biologically, as shown in the section on repro- 

 duction of sponges. It is necessary for each 

 sponge diving crew to gather from 125 to 150 

 sponges per day of effort, or 12,500 to 15,000 

 per year. This number gives only the minimum 

 return required for operation of the boat, with 

 the shares to the members of the crew provid- 

 ing only the minimum for existence. 



Several methods have been tested to esti- 

 mate the average density of the sponges on the 

 rock bars. The best and most practical method 

 found is a calculation based on the area cov- 

 ered daily by the diver, the total number of 

 days of work annually, and the average number 

 of sponges taken by each diving boat. It has 

 been estimated that the diver walks at an aver- 

 age rate of one-half mile per hour and can see 

 about 15 feet on either side. The rate of walk- 

 ing varies greatly according to the concentra- 

 tion of the sponges, slowing down when the 

 sponges are numerous and increasing where 

 the sponges are scarce. Underwater visibility 

 also varies from day to day. 



The two divers aboard each boat work under- 

 water for a total of 10 hours per day out of the 

 12-hour working day. Poor weather conditions, 

 travel time, and time lost in port reduce the 

 number of working days to 100 per year. Multi- 

 plying the distance in feet traveled by the divers 

 in 1 day by the width of the area searched and 

 dividing this total area by 43, 560 (the number of 

 square feet in 1 acre) provides a calculation 

 of the area covered by the divers in 1 day. The 

 area is about 18 acres. At this rate, the area 

 covered in 1 year by the divers of one boat 

 would be 1,800 acres or 2.8 square miles. The 

 1957 fleet of seven diving boats would there- 

 fore cover some 19.6 square miles per year. 



The total sponging area being used by the 

 divers may be estimated as follows: 



1. At the present time the mean diameter of 

 the wool sponges being taken by the 

 divers is 7 inches. Since the smallest 

 legal size is 5 inches, the sponges taken 

 have to range between 5 and 9 or more 

 inches to obtain this 7-inch average. 



2. The growth rate above a 5-inch size is 

 approximately 1 inch per year, and 

 therefore it will be necessary to exploit 



at least part of the total sponging area only 

 once every 5 to 6 years to obtain sufficient 

 numbers of the larger sponges to main- 

 tain the 7-inch average size. As a basis 

 for making a calculation of the total area 

 being worked by the divers, the assump- 

 tion is made that all the sponge-producing 

 bars are worked at least once in every 

 5-year period. 



3. To search all the sponge-producing area 

 once in a 5-year period requires that the 

 total area covered would be three times 

 that covered in any 1 year, and this would 

 represent the total sponging area avail- 

 able and being worked by the diving boats 

 at the present time. This total area would 

 therefore be 59 square miles in extent. 

 All factors considered, this appears to 

 represent a minimum estimate. 



Lack of adequate data makes it Impossible 

 to estimate the area covered by the sponge 

 hookers. Since the average size of wool sponge 

 being taken by the hookers is about 6 inches in 

 diameter, it appears that the available shallow- 

 water hooking area is being covered far more 

 thoroughly than the bottom below the 20-foot 

 depth. Using a total of eight diving- boat equiva- 

 lents' for the number of hooking boats working 

 and the same process of calculation, the area 

 being worked by the hookers is less than 40 

 square miles in extent. 



Average daily take has been estimated by the 

 spongers for each area (see fig. 15 for areas) 

 and is given in table 4 along with the calculated 

 concentration of the legal- sized sponges per 

 acre, based on the above calculations. This 

 concentration agrees very closely with that 

 observed during the present investigation of 

 the sponge beds. 



Based on several counts of the number of 

 sponges on a bar and the measurements of the 

 sponges, there are on the average three sponges 

 less than 5 inches in diameter to each one more 



' Diving-boat equivalents are calculated in section on 

 "present status of the sponge industry." 



26 



