Eggleston et al : Artificial shelters and survival of juvenile Panulirus srgus 



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bay site may not reflect potential predation intensity 

 as previously suggested for the inner-bay site. 



Predation risk on artifical reefs usually decreases 

 with distance from a natural, larger reef. For example, 

 mortality of tethered juvenile grunts (family Poma- 

 dasyidae) in St. Croix, U.S. Virgin Islands, was 40% 

 higher at the reef edge than 20 m away (Shulman 1985). 

 Our results are somewhat consistent with those of 

 Shulman (1985) in that predation of lobsters decreased 

 from 15 to 30 m from the casitas. However, increased 

 predation rates from to 15 m and from 30 to 70 m in- 

 dicate that predation risk does not simply decrease 

 linearly with increasing distance from the artificial reef 

 (casita). We hypothesize that the predator guild orig- 

 inating from the nearby barrier reef at the outer-bay 

 site (see Figs. 3 and 4 for geography) forages within 

 the adjacent seagrass habitat and is attracted to the 

 casitas, thereby leaving a relative "gap" in predator 

 abimdance between 15 and 60 m from the casitas. Thus, 

 predator encounter rates with lobsters tethered only 

 15 m from casitas were probably high relative to 

 lobsters tethered 30 m away. The patterns of survival 

 of small P. argus within close proximity to casitas (i.e., 

 15 m) in this study are consistent with our previous 

 work in seagrass habitats of Bahia de la Ascension, 

 Mexico. For example, survival of small lobsters (46-55 

 mmCL) was significantly higher at medium and large 

 casitas than in seagrass 15m away (Eggleston et al. 

 1990). Predation rates also increased from 30 to 70m, 

 and predators not associated with the casitas, such as 

 Nassau grouper E. striatus, were observed moving 

 from nearby natural reefs to the 70 m no-casita stations 

 rather than from the casitas. 



Resident piscivores set the upper limit of the number 

 and sizes of prey species that can occupy a given reef 

 (Hixon and Beets 1989, Eggleston et al. 1990). For ex- 

 ample, Hixon and Beets (1989) found an inverse rela- 

 tionship between the number of piscivorous fishes on 

 a reef and the maximum number of co-occurring poten- 

 tial prey fishes. The results from our study indicate that 

 large casitas are more effective at reducing mortality 

 on small juvenile lobsters than seagrass habitats, even 

 though seagrass and algal beds provide some refuge 

 for juvenile spiny lobsters (Herrnkind and Butler 1986; 

 R.N. Lipcius et al., unpubl. data). Hence, for small 

 lobsters, our results from both the January and August 

 experiments strongly suggest that artificial lobster 

 shelters such as casitas increase lobster production by 

 enhancing survival in nursery areas. However, our 

 results for the outer-bay site during January indicated 

 that survival of large juvenile lobsters was significantly 

 lower when tethered beneath large casitas compared 

 with nearby seagrass habitats. These results are con- 

 sistent with the notion of building artificial lobster 

 shelters that are scaled according to body size to en- 



hance survival of larger juveniles in nursery habitats, 

 particularly in areas where large conspecifics are 

 removed from large casitas by the fishery (Eggleston 

 et al. 1990, Eggleston and Lipcius 1992). However, fur- 

 ther research on the impact of casitas upon lobster sur- 

 vival, growth rates, local and regional population struc- 

 ture, and benthic community structure will be required 

 to assess the efficacy of this technology as a fisheries 

 enhancement tool. 



Acknowledgments 



We thank L. Coba-Cetina, T. Camarena-Luhrs, and E. 

 Sosa-Cordero with the Centro de Investigaciones de 

 Quintana Roo, and J. Cohen, J. Eggleston, K. Kennedy, 

 and numerous Earth watch volunteers for their able 

 field assistance. Special thanks to Armando Lopez and 

 Sonja Lillvick for the fine accommodations and 

 logistical support, and R. Wicklund and G. Wenz of the 

 Caribbean Marine Research Center for their ad- 

 ministrative guidance. We thank J. Bohnsack, L. Jones, 

 M. Luckenbach, J. van Montfrans, and two anonymous 

 referees for critical comments. This work was funded 

 by Sigma-Xi, Earthwatch-The Center for Field Re- 

 search, the National Undersea Research Program of 

 the National Oceanic and Atmospheric Administration, 

 the Caribbean Marine Research Center, the Common- 

 wealth of Virginia, L.L. Glucksman, and the National 

 Science Foundation (INT-8617945 to D.M. and R.L. 

 and OCE 87-00414 to R.L. and A. Hines). 



Citations 



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