrum spectrum and Binaxia coronata, along 



with sjiecies of Nassa, Murex, Purpura and Turbinella ; 

 (c.) The boring sponge Ctione inclica; 

 (d.) Boring worms (Leucodore) ; 

 (<'.) Star-fishes chiefly Bentaceros and Luidea ; 

 ( f.) Smothering Mollusca such as Modiola barbata, the " Suran," which weaves 



nests and other entanglements around masses of young oysters, and may, 



when present in quantity, cause serious mortality. 



(3.) There are still three other causes of death that require mention, and may on 

 occasions be serious, perhaps disastrous, viz. : 



(a.) Overcrowding. The older are sometimes buried in masses of younger ones. 

 The young are often piled together in such profusion as to interfere with 

 each other's nutrition and growth. Thinning out must and does take place. 

 If it were done artificially all or nearly all might be preserved ; if we leave 

 it to be effected naturally by survival of the fittest the survivors may be 

 very few indeed ; 



(b.) Disease due to the invasion of parasites either (1) worm parasites, which 

 are moderately large and usually not very numerous, and which, unless 

 abnormally abundant, do little harm ; or (2) the more minute Protozoon 

 parasites, which may be present in enormous quantities and probably cause 

 epidemic diseases ; 



(c.) Overfishing. That is, the exhaustion of the breeding stock of the district at 

 a time when no further supplies of young in the larval stages were being 

 brought by currents from neighbouring grounds. This will comparatively 

 rarely happen, and is only likely to be serious during the last year of a 

 series of fisheries. So long as there are three and four-year old oysters on 

 adjoining paars which will be fished in the two succeeding years, it is safe to 

 take every older oyster that can be got off the ground, as those coming on, 

 although not yet ready to fish, are sexually mature and may be relied upon 

 to supply spat ; but in the final year of a series, when no further mature 

 oysters remain for future years, it is important to leave a sufficient stock for 

 breeding purposes. 



In the future, however, if transplanting is adopted, it may be expected 

 that such a state of affairs as the last fishery of a series with no younger 

 oysters growing up in the neighbourhood will be very unlikely to recur. 



R 



