FISHERY BULLETIN: VOL. 80. NO. 1 



not comparable to growth in the field. Despite 

 this, growth increments were visible on the 

 otoliths of over 300 reared larvae. Increments, 

 though extremely narrow and crowded, were 

 even visible on the otoliths of larvae as old as 54 d. 

 In the laboratory, the highest incidence of 

 larvae with one growth increment occurred on 

 days 5 and 6 (Table 1; Fig. la, b). This coincided 

 with the time that larvae first began to swim 

 actively near the surface of rearing containers 

 and search for food. By day 5 larvae had also 

 acquired darkly pigmented, iridescent eyes and 

 functional mouths, and had utilized all or almost 

 all their yolk. Age at first increment formation 

 in the field was ascertained by comparing mean 

 otolith diameter (jum) of field-caught larvae with 

 a single increment, to mean otolith diameter of 

 laboratory-reared larvae of known age (Table 1 ). 

 The otolith diameter, 23.8 nm, of field-caught 

 larvae with only one growth increment (SL = 

 3.7 mm) fell between the mean values for lab- 

 oratory-reared larvae at 5 d, 23.1 (SL = 4.2 mm), 

 and at 6 d, 24.2 (SL = 3.9 mm). Age of all 

 field-caught larvae with one otolith growth 

 increment was, therefore, taken to be 6 d. Age 

 at first increment formation varied among in- 

 dividuals in the laboratory and may, likewise, 

 vary in the field; however, for the purpose of 

 developing a generalized growth model, a single, 

 best estimate of this event was made. The appar- 

 ent smaller size of field-caught larvae with one 

 increment most likely resulted from shrinkage 

 during capture prior to preservation (Theilacker 

 1980). Larvae sampled in the laboratory were 

 pipetted alive directly into preservative, thus re- 

 ducing the amount of handling-induced shrink- 

 age. 



Although laboratory results were somewhat 

 ambiguous, daily periodicity of otolith growth 

 increment formation in P. vetulus was inferred 

 from the following observations: 1) despite less 

 than optimum rearing conditions some 14-, 17-, 

 and 20-d-old larvae had added one increment 

 each day since first formation on day 4 (Table 2); 

 2) no other periodical pattern in increment 

 formation (i.e., other than daily) was observed 

 among laboratory-reared larvae; 3) increment 

 addition among larvae in the sea appeared to 

 follow a stable and uniform pattern. The wide 

 range in number of otolith increments among 

 reared larvae of known age may have been 

 caused by poor growing conditions which re- 

 sulted in stunted body and otolith growth (Table 

 2). Reared larvae of northern anchovy also failed 



a 



Figure 1.— Photomicrographs of Parophrys vetulus otoliths 

 (X 1,000). a. Sagitta (22 ^m in diameter) prior to first in- 

 crement formation from a 4-d-old, laboratory-reared larva; 

 b. Sagitta (24 /iiti in diameter) with two complete increments 

 (highlighted with black lines) from a6-d-old, laboratory-reared 

 larva; c. Sagitta (22 ^m in diameter) with two complete incre- 

 ments (highlighted with black lines) from a 7-d-old, field- 

 caught larva. 



96 



