230 



Fishery Bulletin 103(2) 



Kuwamura and Nakashima, 1998; Koeller et al., 2000; 

 Nakashima et al., 2000). At one extreme, sex change 

 may occur at a fixed size or age threshold. However, 

 sex change is known in many species to be mediated 

 by local factors such as population density, reproduc- 

 tive skew, sex ratio, and size distribution (Warner and 

 Lejeune. 1985; Warner and Swearer, 1991; Lutnesky, 

 1994, 1996; Kuwamura and Nakashima, 1998; Koeller 

 et al., 2000; Nakashima et al., 2000). In many sex- 

 changing species, overlap exists between the sexes in 

 size and age and this overlap indicates that sex change 

 may also depend on individual experience and local 

 conditions (Munoz and Warner, 2003). The pattern of 

 sex change may have important implications for a spe- 

 cies' response to fishing. For example, if the size at sex 

 change is fixed, then the population sex ratio may be 

 affected by size-selective fishing of males, resulting 

 in sperm limitation and decreased larval production 

 (Alonzo and Mangel, 2004). In contrast, if sex change is 

 mediated at the level of the spawning group in single- 

 male harems and mating group size remains the same, 

 sex ratios are maintained if the largest female always 

 changes sex. In such a case, larval production will be 

 reduced only because of the decreased size distribution 

 of the population due to fishing. However, if sex change 

 is controlled by the reproductive skew in the group (e.g., 

 the expected potential for reproduction as a male versus 

 present fecundity as a female), then the largest individ- 

 ual might not change sex and the spawning group could 

 be without a male (Munoz and Warner, 2003). This re- 

 sult would clearly lead to a much greater effect on the 

 productivity of the stock. A detailed understanding of 

 the factors determining sex change and the cascading 

 effects on sperm production, fecundity, and sex ratio can 

 be critical to predicting stock dynamics. Furthermore, 

 most animals have "rules-of-thumb" which determine 

 their behavior and reproduction. Although these rules 

 will have evolved under normal conditions, in the pres- 

 ence of fishing or other human-induced disturbances, 

 animals are likely to continue to use these behavioral 

 rules on ecological time scales even if they no longer 

 function to maximize reproduction. 



Although previous fisheries models have examined 

 sex change, a consensus does not exist regarding how 

 sex change is predicted to affect stock dynamics. Some 

 research has suggested that sex-changing stocks will 

 be more sensitive to fishing and cannot be managed as 

 if they were identical to separate-sex stocks (Bannerot 

 et al., 1987; Punt et al., 1993; Huntsman and Schaaf, 

 1994; Coleman et al., 1996; Beets and Friedlander, 

 1999; Brule et al., 1999; Coleman et al., 1999; Arm- 

 sworth, 2001; Fu et al., 2001). However, it has also 

 been argued that, in the absence of sperm limitation, 

 protogynous stocks should be less sensitive to size-selec- 

 tive fishing because female biomass and thus population 

 fecundity should not decrease as much as in a dioecious 

 population, making traditional management and theory 

 conservative when applied to these species. In general, 

 protogynous stocks have been predicted to be at risk of 

 population crashes because of their potential for nonlin- 



ear population dynamics in the presence of exploitation, 

 yet there is no consensus regarding the importance of 

 the exact pattern of sex change. For example, Arms- 

 worth (2001) examined protogynous stock dynamics 

 when the probability of sex change was a fixed func- 

 tion of individual age and when the probability of sex 

 change depended on the mean age of individuals in the 

 population. He found that these two patterns of sex 

 change had similar general dynamics and argued that 

 management of a protogynous stock might not require 

 knowledge of the precise pattern of sex change. In con- 

 trast. Huntsman and Schaaf (1994) and Coleman et al 

 (1999) have argued that a consideration of the pattern 

 of sex change can be important to managing stocks. 

 But, past theory has generally focused on comparing 

 fixed patterns of sex change with fully compensating 

 reproductive patterns that maintain a fixed sex ratio 

 or ratio of female to male biomass. However, a variety 

 of patterns of sex change exist and there is no reason 

 to believe that all species have evolved to exhibit full 

 compensation under natural conditions, let alone un- 

 der new situations. Thus, it is important to consider 

 how specific sex change rules will affect the dynamics 

 and management of protogynous stocks and whether 

 knowledge of the cues that determine sex change will 

 be important. 



We (Alonzo and Mangel, 2004) developed a general 

 modeling approach for examining the impact of repro- 

 ductive behavior and life history pattern on stock dy- 

 namics. Using this approach, we then compared the 

 dynamics of a protogynous population with fixed size at 

 sex change and an otherwise identical dioecious species 

 (Alonzo and Mangel, 2004). These analyses showed that 

 although dioecious and protogynous stocks clearly have 

 distinct dynamics, simple statements arguing that one 

 life history pattern is more or less sensitive to fishing 

 cannot be made. Protogynous stocks with fixed patterns 

 of sex change were predicted to experience sperm limi- 

 tation and lowered larval recruitment at high fishing 

 pressure, whereas the dioecious stock was predicted to 

 show a large drop in mean population size even at low 

 fishing mortality, but was not predicted to experience 

 lowered fertilization rates due to size-selective fishing. 

 Both stocks were predicted to be sensitive to fishing 

 pattern, but a fixed pattern of sex change was predicted 

 to put a population at risk of crashing if all male size 

 classes were fished even at relatively low fishing mor- 

 tality. Finally, classic spawning-per-recruit (SPR) mea- 

 sures were not predicted to be good indicators of chang- 

 es in the mean population size of protogynous stocks 

 because they cannot indicate whether a population is 

 experiencing sperm limitation and whether this limita- 

 tion may lead to decreased population size or cause the 

 stock to crash with small changes in fishing mortality. 

 Although we found that whether or not a stock changes 

 sex was important, that knowledge alone was not suf- 

 ficient to understand and predict the response of the 

 stock to fishing or management. We also found that 

 sperm production and mating system were important 

 variables affecting the probability that a population 



