688 



Fishery Bulletin 98(4) 



^1986 ,v-^ 



1988^, 



^'o»»»o O O O o 



^:. • • o I, o-VWj > 

  - • • 



Figure 2 



King mackerel larval density at each station sampled during SEAMAP surveys from 1986 to 1995 (c>=zero catch; •= from 

 1 to 82 larvae/10 m- [symbol size scaled accordingly] ). 



duction (number-at-age x relative fecundity-at-age). This 

 measure accounts for the presumably greater contribu- 

 tion of older and larger fish to overall reproductive output; 

 therefore it would seem a better correlate of larval occur- 

 rence and abundance than numbers of fish. However, we 

 found egg production to be somewhat less correlated with 

 larval indices than were numbers of fish, perhaps owing 

 to the use of a constant fecundity-at-age distribution to 

 estimate king mackerel egg production. Significant inter- 

 annual differences in relative fecimdity have been demon- 

 strated for a wide variety of fishes (Bagenal, 1966; 1967; 

 Bagenal and Braum, 1971; Hunter et al., 1985; Rijnsdorp, 

 1991; Koslow et al., 1995). But there is insufficient data 

 on fecimdity of king mackerel over the time series to 

 ascertain the influence of interannual variability on this 

 parameter, and in turn, on larval production. 



Application of growth and mortality rates to refine or 

 adjust larval indices of relative abundance may be moot 

 because determinants of larval survival, i.e. predation or 

 starvation rates, or both, appear to be unrelated to spawn- 

 ing stock abundance (Hunter and Lo, 1993). It has been 

 argued that lai-\'al occurrence provides a more useful index 



of stock size because stock size and the geographic area 

 occupied by eggs and larvae may be correlated, as is the 

 case for Pacific sardines (Sardinops sagax) and northern 

 anchovy {EiigraiiliK mordax). especially at low population 

 levels. (Mangel and Smith, 1990; Smith, 1993; Hunter and 

 Lo, 1993; MacCall, 1988). However, Mangel and Smith 

 (1990) suggest that a switch from presence and absence 

 data to actual counts would be necessary when the spawn- 

 ing biomass increases to a level where "virtually all sta- 

 tions have eggs." 



King mackerel in the Gulf of Mexico are rebounding from 

 the low levels of the late 1970s and early 1980s (Powers 

 and Restrepo, 1993), and the increases are reflected in 

 lai-val occurrence and abundance. A switch to a larval 

 abundance index may be required to follow trends in stock 

 size as larval abundance rises if the uncorrected abun- 

 dance index at some future time no longer corresponds 

 to stock size. Adjustment of the king mackerel larval 

 abundance index would require annual estimates of mor- 

 tality and growth rates by direct aging of survey-caught 

 larvae. For now, both the index of larval occurrence and 

 the unadjusted index of abundance from SEAMAP collec- 



