MARINE MAMMAL COMMISSION - Annual Report for 1991 



support work on mitigating environmental damage and 

 hazards caused by Defense Department activities, the 

 Corps emptied the underground storage tanks, filled 

 them with a concrete slurry to stabilize them, and 

 removed the electrical equipment containing PCBs. 

 Further work to treat or remove soils contaminated by 

 leaking fuel may be undertaken in 1992. 



With regard to repair of the deteriorating seawall, 

 the Fish and Wildlife Service has conducted a bathy- 

 metric survey and provided funds to the Corps for the 

 engineering study. Based on the results, a recom- 

 mended approach will be selected and the Corps will 

 enter a project design phase expected to be completed 

 in 1993. Construction could begin by 1995. 



Male Mobbing Behavior 



As noted above, recovery of Hawaiian monk seals 

 at some of the major breeding colonies is being 

 impaired by the death of females and immature seals 

 as a result of aggressive attacks by groups of up to 25 

 male seals attempting to mate. These incidents are 

 believed to have caused a skewed sex ratio favoring 

 males at some atolls. During mobbing incidents, 

 aggressive males repeatedly bite and scratch their 

 victims on the back and neck, often causing serious 

 injuries. Some female victims die directly from the 

 injuries and others are probably killed by sharks 

 attracted by secretions from open wounds. Mobbing 

 incidents have been most apparent at Laysan Island 

 but have also been seen on Lisianski Island and 

 French Frigate Shoals. The frequency of these 

 incidents appears to have increased in recent years. 



Mobbing behavior threatens the reproductive 

 potential of affected colonies by reducing the number 

 of breeding females. For example, at Laysan seven 

 mature females were killed in 1989, while only one 

 animal was recruited to the breeding population. In 

 1990, two mature females were killed and two recruit- 

 ed. In both years, male and female pups were also 

 killed in mobbing incidents at the island. If the 

 behavior continues, the ratio of males to females will 

 become more strongly skewed towards males, which 

 could exacerbate the problem. 



To address this problem, the National Marine 

 Fisheries Service has investigated the possibility of 

 removing selected male seals known or suspected to 

 have engaged in male mobbings, and administering a 

 drug to suppress testosterone production and reduce 

 their libido, or otherwise treating problem males. 

 Because of risks to the island colonies, including the 

 possibility of removing or otherwise interfering with 

 dominant males responsible for siring pups, the 

 Service has proceeded cautiously. Work to date has 

 been limited to monitoring the nature and frequency 

 of mobbing incidents, identifying male seals involved, 

 collecting tissue samples for analyses to identify male 

 seals responsible for siring pups, and testing on 

 captive males a drug that temporarily suppresses 

 testosterone levels. 



In previous years, the Commission has recom- 

 mended that certain background studies be completed 

 before any field testing to address the problem. 

 Although much background work has been done, all 

 of the recommended studies have not been completed 

 and some critical questions remain unanswered. For 

 example, genetic studies to identify dominant male 

 seals responsible for siring pups have not been com- 

 pleted. Also, while a testosterone suppressant drug 

 has been tested on captive animals and shown to 

 depress testosterone levels, it has not been determined 

 whether doing so will also decrease the libido of 

 treated males. 



Nevertheless, the number of female seals being 

 killed as a result of male mobbing is far out-pacing 

 recruitment at some colonies and thereby is seriously 

 threatening their future reproductive potential. 

 Therefore the Service is considering a limited field 

 trial of the testosterone suppressant drug during the 

 1992 field season to examine behavioral and social 

 structure effects of chemically "removing" males 

 involved in mobbing. Favorable results from the 

 experiment would be followed by further drugging 

 and/or actual physical removal of offending males. 

 At the end of 1991, a decision on whether to proceed 

 had not been made and was to be considered further 

 at a Hawaiian Monk Seal Recovery Team meeting 

 scheduled for 13-15 January 1992. 



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