MORSE: SPAWNING AND FECUNDITY OF ATLANTIC MACKEREL 



stream of water and agitated in a 0.20 mm mesh 

 sieve. Following removal of the ovarian tissue, the 

 eggs were air dried on blotter paper for 2-3 min 

 and weighed (±0.01 g), and two subsamples were 

 removed and weighed (±0.01 mg). All eggs in each 

 subsample were counted and the mean used to 

 calculate total egg numbers based on the weight of 

 all eggs in the ovary. If the two subsample counts 

 differed by 10*^ or more, additional samples were 

 taken until two counts differed by <10%. 



Ages were determined from otoliths as de- 

 scribed by Steven (1952). 



RESULTS 



The allometric relationships of length-weight 

 were expressed by the power function: 



Y =aX^ 



(1) 



where X is length, Y is weight, and a and h are 

 constants. Equation (1) was converted to the 

 linear form by a logarithmic (base 10) transforma- 

 tion to: 



log Y = log a + 61og X 



(2) 



The interrelationships between length mea- 

 surements and between weight measurements 

 were expressed by the linear function: 



Y = a + bX 



(3) 



where Y andX are both length or both weight. All 

 data were fitted using least-squares regression 

 techniques. 



Predictive regression equations were calculated 

 using all observations for males and females and 

 an analysis of covariance applied to determine 

 possible sex related differences. No significant dif- 

 ferences iP = 0.05) were indicated between sexes 

 and sexes were therefore pooled. The pooled re- 

 gression equations and associated statistics are 

 presented in Table 3. 



To determine the peak spawning time the mean 

 gonad somatic index (GSI = percent ovary weight 

 of the gutted weight) was calculated for each week 

 of the sampling period (Figure 3). It appears that 

 individual fish attain their maximum GSI just 

 prior to spawning the first egg batch and a decline 

 in GSI occurs as successive batches are spawned. 

 This was shown by comparing the mean GSI of 

 each maturity stage (Table 4) which showed an 



Table 3. — Length and weight relationships of Atlantic mack- 

 erel collected in the Middle Atlantic Bight, 1977. TW = total 

 weight (grams); GW = gutted weight I grams); TL = total length 

 (millimeters); and FL = fork length (millimeters). Symbols refer 

 to the equation Y = a + bX; n = sample size; r = correlation 

 coefficient; S = standard deviation about the line. 



y a b X n r Sy^ 



Curvilinear relationships between transformed variates 



log TW 

 log GW 

 logTW 

 log GW 



TL 

 TW 



-5.767 

 -5.420 

 -5.780 

 -5.374 



3.275 

 3.106 

 3.334 

 3.140 



log TL 

 log TL 

 log FL 

 logFL 



966 

 966 

 966 

 966 



0.905 

 0.924 

 0905 

 0.924 



Linear relationships between untransformed variates 



1.793 1.098 FL 966 0.986 



-20.410 1.282 GW 966 0.979 



0.036 

 0.030 

 0.036 

 030 



3.594 

 22.397 



19-25 



APRIL 



Figure 3. — Mean gonad somatic index (ovary weight as a per- 

 cent of gutted weight) plotted by week for Atlantic mackerel 

 sampled in 1977. Numbers in parentheses are sample sizes. 



Table 4. — Mean gonad somatic index (GSD and standard devia- 

 tion for each maturity stage of Atlantic mackerel. 



stage 



Mean GSI 



so 



1 . Developing 



2. Ripe 



3. Running ripe 



4. Partially spent 



10.4 



15.0 



24.9 



8.6 



increase from stage 1 to 3 and a rapid decrease at 

 stage 4. Similar results were reported by Kaiser 

 (1973 ) for horse mackerel, Trachurus murphyi. He 

 found that gonad somatic indices reflected mat- 

 uration changes of the ovaries and a sharp decline 

 in the mean GSI coincided with the appearance of 

 the earliest spawning females. In this study the 

 weekly mean GSI increased during the first 3 wk 

 of sampling, peaked between 21 April and 4 May, 

 and then declined steadily through the end of the 

 sampling (22 May). All females examined from the 

 last sampling week were partially spent and indi- 

 cated spawning was nearly completed within the 

 study area. 



105 



