LARIH'HE ETAL.: A(!K AND CROWTH OF PAROPHRYS VETULUS 



model, the parameter L or the y-intercept has 

 been used as an estimator of length at hatching 

 (Zweifel and Lasker 1976). However, the value of 

 this parameter, 2.07 mm SL, for the P. vetulus 

 data set was low compared to mean hatching 

 lengths of reared larvae: 2.60 (N = 11) and 2.91 

 (N = 10) mm SL at 12°-13°C (Laroche unpubl. 

 data); and 2.85 (N = 25) mm TL at 10°-1 1 °C (Orsi 

 1968). Net-caught larvae on which the growth 

 model is based would appear smaller at age be- 

 cause of increased shrinkage during capture 

 (Theilacker 1980). This may account for some of 

 the difference in predicted and observed hatch- 

 ing lengths. Another probable cause of this dis- 

 crepancy is the lack of data points in the <6 d of 

 age region of the plot, i.e., before growth incre- 

 ment formation begins. The value of Lo is based 

 on extrapolation beyond the actual data and may 

 be, therefore, of questionable use as a measure 

 of the appropriateness of this model. 



Comparison with larval growth in the field at 

 similar temperatures of another pleuronectid, 

 Pseudopleuronectes americanus, provided evi- 

 dence that growth rates predicted by the 

 Gompertz model for Parophrys vetulus are 

 realistic. Larval Pseudopleuronectes americanus 

 between the ages of 28 and 42 d, growing in 

 large enclosures in Narragansett Bay at 10°- 

 15°C, had a specific growth rate of 1.9% per day 

 of standard length (Laurence et al. 1979). The 

 predicted specific growth rate of Parophrys 

 vetulus of the same age, growing at9°-ll°C, was 

 2.2%. Larval Pseudopleuronectes americanus be- 

 tween 28 and 42 d of age grew from 6.6 to 8.6 mm 

 SL, while Parophrys vetulus larvae grew from 

 11.2 to 15.3 mm SL. Although these two species 

 differ in size at age, both transform at approxi- 

 mately the same age, 8-10 wk, and appear to 

 grow at similar rates between 4 and 6 wk of age. 

 Since length at hatching, ~2-3 mm SL, is similar 

 for both species, higher rates of growth prior to 

 and after 4-6 wk probably accounts for the 

 greater size at age of P. vetulus and greater size 

 at transformation, >18 mm SL in P. vetulus 

 versus <10 mm for Pseudopleuronectes ameri- 

 canus. 



A comparison of otolith-estimated and length- 

 frequency derived age-at-length data indicated 

 that the latter method overestimated age of 

 Parophrys vetulus larvae >5.5 mm SL by 2-3 

 times. This resulted in a gross overestimate of 

 duration of the pelagic life of this species, 18-22 

 wk (Laroche and Richardson 1979) compared to 

 8-10 wk based on the age data presented here. It 



is unlikely that these large differences are solely 

 the result of different preservatives. Such a large 

 discrepancy between the two methods demon- 

 strates the serious inaccuracies that could result 

 from attempts to estimate age and growth rates 

 from length-frequency data. Such data predict- 

 ably yield low estimates of growth, especially 

 for species with protracted spawning, because of 

 continual recruitment of small larvae to the pop- 

 ulation. Problems of net avoidance by larger 

 specimens further bias length-frequency dis- 

 tributions. 



The otolith aging method developed in this 

 study could be used further to investigate growth 

 and survival among different cohorts of P. 

 vetulus larvae. Spawning in this species is highly 

 variable in both frequency and timing (Laroche 

 and Richardson 1979). Peak spawning can be 

 bimodal in some years with a 2-4 mo separation 

 between peaks (Kruse and Tyler 5 ). Larvae pro- 

 duced in those two peaks could develop and grow 

 under very different temperature regimes and 

 feeding conditions, which could result in two 

 distinct groups of larvae differing in rates of 

 growth, mortality, and relative contribution to 

 that year class. 



ACKNOWLEDGMENTS 



The following individuals are gratefully ac- 

 knowledged for their significant contributions to 

 this study. 



Rindy Ostermann and Betsy B. Washington 

 assisted in all phases of collection of ripe adult 

 specimens, larval rearing, otolith removal, and 

 data compilation. Rae Deane Leatham and Percy 

 L. Donaghay instructed the first author in 

 culture techniques and generously provided the 

 equipment and space in their laboratory for 

 parts of this work. Eric Lynn, National Marine 

 Fisheries Service, NOA A, La Jolla, Calif., sent us 

 cultures of larval food organisms. Gary Hettman, 

 Oregon Department of Fish and Wildlife, kept 

 us informed throughout the fall and winter of the 

 likely locations of English sole spawning con- 

 centrations. Waldo W. Wakefield and Marky 

 Bud Willis assisted at sea in collecting spawning 

 fish. Chip Hogue, Paul Montagua, Gene Ruff, 

 and Andrew Carey provided laboratory space 



5 Kruse, G. H., and A. V. Tyler. 1980. Influence of physical 

 facotrs on the English sole (Parophrys vetulus) spawning 

 season. Unpubl. manuscr., 25 p. Department of Fisheries 

 and Wildlife, Oregon State University, Corvallis, OR 97331. 



103 



