Edwards Duration of unassisted swimming by Stenella atlenuota 



133 



capacities of younger calves, particularly neonates and 

 infants, compared to adults appears to be a reasonable 

 factor contributing to mother-calf separation during or 

 after sets by tuna vessels in the ETP. 



Management and research implications 



From these observations, it appears that energetic limi- 

 tations may contribute significantly to the observed 

 calf deficit in the retrieved purse-seine by facilitating 

 mother-calf separation during purse-seine set activ- 

 ity. Separation risk appears to increase strongly with 

 decreasing calf size and with increased speed and dura- 

 tion of evasion-related swimming. If separation is pro- 

 longed, subsequent mortality of milk-dependent calves 

 appears likely due to predation or starvation because 

 adoption by a surrogate mother is unlikely in the ETP 

 (EdwardsM. Examination of existing aerial photographs 

 of spotted dolphins attempting to evade helicopters and 

 research vessels, as well as directed future experiments 

 in which aerial observations are conducted specifically to 

 identify and monitor mother-calf pairs over time during 

 various chase scenarios, would be very helpful in evalu- 

 ating the extent to which calf size effects predicted here 

 occur during relatively high-speed evasion behaviors by 

 ETP spotted dolphin schools. Estimation of the rate at 

 which dolphins of various ages are likely to encounter 

 tuna purse-seine sets will be another important factor 

 in relating the results of this analysis to potential calf 

 loss during purse-seine sets. 



Assuming that power limitations are a significant 

 factor in calf loss during evasion of tuna purse-seine 

 sets, management strategies that could be implemented 

 to minimize calf loss include 1) avoiding setting on 

 schools that contain calves, particularly young calves, 

 2) minimizing the speed or duration of chase, and 3) 

 minimizing the length of time dolphins are retained in 

 the net so that mothers and calves separated during 

 the set may reunite more quickly, if possible. Minimiz- 

 ing the length of time in the net is already a desired 

 fishery goal, related to minimizing cost and the poten- 

 tial for dolphin mortality in the net. Minimizing chase 

 duration is also already a desired fishery goal, because 

 shorter chases are both less expensive and tend to be 

 more successful. Minimizing chase speed is not a re- 

 alistic goal because it would likely lead to the escape 

 of the dolphins and the targeted tuna. Thus, the only 

 potential improvement that might be made under cur- 

 rent conditions (i.e., while setting on dolphin calves is 

 permitted) would be to concentrate effort on identifying 

 and avoiding dolphin schools with calves, presumably by 

 scrutinizing the school from the vessel's helicopter prior 

 to initiating a set. This may or may not be possible, 

 depending on the ability of observers in helicopters to 

 spot calves from the air. 



Given the model results presented here, it appears 

 that further research and perhaps management actions 

 should be implemented to better understand and reduce 

 the risk of separation of mothers and calves during sets 

 by tuna purse-seiners in the ETP. 



Acknowledgments 



This research was conducted under Section 304 (a)(1): 

 Stress studies, of the 1997 International Dolphin Con- 

 servation Program Act amendments to the U.S. Marine 

 Mammal Protection Act, and subsequent Congressional 

 directives. 



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