Sladek Nowlis and Roberts; Fisheries benefits and optimal design of marine reserves 



613 



ies (Safina, 1993 ), will only fit our adult movement as- 

 sumptions if large reserves are established. Recent 

 studies, though, have shown high site fidelity by fish 

 species previously thought to range widely (Holland et 

 al., 1993; Holland et al., 1996), demonstrating the need 

 for more field data on adult movement patterns. More- 

 over, recent modeling efforts by Holland and colleagues' 

 and others'^ suggest that reserves can benefit highly 

 mobile species through enhanced population fecundity 

 gained fi-om temporary protection. 



If adults do cross reserve boundaries, our predic- 

 tions regarding fisheries benefits from reserves will 

 be influenced in opposing ways. Under heavy fish- 

 ing pressure and intermediate movement tendencies, 

 minor yield enhancements may be possible from this 

 adult spillover (Polacheck, 1990; DeMartini, 1993). 

 However, this same movement would dilute the abil- 

 ity of reserves to enhance larval transport to fishing 

 areas. As Polacheck showed (1990), spawning stock 

 biomass, or the potential for fisheries enhancement 

 through larval transport, is highest at lowest levels 

 of adult movement. Because the potential benefits 

 from larval transport presented here far outweigh 

 those predicted from adult spillover (Polacheck, 1990; 

 DeMartini, 1993), it is likely that adult movement 

 across boundaries will decrease the predicted yields 

 from reserves. Consequently, reserves will have the 

 highest potential for enhancing surrounding fisher- 

 ies if they are designed as a collection of units large 

 enough to contain populations of adults with rela- 

 tively little movement across boundaries. 



Our assumptions regarding larval transport have 

 less supporting evidence. Most aquatic species dis- 

 perse more widely as larvae than as adults (Boehlert, 

 1996), and the potential for long-distance dispersal 

 across reserve boundaries is great for species with 

 long-lived larvae ( Roberts, 1997 ), including most food 

 fish. Consequently, lai-vae are likely to move from 

 reserves to fishing areas as long as oceanographic 

 conditions and larval behavior permit. Without lar- 

 val transport, the potential for fisheries benefits fi'om 

 reserves is more limited, although Holland and col- 

 leagues' did show that a reser\'e system in which lar- 

 vae stayed in place but adults moved widely across 

 boundaries could produce some benefits. Reser\'es, es- 

 pecially in heavily overfished or large management 

 areas, may need to be partitioned into several subunits 

 that maintain adult populations within them but al- 

 low larvae to disperse to remaining fishing areas. 



We also assumed a stock-recruitment relationship, 

 implying that a significant portion of the population 



^ Guenette, S. 1998. Fisheries Centre, University of British 

 Columbia, 2204 Main Mall, Vancouver. BC V6T 1Z4. Canada. 

 Unpublished data. 



fecundity from reserves stays in or returns to the 

 management area. The degree to which marine popu- 

 lations are locally sustained remains an active area 

 of debate in marine ecology. Larvae of most tropical 

 food fish are often found in greatest quantities off- 

 shore (Boehlert, 1996), suggesting the possibility of 

 long-distance dispersal. However, studies that show 

 this result may be biased because sampling within 

 the complex structure of the reef itself is difficult 

 ( Boehlert, 1996 ). Therefore, reefs may harbor greater 

 concentrations of larvae than are measured above 

 the reef This complexity (Wolanski and Sarsenski, 

 1997), along with potential for larval behavior to in- 

 fluence their distribution (e.g. Breitburg et al., 1995), 

 suggests that larvae may be retained at higher con- 

 centrations than predicted by simple oceanographic 

 models (e.g. Roberts, 1997). If recruitment dynam- 

 ics are influenced on a much larger spatial scale than 

 encompassed by the management area, such that the 

 stock in the management area has a minimal im- 

 pact on recruitment back to it, reser\'e benefits to 

 the management area are likely to be much more 

 limited. Cohort models, including those by Polacheck 

 (1990) and DeMartini (1993), can be interpreted as 

 situations in which larval supply is constant and not 

 influenced by local stock. As has been discussed, these 

 models show limited potential for fisheries benefits 

 from reser\'es. It is necessary to think of reserve sys- 

 tems at a scale that fits stock-recruitment relation- 

 ships. Yet our knowledge of these relationships re- 

 mains poor. Even if larvae have the potential to dis- 

 perse over large distances, stock-recruitment rela- 

 tionships could still exist on a local level if a signifi- 

 cant portion of larval production is retained. The 

 safest approach to this uncertainty is to design re- 

 serve systems at large scales. However, there is still 

 the potential for reserves to produce fisheries ben- 

 efits on small scales if lai-vae have the capacity to be 

 retained. Further research on stock-recruitment re- 

 lationships in marine populations will be invaluable 

 for resolving this pressing issue along with many 

 others in fisheries management. 



Field needs and testable predictions 



Our results identify areas in need of additional field 

 work and make testable predictions. The needs in 

 regard to field work differ for our quantitative and 

 qualitative predictions. The quantitative predictions 

 were highly sensitive to all parameters that affected 

 intrinsic population growth potential. The most im- 

 portant and least understood of these parameters is 

 lai'val sui-v'ivorship. We need significantly better in- 

 formation about the duration of the egg and larval 

 stages of coral reef fishes and their daily mortality 



