.AROC'HK ETAL.: AGE AND GROWTH OF PAROPHRYS VETULUS 



Table 1. — Comparison of mean otolith diameters (OD) of laboratory-reared 

 and field-collected Pnrophrys vetulus larvae. Age of reared larvae represents 

 days from hatching. 



to consistently form growth increments when 

 maintained on low rations (Methot and Kramer 

 1979). In P. vetulus, delayed inception of incre- 

 ment formation, up to 8 d after hatching, may 

 also have accounted for some of the apparent 

 irregularity in increment formation in the lab- 

 oratory (Table 2). Another factor contributing to 

 ambiguity of laboratory results was the diffi- 

 culty in counting otolith increments in older 

 larvae. Increments in most laboratory-reared 

 fish after 16-25 d were exceedingly faint and, in 

 some fish, no increments could be discerned 

 (Fig. 2a, b). Growth increments were, in gen- 

 eral, clearer and more distinct on the otoliths of 

 field-caught P. vetulus larvae than on otoliths of 

 laboratory-reared fish (Figs, lc, 2c). The steady 

 increase in number of increments with increas- 

 ing otolith diameter and length of pretransfor- 

 mation larvae in the field is evidence that the 

 irregularity in increment formation observed in 

 the laboratory did not occur under natural feed- 

 ing conditions (Figs. 3, 4). 



Age and Growth 



Age of field-caught P. vetulus larvae in days 

 from hatching was estimated by adding 5, the 

 number of days prior to appearance of the first 



otolith growth increment, to the number of in- 

 crements counted on sagittae. Counts of growth 

 increments were obtained from 338 larval and 

 transforming, pelagic specimens ranging from 

 2.4 to 20.0 mm SL (Fig. 4). But age could be esti- 

 mated for only 331 larvae because increment 

 formation had not yet begun in seven small speci- 

 mens, 2.4-3.7 mm SL (Fig. 5). The oldest P. 

 vetulus taken in plankton samples during 1977- 

 78 was 74 d (2.4 mo) old and 17.8 mm SL. The 

 next oldest larvae ranged from 65 to 70 d old and 

 were 19-20 mm SL. The length of pelagic life of 

 P. vetulus can be estimated directly from these 

 data to be 2-2.5 mo. Few P. vetulus larvae >20 

 mm SL, the size at which larvae transform to 

 benthic juveniles (Ahlstrom and Moser 1975; 

 Rosenberg and Laroche footnote 3), were taken 

 in extensive plankton collections off Oregon 

 during the spring months in 1972-75 (Laroche 

 and Richardson 1979). The largest larva taken in 

 those collections was 22 mm SL. 



Behavior of reared P. vetulus larvae further 

 supports a pelagic phase of 2+ mo. At approxi- 

 mately 60 d of age, larval P. vetulus maintained 

 in the laboratory first exhibited the tendency to 

 rest on their sides on the bottom and to swim with 

 their bodies at an angle to the vertical (J. L. 

 Laroche unpubl. data). 



Table 2.— Summary of growth in body length (SL) and otolith 

 diameter (OD), and counts of growth increments on otoliths of 

 laboratory-reared Parophrys vetulus larvae. N= number of larvae 

 from which growth increment counts were taken; (N) - number of 

 larvae used in mean otolith diameter calculation. 



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