RATES OF ATRESIA IN THE OVARY OF CAPTIVE AND WILD 

 NORTHERN ANCHOVY, ENGRAULIS MORDAX 



J. Roe Hunter and Beverly J. Macewicz ' 



ABSTRACT 



The process of ovarian atresia was described for northern anchovy using a laboratory group in which 

 atresia was induced by starvation. Atretic characteristics of the ovary were described and illustrated, 

 atretic ovarian states defined, and the rate that the ovary passed from one atretic stage to another was 

 measured. The ovaries of starved females regressed rapidly; 3 days after the onset of starvation the 

 ovaries of about half of the females contained yolked oocytes undergoing resorption of yolk (alpha stage 

 of oocyte atresia) and by 23 days after the onset of starvation no yolk remained in the ovaries of any of 

 the females. Gamma+delta stages of atretic follicles persisted in the ovary for over a month, but their 

 decline in abundance indicated that eventually all signs of past reproductive activity would be lost in 

 regressed ovaries. 



In the natural population, rates of ovarian atresia increased seasonally from only a few percent of the 

 females showing some atresia in peak spawning months to over 50% near the end of the season. 

 Females with low levels of alpha stage atresia (<50% yolked oocytes affected) spawned about half as 

 frequently as did those with no alpha stage atresia. Spavraing was rare (1% of the females) or absent in 

 females with high levels of alpha stage atresia (>50% yolked oocytes affected). Late in the spawning 

 season, it may be possible to forecast the end of spawning in the populations using the frequency of 

 females in the populations with high levels of alpha stage oocyte atresia. Throughout the spawning 

 season atretic rates were higher in small females (standard length <10 cm) than in larger ones 

 indicating that 1-year-old females spawning for the first time have a much shorter spawning season 

 than do older females. 



Four approaches commonly used to determine the 

 reproductive state of female fishes are 1) staging 

 of ovaries using gross anatomical criteria such as 

 the international Hjort scale (Bowers and Holli- 

 day 1961); 2) calculation of the gonosomatic index 

 (GSI), i.e., gonad weight divided by female weight 

 or the equivalent (de Vlaming et al. 1982); 3) es- 

 timating the mean diameter of the oocytes in the 

 most advanced mode of oocytes (Hunter and 

 Goldberg 1980; Hunter and Leong 1981); and 4) 

 classifying ovaries histologically. Histological 

 classification is superior to all other methods. Two 

 of its great strengths are that the frequency of 

 spawning of multiple spawning fish populations 

 can be accurately estimated using the presence of 

 postovulatory follicles (Hunter and Goldberg 

 1980) and that regressing ovaries can be distin- 

 guished from immature and from postovulatory 

 ovaries. The histological criteria used to identify 

 regressing ovaries is the presence of many oocytes 

 and follicles undergoing resorption, a process 

 known as atresia. 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 

 92038. 



Manuscript accepted April 1984. 



FISHERY BULLETIN: VOL. 83, NO. 2, 1985. 



The interpretive power of histological analysis 

 could be enhanced if the process of ovarian atresia 

 were better documented. Specifically, ovarian 

 atretic stages need to be defined, rates of atresia 

 and duration of stages estimated, and the relation 

 between ovarian atretic state and the probability 

 of spawning determined. Such information would 

 facilitate process oriented field studies on re- 

 productive biology, and increase the accuracy of 

 estimates of size at first maturity and size- or 

 age-specific duration of the annual spawning sea- 

 son. 



This study provides the laboratory and field 

 calibration necessary for the assessment of the 

 reproductive state of northern anchovy, Engraulis 

 mordax, using the atretic condition of the ovary. 

 We identify a range of ovarian atretic characteris- 

 tics that define the atretic condition of the ovary, 

 estimate rates of atresia, and estimate the dura- 

 tion that atretic characters persist in the ovaries of 

 starving females in the laboratory. We use this 

 information to classify ovaries of sea-caught 

 females and estimate the probability of spawn- 

 ing for females with various levels of ovarian atre- 

 sia. 



We know of no similar work. A large descriptive 



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