Alonzo and Mangel: The effects of size-selective fisheries on the stock dynamics of and sperm limitation in sex-changing fish 



species with sperm competition because of decreased 

 allocation to sperm production. Pair spawning among 

 individuals could also lead to decreased fertilization 

 rates. Reproductive behaviors often found in sex- 

 changing species, such as territoriality, female choice, 

 resource-defense polygyny, and mate monopolization, 

 all lead to skewed reproductive success for males and 

 could further decrease fertilization rates. Sperm limita- 

 tion is predicted to occur, and an understanding of such 

 factors as fertilization rate, sperm production, mating 

 skew, and mating group size will increase our ability to 

 understand and predict stock dynamics. 



Traditional spawning-per-recruit measures can fail 

 in the presence of sperm limitation 



Although problems exist with traditional spawning- 

 per-recruit measures in general (Parkes, 2000), they 

 are especially problematic for sex-changing stocks. In 

 the dioecious stock, the relationship between female 

 and total spawning stock biomass per recruit exhibits a 

 roughly linear relationship with population size. In the 

 sex-changing stock, female fecundity does not reflect 

 the changes in mean population size. Although total or 

 male spawning stock biomass per recruit did decrease 

 with decreased population size, the fit between these 

 measures will depend greatly on the size-dependent 

 sperm production of males, mating aggregation size, 

 and other factors determining the potential for sperm 

 limitation. Male or total spawning stock biomass 

 per recruit alone cannot predict sperm limitation 

 and thus will fail to predict the potential population 

 crashes that may result. We conclude that any mea- 

 sure of spawning per recruit in a sex-changing species 

 that does not consider sperm limitation and reduced 

 fertilization rates has the potential to underestimate 

 the impact of fishing on the population. The number 

 of eggs produced or female spawning stock biomass 

 can remain relatively unchanged in the face of high 

 fishing mortality even as the population is predicted 

 to decline. However, the failure of classic spawning- 

 per-recruit measures in the presence of declines due 

 to sperm limitation or decreased fertilization rate will 

 not be limited to protogynous stocks. Although sperm 

 production patterns and fertilization rates are not known 

 for many commercially important species, this information 

 can be collected to develop a general sense of how sperm 

 production depends on individual size. We also have a 

 general sense of the factors that are expected to affect fer- 

 tilization rates (Birkhead and Moller, 1998) and these can 

 be easily studied in any species where spawning grounds 

 are accessible to researchers. It is clear that new manage- 

 ment measures must be developed for sex-changing species 

 that consider the potential for sperm limitation because 

 biomass alone may miss the potential for rapid population 

 crashes. One purpose of theory is to tell us what we need 

 to know more about and to stimulate further research. Our 

 results clearly indicate that we need to know more about 

 sperm production and fertilization rates when managing 

 protogynous stocks. 



"P V 



<= s 



- oi 



-^ <D 



CD CD 



0.75 



0.5 



& 0.25 



i- c\j 



Protogynous 



Dioecious 



B 



600,000 

 500.000 



400.000 

 300.000 



200,000 



100,000 



Protogynous 



Dioecious 



Figure 8 



The effect of marine reserves on protogynous and dioecious 

 populations when fishing effort is redistributed (case 2). iAi 

 Percent change in the presence of fishing iF=ll in the produc- 

 tion of fertilized eggs compared to percent change in the absence 

 of fishing. (Bl Annual biomass removed by the fisheries varies 

 with marine reserve and sex-change pattern. Numbers shown 

 are for 10 mating sites when F=l. 



Marine reserves and size-selective fishing can be used 

 to manage protogynous stocks 



Marine reserves clearly have the potential to decrease the 

 impact of fishing on populations. Large highly fecund or 

 fertile individuals may be protected from size-selective 

 fisheries. However, the benefits of a marine reserve will 

 be significantly decreased if fishing effort is simply redis- 

 tributed to unprotected sites (Figs. 7 and 8; Guenette and 

 Pitcher, 1999; Apostolaki et al„ 2002). It is usually rec- 

 ognized that the larval export and import dynamics will 

 be crucial to whether reserves increase mean population 

 size. We predict that the degree to which stocks respond 

 to no-take reserves will also depend on their life-history 

 pattern, mating system, and size-dependent fecundity 

 and fertility. The protection of large and fecund (or fertile) 



