HOUDE: ABUNDANCE AND POTENTIAL YIELD OF THREAD HERRING 



ing only 1971 and 1973 estimates of egg abun- 

 dance and their respective variances, the 0.95 

 confidence intervals on thread herring biomass 

 during those years ranged from 72,814 to 428,758 

 metric tons. 



The area adjustments that corrected egg abun- 

 dance estimates for four 1971 and 1972 cruises had 

 a relatively minor effect on biomass estimates in 

 those years. Corrected estimates, presented in 

 Table 3, exceeded uncorrected estimates by 3,060 

 metric tons in 1971 and by 11,946 metric tons in 

 1972. 



Method II 



An estimate of annual spawning also was ob- 

 tained by a modification of Simpson's (1959) 

 method (Houde 1977a). Biomasses of adult thread 

 herring were then estimated (Table 4); they were 

 108,139 metric tons in 1971, 45,048 metric tons in 

 1972, and 325,803 metric tons in 1973. 



Most Probable Biomass 



If the 1972 estimates are not considered, the 

 most likely adult thread herring biomass in the 

 eastern Gulf during 1971-73 was between 100,000 

 and 400,000 metric tons. Yearly fluctuations in 

 thread herring biomass may be significant in the 

 eastern Gulf of Mexico but the size of such fluctua- 

 tions could not be determined. Severe red tides, 

 which are common in the area, and hurricanes are 

 just two phenomena occurring during summer 



months that might affect annual recruitment, 

 causing significant year-class fluctuations. But, 

 during the years of this study it seems unlikely 

 that the stock of adult thread herring exceeded 

 430,000 metric tons and it probably was less than 

 that amount. These estimates represent only a 

 part of the Gulf of Mexico thread herring popula- 

 tion. Large sotcks exist in the northern and west- 

 ern Gulf that are not included in the estimates. 

 Also, juvenile thread herring biomass is not in- 

 cluded and it may constitute a significant part of 

 the population that could be harvested by a 

 fishery. 



Concentration of Biomass 



If thread herring adults were evenly distributed 

 from the coastline to the 50-m depth contour in 

 1971 and 1973, an area of 106.7 x 10 5 ha, the 

 concentration of biomass would be in the range of 

 6.8 to 40.2 kg/ha, based on adult biomass esti- 

 mates and the 0.95 confidence interval on those 

 estimates. The estimated thread herring biomass 

 concentration is less than that for round herring 

 (Houde 1977a) which ranged from 14.1 to 102.3 

 kg/ha. Round herring occur in a smaller area of 

 the eastern Gulf than thread herring; the round 

 herring being mostly confined to the 30- to 200-m 

 depth zone which is 76.5 x 10 5 ha. Thread herring, 

 although less concentrated, are highly visible be- 

 cause of surface schooling behavior and also are 

 presumably more accessible to a potential fishery 

 because they are found nearer to the coast in shal- 

 lower water. 



TABLE 4. — Annual spawning and biomass estimates for thread 

 herring from the eastern Gulf of Mexico during 1971, 1972, and 

 1973. Estimates are based on the method described by Simpson 

 (1959). The 1972 estimate is unreliable because a hurricane cur- 

 tailed survey cruise GE7210 during the peak of the spawning 

 season. 



Year 



1971 



1972 



1973 



Cruise 



Daily spawning 



estimate 

 (eggs x 10") 



Annual spawning 



estimate 



(eggs x 10") 



321.172 



133.793 



967.636 



Adult biomass 

 (metric tons) 



108,139 



45,048 



325,803 



Potential Yield to a Fishery 



Estimates of annual potential yield of adult 

 thread herring from the eastern Gulf range from 

 27,506 to 186,184 metric tons (Table 5). Estimates 

 were obtained from C max = XMB where M, the 

 natural mortality coefficient, was assigned three 



TABLE 5. — Range of potential yield estimates for eastern Gulf of 

 Mexico thread herring, based on biomass estimates in 1971 and 

 1973 by the Sette and Ahlstrom (1948) method. Yields are pre- 

 dicted at three possible values of M, the natural mortality coef- 

 ficient. Biomass estimates were obtained from values in Table 3. 



Year 



Biomass 



estimate 



(metric 



tons) 



Estimated potential annual yields 

 (metric tons) for given values of M 



M=0.5 



M=0.75 



M = 1.0 



1971 

 1973 



Mean of 1971 

 and 1973 



110,024 

 372,367 



241,196 



27,506 

 93,092 



60,299 



41,259 

 139.638 



90,448 



55,012 

 186,184 



120,598 



503 



