HOUDE: ABUNDANCE AND POTENTIAL YIELD OF THREAD HERRING 



losses per millimeter increase in length are 29.9% 

 in 1971 and 32.6% in 1973. Confidence limits on Z 

 at the 0.95 probability level were Z ± 0.0816 in 

 1971 and Z ± 0.1385 in 1973. The mortality 

 coefficients did not differ significantly between 

 years tf-test; P>0.50). 



Mortality coefficients for round herring larvae 

 per millimeter increase in length were Z = 0.2269 

 in 1971-72 and Z = 0.3647 in 1972-73 in the east- 

 ern Gulf of Mexico (Houde 1977a). Larval mortal- 

 ity of scaled sardines in 1973 was Z = 0.3829 

 (Houde 1977b), which is nearly identical to that 

 for thread herring. Lenarz (1973) reported ranges 

 of instantaneous coefficients for abundance at 

 length data to be 0.15 to 0.33 for Pacific sardine, 

 Sardinops caeruleus, and from 0.32 to 0.46 for 

 northern anchovy, Engraulis mordax, larvae. The 

 Pacific sardine coefficients were lower than those 

 for thread herring, but the anchovy coefficients 

 were similar to thread herring coefficients. Ma- 

 tsuura (in press) obtained a high instantaneous 

 coefficient of Z = 0.4962 for Brazilian sardine, 

 Sardinella brasiliensis, which is higher than any 

 values observed for Gulf of Mexico clupeid larvae. 



To obtain estimates of larval mortality relative 

 to age rather than length, an exponential growth 

 model was used to estimate age at length for 

 thread herring larvae, given various mean daily 

 growth increments during the larval stage. Mean 

 daily growth increments of eastern Gulf clupeid 

 larvae probably range from 0.3 to 1.0 mm based on 

 laboratory rearing experiments for some species 

 (Richards and Palko 1969; Saksena and Houde 



1972; Saksena et al. 1972; Houde 1973b; Houde 

 and Swanson 1975). At temperatures above 26°C, 

 healthy larvae grew, on average, more than 0.5 

 mm/day. Duration of the egg stage for thread her- 

 ring is about 0.84 days. The duration of nonfully 

 vulnerable length classes also was estimated be- 

 fore mean age of each fully vulnerable 1-mm 

 length class was calculated. Nonfully vulnerable 

 length classes were 1.1 to 4.0 mm in 1971 and 1.1 

 to 5.0 mm in 1973. The duration of these stages in 

 thread herring probably is from 1.0 to 3.0 days and 

 4.0 to 6.0 days, respectively, based on evidence 

 from laboratory rearing of similar clupeid larvae 

 (Houde et al. 1974; Houde and Swanson 1975). 

 Eastern Gulf clupeid larvae quickly attain 4.0 mm 

 length during the first day after hatching, but 

 show no further growth in length until the fourth 

 day after hatching. No direct observations of stage 

 duration for thread herring larvae 5.0 mm or less 

 in length were available from laboratory experi- 

 ments but their growth pattern during this stage 

 probably does not differ from that of other 

 clupeids. Stage durations of nonfully vulnerable 

 length classes were assigned based on observa- 

 tions of the other species. Methods and details of 

 the mortality estimating procedure were given by 

 Houde (1977a). 



Two examples of duration-corrected abundance 

 data assuming exponential growth of fully vul- 

 nerable larval length classes up to 19.0 mm in 

 1971 and 20.0 mm in 1973 are given in Table 7. In 

 these examples, the mean daily growth increment 

 was assumed to be 0.8 mm. Sets of such abundance 



TABLE 7. — Two examples of data from 1971 and 1973 used to obtain stage duration, mean age, and duration-corrected abundance of 

 thread herring eggs and larvae. Duration-corrected abundances were subsequently regressed on mean ages to obtain mortality rates 

 (Table 8). Abundance estimates in the second column of the Table were previously corrected for daytime avoidance. In these examples, 

 the mean daily growth increment (6) was set at 0.80 mm. The nonfully vulnerable size classes were 1.1 to 4.0 mm in 1971 and 1.1 to 

 5.0 mm in 1973. Calculating procedures were given in Houde (1977a), Equations (12) to (16). Regressions for these data are presented 

 in Figure 18. 



507 



