Figure 20.— Take of wool sponges per unit of effort, 1935-56. 



This fishing effort was more than the spong- 

 ing area could bear. In the period 1941-46 we 

 find that the size of the sponging fleet increased 

 22 percent. If it is assumed that the number of 

 sponges on the bars remained the same, it can 

 be calculated that the return per unit of effort 

 would drop by 18 percent. In actual fact the 

 return per unit of effort dropped by 20 percent 

 (by weight). When it is considered that during 

 this time more and more smaller sponges were 

 being harvested before they could produce eggs, 

 the fishing intensity would probably have 

 severely damaged the sponging grounds. The 

 sudden drop in price immediately after the war 

 in 1946 and the disease of 1947 brought disaster 

 to the industry instead. 



It may be assumed from the above discussion 

 that from 1941 to 1946 the number of sponges 

 on the bottom was the same or nearly so at all 

 times. It is possible, therefore, to construct a 

 graph to represent the return per unit of effort 

 for varying sizes of a sponge fleet when this 

 level of concentration of sponges is found. This 

 has been done in figure 21, line A, and the line 

 interpolated to give the return per unit of effort 



if the sponge fleet in terms of diving boat 

 equivalents were 68. Sixty-eight boats prob- 

 ably represent the average size of the fleet 

 before 1938. Line A thus represents the ap- 

 proximate level of return per unit of effort that 

 is taking place when the concentration of 

 sponges on the sea bottom is below replace- 

 ment level. 



Lines C, D, and E in figure 21 represent the 

 low, average, and high returns per unit of effort 

 found before 1938. Only the solid part of the 

 line is supported by data, the dashed portions 

 being extrapolated. The low return per unit of 

 effort before 1938, line C, may be interpreted 

 as being the lowest possible level of return per 

 unit of effort, and by extension, of the concen- 

 tration of sponges on the bottom which will 

 allow rapid recovery of the sponging beds. Any 

 return per unit of effort, and concentration of 

 sponges on the bottom, below that represented 

 by line D would be less than the desired level. 

 At this level of concentration and a 68- boat 

 fleet, each boat would be taking about 5,100 

 pounds of sponges per year, this representing 

 a concentration of 2.8 pounds per acre (5,100 



62 



