HOUDE: ABUNDANCE AND POTENTIAL YIELD OF ROUND HERRING 



5) Abundance estimates for each class are cor- 

 rected for duration. This is necessary to esti- 

 mate the number present at mean age in that 

 class. If exponential growth holds, the number 

 of larvae in each successive age group will have 

 been underestimated before the duration cor- 

 rection was made, because the time spent by 

 larvae in successive length classes is decreas- 

 ing. The correction is made by dividing the 

 abundance estimates of each class (including 

 the egg stage) from step 2 above by their dura- 

 tions, given in step 3. 



6) The instantaneous mortality coefficient is then 

 calculated for each combination of mean daily 

 growth increment, assigned egg stage dura- 

 tion, and assigned nonfully vulnerable larvae 

 duration. It is estimated from the exponential 

 regression of night-day-corrected and 

 duration-corrected abundances on mean age 

 and is fitted for all age-classes that were 

 adequately represented in the data, excluding 

 nonfully vulnerable larvae. For round herring 

 the regression was fit for age-classes including 

 the egg stage and larvae ranging from 4.1 to 

 16.0 mm SL. 



N, = N exp( -Zt) 



(16) 



where Z = the instantaneous coefficient of rate of 

 decline in catch. It is the instantaneous 

 mortality coefficient if factors such as 

 gear avoidance are not significant con- 

 tributors to the decline in catch as lar- 

 vae grow older 

 N t = the number of eggs or larvae at time t 

 N — they-axis intercept; it is an estimate of 

 abundance at time (i.e., the number 

 of eggs that was spawned) 

 t = the time (in days) from spawning. 



7) Mortality with respect to length also is esti- 

 mated in the exponential regression of night- 

 day-corrected abundance on length. Only fully 

 vulnerable length classes were used in this cal- 

 culation. For round herring, larvae from 4.1 to 

 16.0 mm SL were included in the analysis. 



N L = N A exp(-ZL) 



(17) 



where Z = the instantaneous coefficient of rate of 

 decline in catch. It is the instantaneous 

 mortality coefficient per millimeter of 

 standard length if factors such as gear 



avoidance do not contribute sig- 

 nificantly to decline in catch as larvae 

 grow. 



Nl = the number of larvae of length L 



N A = they-axis intercept 

 L = the standard length (millimeters) of 

 larvae. 



RESULTS 



Occurrence of Eggs and Larvae 



Eggs and larvae of round herring were collected 

 on cruises from November to May (Table 1), and 

 were most common in January and February. 

 They did not occur in cruises from June through 

 September, indicating that there is no spawning 

 during summer in the eastern Gulf of Mexico. 

 Most eggs and larvae were collected on the outer 

 continental shelf (Figure 2) where depths ranged 

 from 30 to 200 m. Eggs occurred on only two occa- 

 sions at stations less than 30 m deep and on a 

 single occasion at a station deeper than 200 m 

 (Figure 2), although relatively little sampling ef- 

 fort was made at stations beyond the 200-m depth 

 contour. Occurrences of larvae were more wide- 

 spread (Figure 2), as expected due to dispersal by 

 water currents, but most occurrences remained 

 within the 30- to 200-m depth zone. The observed 

 distribution suggests that most of the adult popu- 

 lation is found on the outer shelf, at least during 

 the spawning season. A major spawning center is 

 located between long. 082°30' and 084°30'W and 

 lat. 27°00' and 28°00'N (Figures 2-6). The location 

 is about 150 km from Tampa Bay in a west by 

 southwest direction. This is the same general area 

 where round herring adults were trawled in 

 exploratory fishing surveys (Anonymous 1958; 

 Salnikov 1969). There is evidence that a second 

 minor spawning center is found between long. 

 082°00' and 083°30' W and lat. 24°45' and 25°30'N. 

 This location is just north of the Dry Tortugas 

 Islands. 



The cruise means for numbers of round herring 

 eggs under 10 m 2 ranged from 0.00 to 151.20 for 

 the 17 cruises in the survey (Table 1). Considering 

 only positive stations (i.e., stations where round 

 herring eggs were collected on a cruise), cruise 

 means ranged from 8.30 to 604.81 under 10 m 2 of 

 sea surface (Table 1). Catches at individual sta- 

 tions frequently ranged from 11 to 1,000 under 10 

 m 2 but exceeded 1,000 on only three occasions 

 during the 17 cruises (Figures 3-6). Round herring 



69 



