Vaughan and Prager: Decline in abundance of Pagivs pagrus off the soutfieastern United States 



371 



Full(F) 



 Total SSB 



-Yield 



Figure 10 



Sustainable yield and spawning stock biomass at varying 

 levels of fishing, projected for red porgy off the southeastern 

 United States from selectivity pattern estimated for 1992-96. 

 Selectivity pattern estimated through calibrated VPA of (A) 

 primary catch matrix, iBi alternate catch matrix. 



below MSST. 1992-96 (Table 8). Still lower estimates of 

 spawning-stock biomass were obtained for the terminal year, 

 although we view terminal-year estimates with scepticism 

 because of retrospective patterns. The production model, not 

 commonly subject to such retrospective patterns, also esti- 

 mates current biomass as well below MSST (Table 9). 



Current estimates of full F (Table 5 for 1992-96) are 

 considerably higher than estimates of F^c^y (Tables 8 and 

 9). In addition, estimates of F^jgy are considerably lower 

 than F values corresponding to static SPR of SO'/; to 40^* . 

 If static SPR is used as a reference point, a value higher 

 than SPR = 40'> will be necessary to maintain the stock at 

 reasonably productive levels. 



It seems inescapable that heavy fishing mortality has 

 been a major cause of the decline in recruitment (Fig. 4B), 

 particularly in the late 1970s and 1980s. Although fishing 

 mortality was lower in the mid-1980s, it increased again 

 in the 1990s for fully-recruited ages (Fig. 4A), and thus 

 static SPR has also declined (Fig. 7). Patterns of increas- 

 ing F and declining SPR are even more pronounced in es- 

 timates from the alternate catch matrix (Figs. 4A, 7). Even 

 though spawning-stock biomass remained high in the late 

 1970s, recruitment began to decline (Fig. 9). This finding 

 suggests that the usual effects of heavy fishing pressure 



may have been compounded by other population-specific 

 factors (e.g. those relating to sex structure) or extrinsic 

 factors (e.g. unfavorable environmental conditions! affect- 

 ing the reproductive success of the population. 



Management of hermaphroditic reef fishes is difficult, 

 and it has had few successes (Coleman et al., 1999). As 

 we have shown, interaction of a population's size and sex 

 structures complicates the attempt to model reproductive 

 capacity (e.g. SPR) as a function of fishing mortality rate. 

 Size limits can have subtle side effects if they result in 

 a deficit of males and if larger females in the population 

 consequently change sex at a higher rate. Use of bag lim- 

 its and size limits in management makes it likely that 

 fish will be caught and released, imposing discard mortal- 

 ity that is incompletely observable. Discard mortality has 

 been found an important factor in collapse of at least one 

 fish stock (Myers et al., 1997). Fish that aggregate become 

 more catchable (increased catchability coefficient q = F/f) 

 as abundance declines, another phenomenon associated 

 with risk of collapse (Clark, 1974; Gulland, 1975). Ludwig 

 ( 1998) has suggested on theoretical grounds that the best 

 management strategy for stocks that may collapse is rapid 

 adjustment of harvest size in response to changes in stock 

 abundance. Ludwig noted: "It is noteworthy that the com- 



