HOUDE: ABUNDANCE AND POTENTIAL YIELD OF ROUND HERRING 



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2.1- 4 1- 6.1- 8.1- 10.1- 12.1- 14.1- 161- 181- 

 30 50 70 90 110 13 15 17.0 19 

 STANDARD LENGTH CLASSES (mm) 



FIGURE 9. — Length-frequency distributions of round herring 

 larvae for 1971-74 cruises to the eastern Gulf of Mexico. Fre- 

 quencies are expressed as estimated abundance of larvae in each 

 length class within the area represented by the cruise. 



by length classes were examined over all cruises 

 and they indicated that considerable net avoid- 

 ance was occurring in the day relative to that 

 occurring at night. The data were plotted by 2-mm 

 length classes (Figure 10), and functions were 

 fitted to allow estimation of the night-caught to 

 day-caught ratio for larvae in any length class. 

 The ratio increased rapidly for larvae of 4.0-13.0 

 mm, but then decreased from a factor of more than 

 3.0 to about 1.0 when larvae had grown to 18.0 mm. 

 Two power functions were fitted: for larvae 2.1- 

 14.0 mm SL the function was R = 0.3041 X° ' 9115 , 

 where R is the ratio of night-caught to day- 

 caught larvae andX is standard length of larvae; 

 for 12.1- to 20.0-mm SL larvae the function was 

 R = 44,521.54 X" 37298 . Larva catches made at 

 daytime stations were adjusted by R (Equation 

 (ID). Exponential functions or a single poly- 

 nomial could have been used in place of the power 

 functions to describe the relationship, but the 

 power functions provided reasonably good fits to 

 the data and were acceptable for correction pur- 

 poses. No adjustments were made for larvae <4.0 

 mm or > 18.0 mm because there was no observable 

 difference in night or day catches for larvae of 

 those lengths. 



The round herring larvae night to day catch 

 ratios are unusual with respect to the observed 



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Y- 0.3046X 09 " 5 



Y- 44521. 54X 



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7.0 9.0 110 130 150 



MIOPOINT OF LENGTH CLASS (mm) 



17 19 



FIGURE 10. — Night to day ratios of sums of catches, standardized 

 to numbers under 10 m 2 of sea surface, for round herring larvae 

 collected in 1971-73 in the eastern Gulf of Mexico. The ratios 

 were calculated for larvae within each 2-mm length class from 

 2.1 to 20.0 mm SL. Fitted power functions describe the relation- 

 ships for larvae from 2.1 to 13.0 mm SL and for larvae from 

 13.1 to 20.0 mm SL. Larval abundance estimates for each length 

 class at stations occupied during daylight were corrected by the 

 appropriate ratio factor for each length class to account for 

 daytime avoidance. 



81 



