ANDERSON and LINDNER: BIOLOGY OF ROYAL RED SHRIMP 



vember the females show a mode at about 80 mm 

 total length that can be followed readily through- 

 out the year to a second mode at about 120 mm. 

 The mode at 120 mm can also be traced through- 

 out the year to the bimodal group with modes 

 at 143 and 153 mm. The bimodality of this 

 latter group we believe to be fortuitous, but it 

 can be followed to a hump between 153 and 158 

 mm in the January-February distributions. 

 Thereafter the modes become lost in the large 

 group of mature females. We believe that the 

 two additional modes in our November distri- 

 butions (173 and 183 mm) also are fortuitous 

 and result from the sampling procedures. 



A group of small male shrimp also appears 

 in November, forming a mode at about the same 

 length as that for the females, which can be 

 traced throughout the year to the second mode 

 at about 115 mm. This second mode can be 

 traced as a hump to the left of the main mode 

 from January through September to the third 

 mode at 128 mm, beyond which it is lost. 



We made various attempts to fit Von Bert- 

 alanfy growth curves to the data without sat- 

 isfactory results other than it was apparent the 

 first two modes could be attributed to 1- and 2- 

 year-old shrimp. Evidently morphometric 

 changes associated with maturity preclude the 

 use of total length as a means of determining 

 age of H. robustus after they reach maturity. 



We believe the first three groups of males and 

 females we have cited are 3 distinct age classes. 

 If the peak of spawning is in Maixh, both sexes 

 would be about 100 mm total length at 1 year of 

 age. From our data it is impossible to disting- 

 uish more than the first 3 age classes. The older 

 age classes, which probably represent 2 or more 

 additional years, would give a minimum life span 

 of 5 years. Probably, however, at least some 

 of the largest shrimp are older than this. 



SIZE AND AGE AT RECRUITMENT 



When we consider all of our length measure- 

 ments as a unit, either unweighted, weighted 

 to give each cruise equal weight, or weighted 

 to give each year equal weight, we obtain almost 

 identical distributions. In Figure 14 we show 



'°  y  y  *°  ^° .^^ ^° ^ ^'^ '°° "° '^ '^° '*° '^° '^° '^° '»° '^° MO 210 



I I 1 I I I I I I I 



ANNUAl 

 — 7,734 MAltS 

 --10,441 FEMALES 



/ 



A 



/.-.'— -V 



■V 



rr^ 



TOTAL LENGTH if/,f/t) 



Figure 14. — Length distributions of H. robustus by sex 

 for all samples combined. 



these length distributions weighted to give equal 

 weight to collections during each cruise. It is 

 readily evident from these curves that only the 

 groups presumed to represent the second, third, 

 and fourth and older age groups are present 

 in substantial numbers and that the population 

 is composed largely of mature shrimp. 



Although the data are not adequate, they sug- 

 gest that recruitment starts at about 1 year of 

 age, but the shrimp are not fully recruited until 

 about 2 years of age, and recruitment may not 

 be complete until the shrimp are mature — about 

 3 years old. In the combined length distribu- 

 tions (Figure 14), 55% of the females were 

 longer than 160 mm and 61% of the males were 

 longer than 125 mm (the lengths at which we 

 believe each sex is fully mature). Only 6% of 

 the males and 4'/r of the females were less than 

 100 mm long. The smallest shrimp we sampled 

 was in the 56- to 60-mm length class. As we 

 mentioned earlier, we do not believe that gear 

 selectivity causes the scarcity of shrimp under 

 100 mm long and the lack of them under 56 mm 

 long. Royal red shrimp do not appear on the 

 fishing grounds at sizes smaller than about 55 

 mm. The observations of Anderson and Bullis 

 (1970) who had clear visibility of the bot- 

 tom from a distance of less than 1 m, substanti- 

 ated the lack of small shrimp on the St. Augus- 

 tine Grounds. Furthermore, 37 H. modestm, 

 43 to 93 mm long (mode, 63 mm) were taken 

 on April 20, 1957, in 225 fm with a 40-ft flat 

 shrimp trawl fitted with commercial 2-inch 

 stretched mesh in the body and 11/2-inch mesh 

 in the cod end. In addition, H. R. Bullis, BCF 

 Pascagoula, Miss, (personal communication), 

 concerning trawling on the H. robustus grounds 



323 



