620 U. S. BUREAU OF FISHEMES 



by Lieut. M. M. Nelson, commanding officer of the U. S. S. Whip- 

 poorwlll, who made astronomical observations and determined the 

 longitude and latitude of several points on the islands. Three high 

 signal towers built on the island were carried on the motor boat and 

 placed in shallow water in the middle of the lagoon. They were 

 supplied with platforms from which the location of pearl-oyster 

 beds, marked by buoys, was determined by means of compass and 

 range finder. 



Biological observations consisted of the following : Sounding ; tak- 

 ing of samples of bottom and water; temperature readings (top and 

 bottom) ; and determinations of the alkalinity of the water, of its 

 transparency, and of its food content. For the examination of pearl- 

 oyster beds three divers were employed, who were directed to swim 

 and dive for a period of time varying from half an hour to an 

 hour and a half. Because of the irregular distribution of pearl- 

 oyster beds the total number of the present oyster population of 

 the lagoon could not be determined. However, from the data 

 obtained during the investigation it is possible to determine the rela- 

 tive abundance of oysters; that is, the number of oysters found by 

 the average diver during a given period of time. 



Biological observations just described were carried out at 70 sta- 

 tions uniformly distributed over the whole area of the lagoon, and 

 from an anaylsis of the data the following conclusions can be 

 drawn : 



Pearl oysters grow at a depth of from 10 to 47 feet, attached 

 almost exclusively to live corals. The average weight of the adult 

 oyster is about 2 pounds, and the maximum weight is 7 pounds, 

 but at present those weighing over 3 pounds are rather scarce. All 

 the oyster reefs examined in the course of the investigation show 

 signs of depletion. On most of them, 3 divers were able to find in 

 one hour only 3 or 4 oysters. 



One-year-old oysters were almost entirely absent. This fact indi- 

 cates that either oysters failed to spawn and set in 1929 or that last 

 year's crop was destroyed by the fishermen. Because small oysters 

 have a tendency to attach themselves to the shells of the adult ones 

 the second explanation seems to be very probable. 



Pearl oysters spawn and set in Pearl and Hermes Lagoon in July 

 and August. There are two factors which are essential for a success- 

 ful spawning: High temperature of the water (27° C.) and abund- 

 ance of adult oysters on the reef. The latter factor is as important as 

 the first one. When oysters are scarce and single individuals are 

 scattered over the large area, the two sexes fail to stimulate each 

 other and chances for spawning and fertilization of their eggs are 

 very small. Thus, when because of overfishing the number of the 

 adult oysters on the reef is reduced to a certain low limit, the 

 oysters fail to spawn and their propagation ceases. The evidence 

 supporting this statement is found in the fact that no young oysters 

 (spat) were found on single individuals taken from badly depleted 

 reefs, whereas on the reefs that were not so badly overfished the 

 spat was abundant. 



Pearl oysters of Pearl and Hermes Reef produce valuable shells 

 (mother-of-pearl) and bear large numbers of pearls. It has been 

 estimated that since 1927, when pearl beds were discovered, not less 

 than 100 tons of shells (about 106,000 oysters) were taken from the 



