FISHERY BULLETIN: VOL. 75, NO. 3 



98% in 1971. Estimated percentage mortalities 

 from spawning to hatching (Table 9) were lower 

 for thread herring than those estimated previ- 

 ously for round herring (35 to 90% ) from the east- 

 ern Gulf (Houde 1977a). They also were lower 

 than those (>85%) estimated for scaled sardines 

 (Houde 1977a) in 1973. The 5.5 mm SL stage rep- 

 resents postyolk-sac thread herring larvae that 

 had succeeded in starting to feed; percentage mor- 

 tality to that stage was estimated to range from 

 62.6 to 84.5% (Table 9). 



The 15.5-mm stage would be attained at 18.5 to 

 19.0 days if the instantaneous growth coefficient 

 was 0.0797 (equals 0.80-mm mean daily growth 

 increment) (Table 7). At that growth rate 20 

 larvae/ 1,000 spawned eggs would have survived 

 to 15.5 mm SL in 1971, but only 6 larvae/1,000 

 eggs would have survived to 15.5 mm in 1973 

 (Table 9). The expected number of thread herring 

 survivors at 15.5 mm/1,000 spawned eggs was 

 similar to that estimated for round herring from 

 the eastern Gulf (Houde 1977a), but greater than 

 the number estimated for scaled sardines (Houde 

 1977b). 



SUMMARY 



5. Estimates of annual potential yield to a 

 fishery, based on 1971 and 1973 biomass esti- 

 mates, ranged from 27,500 to 186,200 metric tons 

 of adult thread herring. The potential yield, based 

 on the mean of 1971 and 1973 biomass estimates, 

 was between 60,300 and 120,600 metric tons. 



6. Larval abundance was greater in 1973 than 

 in 1971. Mortality rates for larval thread herring 

 were estimated by length and for estimated ages. 

 For lengths, the instantaneous coefficients of de- 

 cline in catches wereZ = 0.3545 in 1971 andZ = 

 0.3942 in 1973, corresponding to 29.9 and 32.6% 

 losses per millimeter of growth. For age, the most 

 probable daily mortality estimates were Z = 

 0.2124 in 1971 and Z = 0.2564 in 1973, which 

 correspond to daily loss rates of 19.1 and 22.6%. 



7. It is probable that >99% mortality occurred 

 between spawning and the 15.5-mm stage in 1973, 

 and that approximately 98% mortality occurred in 

 1971. About 20 larvae/1,000 spawned eggs were 

 estimated to have survived to 18.5 to 19.0 days 

 after hatching and 15.5 mm SL in 1971, but only 6 

 larvae/ 1,000 eggs were estimated to have sur- 

 vived to that stage in 1973. 



ACKNOWLEDGMENTS 



1. Spawning by thread herring in the eastern 

 Gulf of Mexico occurred from February to Sep- 

 tember, based on catches of larvae from March 

 through September and eggs from May through 

 August. Most spawning took place from April to 

 August in depths <30m, within 50 km of the coast. 

 Spawning was most intense between lat. 26°00'N 

 and 28°00'N (Fort Myers to Tampa Bay, Fla.). 



2. Eggs were collected when surface tempera- 

 tures ranged from 22.5° to 30.3°C and when sur- 

 face salinities were 32.4 to 36.8%o. Larvae ^=5.0 

 mm SL were collected at surface temperatures 

 from 18.5° to 30.9°C and at surface salinities from 

 27.3 to 36.9%o. Most eggs and =£5.0-mm larvae 

 were taken when surface temperature exceeded 

 25°C and when surface salinity was above 35.0%o. 



3. Estimates of adult biomass ranged from 

 108,000 to 372,000 metric tons in 1971 and 1973. 

 The 0.95 confidence intervals on 1971 and 1973 

 estimates range from 72,800 to 428,800 metric 

 tons. 



4. The estimated concentration of adult thread 

 herring biomass from the coast to the 50-m depth 

 contour was in the range of 6.8 to 40.2 kg/ha. The 

 total area in which thread herring occurred was 

 106.7 x 10 5 ha. 



People and agencies that were acknowledged for 

 their support of this project by Houde (1977a) are 

 thanked once again. Harvey Bullis reviewed an 

 early draft of the paper. This research was spon- 

 sored by NOAA Office of Sea Grant, U.S. Depart- 

 ment of Commerce, under Grant 04-3-158-27 to 

 the University of Miami. 



LITERATURE CITED 



AHLSTROM, E. H. 



1954. Distribution and abundance of egg and larval popu- 

 lations of the Pacific sardine. U.S. Fish Wildl. Serv., 

 Fish. Bull. 56:83-140. 



1959. Vertical distribution of pelagic fish eggs and larvae 

 off California and Baja California. U.S. Fish Wildl. 

 Serv., Fish. Bull. 60:107-146. 



1968. An evaluation of the fishery resources available to 

 California fishermen. In The future of the fishing indus- 

 try of the United States, p. 65-80. Univ. Wash. Publ. 

 Fish., New Ser. 4. 



ALVERSON, D. L., AND W. T. PEREYRA. 



1969. Demersal fish explorations in the northeastern 

 Pacific Ocean — an evaluation of exploratory fishing 

 methods and analytical approaches to stock size and yield 

 forecasts. J. Fish. Res. Board Can. 26:1985-2001. 



BANNISTER, R. C. A., D. HARDING, AND S. J. LOCKWOOD. 

 1974. Larval mortality and subsequent year-class 



510 



