FISHERY BULLETIN: VOL. 84, NO. 3 



apparently spawned all of their hydrated eggs and 

 they are then classified as stage 1 or 2. The mean- 

 ing of stage 7 in female anchovy remains a mystery. 



The seasonal and diurnal patterns described above 

 indicate that, with the exception of stages defined 

 by the presence of hydrated eggs, gross anatomical 

 maturity stages have little utility other than describ- 

 ing the seasonality of spawning. However, field iden- 

 tifications of the presence of hydrated eggs, if they 

 are calibrated with histological data and if the diur- 

 nal pattern of hydration is known, can potentially 

 be used to determine spawning incidence. 



Several authors have pointed out that females with 

 hydrated eggs and actively spawning females were 

 more numerous than females with day-1 postovula- 

 tory follicles, and have suggested that hydrated and 

 actively spawning females may be more susceptible 

 to capture due to behavioral or physiological factors 

 (Hunter and Goldberg 1980; Stauffer and Picquelle 

 1980; Alheit et al. 1984). Previous workers have 

 therefore used the percentage of day-1 females as 

 the index of the daily spawning incidence The over- 

 all percentages of day-0 and day-1 females in the 

 histology data set (8,672 females) used in our 

 analyses are 15.9 and 11.5%. Alheit et al. (1983) 

 found the overall percentages of day-1 and day-2 

 Peruvian anchoveta females to be 17.26 and 14.81%. 

 Hunter and Goldberg (1980), and subsequent 

 workers on the northern anchovy, took their samples 

 at night whereas Alheit et al. (1983) took their Peru- 

 vian anchoveta samples primarily during the day. 

 Therefore the definition of day-1 is somewhat dif- 

 ferent in studies of the two species. In our analysis 

 both day-0 and day-1 females appear to be more 

 susceptible to capture by midwater trawl in the early 

 evening than later at night. The percentages of both 

 decline as the night progresses; however, the decline 

 is more extreme in the day-0 females (Fig. 8). 



The use of maturity stages 5 + 6 could result in 

 several sources of bias that would tend to produce 

 overestimates of the spawning incidence of northern 

 anchovy. If females with hydrated eggs are more 

 susceptible to capture, there will be a tendency to 

 produce biased estimates. However, this bias would 

 not be expected to be size or age dependent, nor 

 would it be expected to vary during the spawning 

 season. The same bias would be expected to occur 

 in 1- and 4-yr-old hydrated females and the same bias 

 would be expected in February and April. Therefore 

 the use of the percentages of hydrated females or 

 maturity stages 5 + 6 females may result in over- 

 estimates of the total spawning incidence or annual 

 fecundity, but the relative spawning incidence or 

 relative annual fecundity of the different age groups 



would not be biased. A second source of bias is that 

 an unknown number of females have ovaries so small 

 that visual determination of sex is impossible with- 

 out magnification. Therefore, the incidence of spawn- 

 ing is overestimated because it is calculated by 

 dividing the number of stages 5 + 6 females by less 

 than the total number of females. This bias is size 

 and age dependent, being much more common in 

 smaller and younger anchovy, but not month depen- 

 dent. Note that the various studies of the spawning 

 incidence in northern anchovy and Peruvian an- 

 choveta have defined the spawning incidence to be 

 the number of females spawning per day divided by 

 the number of mature females, i.e., these studies ex- 

 clude immature females, which are primarily the 

 smaller and younger fish, from the calculation. 



There are also several sources of bias that would 

 tend to produce underestimates of the spawning in- 

 cidence The anchovy fishery in southern California 

 primarily occurs at night during the fall months and 

 during the daylight hours in the spring. A period of 

 low availability to the commercial fishery is asso- 

 ciated with the spawning season. Mais (1974) asso- 

 ciated this phenomenon with variation in schooling 

 behavior and showed that acoustic surveys detect 

 relatively few "commercial-sized" anchovy schools 

 during the spawning season. If low availability to the 

 commercial fishery is associated with spawning ac- 



t 1 — i 1 1 1 1 1 1 1 r 



1800 2000 2200 2400 200 400 



TIME OF NIGHT 



Figure 8.— The percentages of day-0 and day-1 female northern 

 anchovies by time of night. 



512 



