FISHERY BULLETIN: VOL. 75, NO. 1 



TABLE 8. — Range of potential yield estimates for eastern Gulf of 

 Mexico round herring, based on biomass estimates by three 

 methods. Yields are predicted at three possible values of M, the 

 natural mortality coefficient. Biomass estimates were obtained 

 from values in Tables 5-7. 



TABLE 9. — Abundance estimates of round herring larvae for 

 each cruise. Estimates include larvae in all size classes and 

 were obtained using Equations (2) and (3). 



Biomass estimating 



method and 

 spawning season 



Biomass Estimated potential annual 



estimate yields (metric tons) for 



(metric given values of M 



tons) M = 0.5 M = 0.75 M = 1.0 



I 1971-72 717,815 



I 1972-73 131,136 



I Mean of 1971-72 



and 1972-73 424,476 



II 1971-72 698,045 

 II 1972-73 130,995 



II Mean of 1971-72 



and 1972-73 414,520 



III 1971-72 mean 841,373 



III 1972-73 mean 164,474 

 III 1971-72 cruises 



8B 7201 and 



GE 7202 673,481 

 III 1972-73 cruise 



IS 7303 136,330 

 III 1 970-74 geometric 



mean of 8 estimates 1 81 , 1 20 

 III 1970-74 arithmetic 



mean of 8 estimates 415,175 



179,454 

 32,784 



269,181 

 49,176 



358,908 

 65,568 



106,119 159,179 212,238 



174,511 261,767 349,022 



32,749 49,123 65,498 



103,630 155,445 207,260 



210,343 315,515 420,687 



41,118 61,678 82,237 



168,370 252,555 336,740 



34,082 51,124 68,165 



45,280 67,920 90,560 



103,794 155,691 207,588 



the range that I believe represents the mean po- 

 tential annual yield of the stock. This is equiva- 

 lent to a potential harvestable yield of 6.5-32.7 

 kg/hectare in the 76.5 x 10 9 m 2 of round herring 

 habitat in the eastern Gulf. If stock size fluctuates 

 greatly from year to year then the harvestable 

 yield also may vary. As Alverson (1971) has 

 pointed out, the biological potential yield is not 

 necessarily the realizable yield. The realizable 

 yield will depend upon the availability of the stock 

 and its vulnerability to fishing gear. Neither of 

 these factors has been evaluated for eastern Gulf 

 round herring. It is possible that large year to year 

 fluctuations in round herring biomass do occur, as 

 suggested by the great differences in 1971-72 and 

 1972-73 biomass estimates. Such variation could 

 reflect year class fluctuations or yearly changes in 

 distribution of parts of the stock between the 

 north-central and eastern Gulf. Although they are 

 abundant, there is no reason to believe that round 

 herring in the eastern Gulf constitute a stock as 

 large as the Gulf menhaden stock in the north- 

 central Gulf of Mexico, which produces a mean 

 annual yield of more than 550,000 metric tons. 



Larval Abundance Estimates 



Larvae occurrence and abundance varied sea- 

 sonally in the same manner as eggs (Table 9). The 

 range of larvae abundances for positive cruises, 

 including larvae in all length classes, was 0.47- 

 31.95 x 10 10 . In subsequent estimates of larval 



Cruise 



Area 



represented 



by the cruise 



(m 2 x 10 9 ) 



Positive area' 

 (m 2 x 10 9 ) 



Cruise larvae 



abundance 



(larvae x 10 10 ) 



GE 7101 

 8C7113 



Tl 7114 

 GE7117 

 8C 7120 



Tl 7121 

 GE 7127 



Tl 7131 



8B7132 

 8B 7201 



GE 7202 

 GE 7208 

 GE 7210 

 IS 7205 

 IS 7209 

 IS 7303 

 IS 7308 

 IS 7311 

 IS 7313 

 IS 7320 

 CL 7405 2 

 CL 7412 



25.79 



120.48 

 101.10 



189.43 



72.99 



148.85 



124.88 



48.43 



104.59 



149.80 



149.80 



151.42 



156.50 



153.18 



153.89 



52.00 



91.33 



13.69 



21.80 

 0.00 



0.00 



21.58 



78.43 



15.79 



0.00 



0.00 



17.79 



78.19 



10.52 



0.00 



0.00 



31.34 



0.00 



2.91 



2.58 



3.60 

 0.00 



0.00 



2.92 



26.55 

 0.47 

 0.00 

 0.00 

 2.70 



31.95 

 3.99 

 0.00 

 0.00 

 1.71 

 0.00 

 5.09 



1 Positive area is defined as the area representing stations where either eggs 

 or larvae of round herring were collected. 



2 No stations on this cruise were located far enough offshore for round herring 

 eggs or larvae to have been collected. 



abundance by length classes and in mortality es- 

 timation procedures, larval abundance by each 

 1-mm length class was adjusted for cruises GE 

 7127-TI 7131-8B 7132 and GE 7208 to account for 

 only part of the potential round herring spawning 

 area having been sampled. The adjustment factors 

 were 0.655 and 0.839, the same factors that were 

 used to adjust egg abundance for those cruises. 



Larvae that were collected ranged from 2.1 to 

 30.0 mm SL during the survey. Length frequen- 

 cies of larvae in the 2.1-20.0 mm SL range are 

 illustrated in Figure 9. Larvae >20.0 mm were 

 rarely collected during the survey. Frequencies for 

 each length class in Figure 9 are given as esti- 

 mated abundance during each cruise (Equation 

 (3)). No area adjustments have been made in Fig- 

 ure 9 for the two cruises that did not cover the 

 entire spawning area. Round herring larvae <4.0 

 mm SL usually were in poor condition, with 

 curved or deformed bodies, and their measure- 

 ments are underestimates of true length. O'Toole 

 and King (1974) hatched eggs that they had col- 

 lected and reported that preserved, newly hatched 

 round herring larvae were 3.75-4.00 mm long. The 

 4.1- to 5.0-mm SL length class was the most abun- 

 dant class in my survey (Figure 9). I assumed that 

 this length class was fully vulnerable to the sam- 

 pling gear, although some escapement may have 

 occurred for larvae of this size. 



The ratios of night-caught to day-caught larvae 



80 



