446 



Fishery Bulletin 89|3), 1991 



Length-weight relationships Significant changes in 

 body form usually occur at or near sexual maturity for 

 most teleosts. The smallest mature male blue marlin 

 reported in the literature is about 166 cm long (Erd- 

 man 1968). However, we established the upper length 

 limit for immature fish at 140 cm because the length- 

 weight relationship offish between 140 and 166cm in 

 our sample appeared to be closer to that for larger fish. 

 Differences in the slopes and Y-intercepts of the 

 length-weight relationships for each sex category (i.e., 

 males, females, and unknown sex) were tested using 

 covariance analysis. Mature fish in our age analysis are 

 a small and size-selected subsample of those available, 

 since a majority of the adult blue marlin population is 

 over 200 cm. To maximize the amount of information 

 for length-weight analysis of mature adults (>140cm 

 long), we used all available length and weight data col- 

 lected by the SEFSC recreational billfish survey pro- 

 gram from 1972 to 1988 (1969 males, 3260 females). 

 Covariance analysis was used for fish in the common 

 length range 140-277 cm (maximum length of males in 

 the sample) to compare the length-weight relationship 

 of mature male and female blue marlin. Separate 



length-weight equations were developed for the entire 

 mature size range of both sexes, and a single length- 

 weight equation was used for immature fish (< 140 cm). 



Weight-at-age The variability of weight-at-age in our 

 data was large and sample sizes for each sex were 

 small, a common occurrence in marlin studies. Since 

 variability in length is much less than for weight and 

 our length-weight relationships are based upon large 

 numbers of fish, we estimated weight-at-age by con- 

 verting individual lengths to weights. For larvae and 

 juveniles, we converted parameters of the Gompertz 

 length-at-age equation directly using equations 4a-d 

 below. 

 From equation 3a and using the allometric equation 



In W = a + b In L, 



we obtain 



In W = a + b * In Pj - b * P 2 * exp [ - P 3 * t] 



which allows us to derive the weight-at-age equation 



