Goldfish, anchovies, and charr are physostomes, that is, their swimbladders have an opening into their 

 stomachs. Hence, based on consideration of equation 11-115, it is to be expected that the tension in 

 physostome swimbladders would be relatively low. The data show that this is so. The other fish are 

 physoclists, that is, their swimbladders are completely closed. These fish may well obtain some benefit, either 

 hydrostatic or otherwise, by maintaining a tension in the swimbladder wall. The data indicate that cod, ling, 

 pollack, and coalfish maintained relatively high values of s. Physiologically, it should be possible for a 

 physoclist to vary the tension in the swimbladder wall from very taut to flaccid. It appears that the crappie 

 swimbladder was in a flaccid condition. 



Since physoclists probably can vary the tension in their swimbladders, any shock to their systems, such as 

 a rapid increase in pressure, might cause the swimbladder tension to change dramatically. This seems to be 

 the situation for the cod examined by Sand and Hawkins. This fish had been allowed to become adapted to a 

 depth of 1 1 m for at least 48 hours. At this depth, its measured resonant frequency was several times higher 

 than that of a bubble of the same presumed size and shape. This difference was attributed to a high 

 swimbladder tension. Rapidly shifting this fish to a depth of 25 m significantly decreased the measured resonant 

 frequency to a value only about 1 percent greater than that expected for a bubble of the same presumed size and 

 shape. This ratio remained essentially constant as the fish was shifted in 5 m increments to 50 m. The 

 decrease in a) 0m as the fish was shifted from 11 m to 25 m is readily explained by the new model by a 

 sudden decrease in s, which seems quite feasible from a physiological standpoint. 



Thus, the variations in the values of s calculated using the new model can be explained by the 

 differences between physostomes and physoclists and by the ability of physoclists to vary swimbladder 

 tension. Hence, the new model is an improvement over the models of Andreeva and Lebedeva. By using a 

 variable swimbladder tension, rather than a fixed shear modulus, to model tissue stiffness, the new model 

 can predict resonant frequencies significantly higher than those expected for a free bubble, whereas the 

 other models cannot. 



As a practical matter, a parameter that can be randomly varied over several orders of magnitude is not 

 very useful in a predictive model. However, the variations in tension indicated in Table II were obtained from 

 fish which were subjected to other than natural conditions, such as rapid changes in depth. It is quite 

 possible that, under more natural conditions, values of swimbladder tension would be much more uniform. 

 Thus, it could very well be that further experiments, in which the experimental conditions more closely 

 approximate natural conditions, could provide much information about swimbladder tension. This would 

 enhance the value of the new model as a predictor of resonance in large physoclists. 



Although the variations in s can be adequately explained by variations in fish physiology, the uncertain 

 variation of a with depth definitely causes some degree of variation in s. In order to calculate s, the difference 

 between u) 0m and co for a bubble of the same assumed size and shape is required. Thus, the accuracy of the 

 estimate of a greatly affects the accuracy of the calculated value of s. 



Sundnes and Sand measured the resonant frequencies of charr in order to determine their swimbladder 

 volumes. In the case of these physostomes, this was a valid procedure. However, in the case of large 

 physoclists, where swimbladder tension affects a) 0m , this procedure would be invalid. Hence, experiments 

 which might be designated to acoustically examine swimbladder tension require an accurate and 

 independent measurement of swimbladder volume at depth. 



The values of ^ given in Table II are also grouped on the basis of the magnitude of swimbladder tension. 

 For the cases where 



s < 10 5 dyne/cm, 



the values of ^ range from 1 30 to 300 poise, with the values being quite consistent for a particular set of data. 

 For the cases where 



s > 2 x 10 5 dyne/cm, 

 the values of ^ range from 370 to 2,600 poise, with wide variations within a particular set of data. Based on the 

 consistent results obtained, and the reasonableness of their values, it appears that the new model can be 

 used to predict the damping for the first group. However, the high values and variability of ^ in the second 

 group must be examined more closely before any conclusion can be reached. 



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