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Fishery Bulletin 90(4), 1992 



in premolt condition died suggests that lobsters at this 

 stage are less fit. The probability that this mortality 

 would occur only with premolt individuals by chance 

 alone is < 0.001 (Agresti 1990). Increased vulnerabil- 

 ity to a poor physical environment, conspecific aggres- 

 sion, or predation have been associated with molting 

 (Conan 1985). It seems likely that the higher percent- 

 age of spiny lobster in premolt stage accounts for some 

 of the difference in mortality between spiny and slip- 

 per lobsters. 



The significantly higher mortality observed in the 

 laboratory compared with the field does not support 

 the idea that undetected predation substantially af- 

 fected the field results. The relatively low absolute level 

 of total mortality suggests that such predation is prob- 

 ably minimal at the field site. However, with mortal- 

 ity being higher in the laboratory than in the field, no 

 estimate of predation is possible. 



More than twice as many deaths were observed in 

 the laboratory as in the field, even though the field 

 trials involved twice as many lobsters and three times 

 as many were in the premolt stage. This suggests a less 

 healthy or fit laboratory population, which is consis- 

 tent with the significantly slower exit of pooled species 

 from traps in the laboratory versus in the field. Aspects 

 of the laboratory environment (e.g., water quality, 

 lighting, diet) may have degraded the physical condi- 

 tion of the lobsters or affected their behavior, inhibiting 

 their exit, or providing less inducement to leave the 

 traps than that encountered in the field. It seems like- 

 ly that our field assessment provides a better estimate 

 of natural exit patterns. 



With a study design similar to ours, Munro (1974) 

 examined the rate of fish exiting unbaited traps. His 

 theoretical model suggests that the number exiting per 

 day may be a fixed fraction of the current trap occu- 

 pancy, and that catch eventually reaches an asymptote 

 when trap entrances are balanced by exits. Our number 

 of stocked lobsters declined approximately exponential- 

 ly, approaching zero asymptotically. However, the total 

 occupancy of traps declined daily until it reached a low 

 and varying level at which exits were roughly matched 

 by entrances of lobsters. This final, low level of trap 

 occupancy at the end of the stocking experiment seems 

 consistent with native occupancy observed during the 

 monthly field monitoring. 



In our stocking tests, some individuals likely left a 

 trap and reentered it undetected between censuses, 

 particularly in the field test where the observation in- 

 terval was 48 hours. Based on independent probabilities 

 of exit and entry estimated from our field data, the 

 theoretical joint probability of such reentry was as high 

 as 0.06, and probably about 12 individuals left and 

 reentered the same trap undetected during the full 

 26-day field stocking experiment. 



Conclusion 



Our results indicate that spiny and slipper lobsters are 

 not restrained by lost molded-plastic traps for periods 

 long enough to cause serious harm. There is no evi- 

 dence that such lost traps result in increased mortal- 

 ity. The absence of any apparent trap-induced mortality 

 and the low incidence of identifiable in-trap mortality 

 due to predation suggest that ghost fishing by these 

 traps contributes little to the total mortality of the 

 population. Such traps, when unbaited and intact, may 

 best be considered short-term artificial shelters that 

 lobsters enter and exit occasionally, more or less at will. 



Acknowledgments 



Thanks are due to Steve Kaiser for advice on selection 

 of study sites and to Bill and Joanne Goebert for pro- 

 viding ready access to those sites. Ray Boland, Karl 

 Bromwell, Theresa Martinelli, and Leslie Timme 

 assisted in the rigorous program of field monitoring. 

 Greatly appreciated are the statistical and substantive 

 comments of Deborah Goebert, Robert Moffitt, James 

 Parrish, and Jeffrey Polovina. 



Citations 



Agresti, A. 



1990 Categorical data analysis. John Wiley, NY, 558 p. 

 Bathen, K.H. 



1978 Circulation atlas for Oahu, Hawaii. Misc. Rep. UNIHI- 

 SEAGRANT-MR-78-05, Univ. Hawaii Sea Grant Coll. Prog., 

 Honolulu, 22 p. 

 Breen, P.A. 



1987 Mortality of Dungeness crabs caused by lost traps in the 

 Fraser River estuary, British Columbia. N. Am. J. Fish. 

 Manage. 7:429-435. 

 1990 A review of ghostfishing by trap and gill nets. In Sho- 

 mura, R.S., and M.L. Godfrey (eds.). Proceedings, Second 

 international conference on marine debris, 2-7 April 1989, 

 Honolulu, p. 571-599. NOAA Tech. Memo. NMFS-SWFSC- 

 154, NMFS Southwest Fish. Sci. Cent., Honolulu. 

 Conan, G.Y. 



1985 The periodicity and phasing of molting. In Wenner, 

 A.M. (ed.), Crustacean issues, factors in adult growth, vol. 3, 

 p. 73-99. A. A. Balkema, Boston. 

 Drach, P. 



1939 Mue et d'intermue ehez les Crustaces Decapodes. Ann. 

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 Fienberg S.E. 



1987 The analysis of cross-classified categorical data, 2d 

 ed. MIT Press, Boston, 198 p. 

 Haight W.R., and J.J. Polovina 



1992 Status of lobster stocks in the Northwestern Hawaiian 

 Islands, 1991. Admin. Rep. H-92-02, NMFS Southwest Fish. 

 Sci. Cent., Honolulu. 19 p. 

 High. W.L. 



1976 Escape of Dungeness crabs from pots. Mar. Fish. Rev. 

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