Murphy and Chittenden Reproduction, age and growth, and movements of Pepnlus burti 



103 



stage were also determined for randomly selected 

 fish in October 1978 and during December 1979-Feb- 

 ruary 1980 to verify spawning seasons. All lengths 

 presented are fork lengths, and all length frequen- 

 cies are moving averages of three, unless specified 

 otherwise. 



We also made collections in the northcentral Gulf, 

 at depths of 9-91 m, aboard the FRS Oregon II east 

 (89°00'W-89°30'W) and west (89°30'W-91°30'W) of 

 the Mississippi River during 10-19 April 1980 and 21 

 April- 1 May 1980, respectively (Rohr et al. 1980). Fish 

 were sampled from these catches without randomiza- 

 tion and measured (FL) to compare with size and age 

 compositions in the northwestern Gulf. 



Age was determined by length-frequency analysis, 

 that is, the modal group progression analysis modifica- 

 tion (Jearld 1983) of the Petersen method (Lagler 1956, 

 Tesch 1971). Cohorts were specified by the season and 

 year when they hatched, for example, Fall 79 . Cohorts 

 hatched during the periods January-May and Septem- 

 ber-October are hereafter referred to as Winter and 

 Fall cohorts, respectively; this abbreviated terminology 

 corresponds with the major spawning each period (see 

 "Maturation and Spawning Periodicity"). Arithmetic 

 means were used to describe central tendencies in 

 cohort length frequencies. 



Mean hatching dates used to approximate time scales 

 to calculate growth were determined by a one-step 

 iterative process following Standard and Chittenden 

 (1984). A hatching date of 1 March was assigned to 

 start iterations for the Winter 79 and Winter 80 cohorts 

 because, in those groups, fish 30-75 mm, assumed to 

 be 2-4 months old, first appeared mid-May to mid-June. 



A hatching date of 1 February was assigned to start 

 iterations for the Winter 78 cohort because, in that 

 group, fish 20-50 mm, assumed to be 1-3 months old, 

 became abundant in mid- April. A hatching date of 1 

 September was assigned to start the approximation for 

 Fall cohorts because fish 30-75 mm, assumed to be 2-4 

 months old, first appeared November through early 

 December. Quadratic regression of length-on-age in 

 days was then used to estimate initial x-intercepts for 

 each cohort. Final hatching dates were calculated by 

 using x-intercepts to readjust the initial x-variable 

 (time) scale, so that each final quadratic growth curve 

 passed through the origin (Table 2). Descriptions of 

 spawning periodicity using length frequencies assume 

 early size and age combinations (see Results) predicted 

 from quadratic regression of length-on-age pooling all 

 Winter and Fall cohorts (Table 2). Von Bertalanffy 

 growth equations were fit to length and age (in years) 

 data using the nonlinear least-square parameter esti- 

 mation procedure in FSAS (Saila et al. 1988). Data 

 points described a curvilinear regression and evidenced 

 an asymptote, so equations presented met the mini- 

 mum requirements for a von Bertalanffy fit (Knight 

 1968, Gallucci and Quinn 1979). 



Maximum age was approximated by the Beverton- 

 Holt model parameter t L (Gulland 1969) following the 

 definition that only 0.5-1.0% of the catch exceeds age 

 t L or its corresponding length (Alverson and Carney 

 1975, DeVries and Chittenden 1982). Maximum sizes 

 and ages, and sizes-at-age, presented are termed ap- 

 parent, because they may be affected by emigration of 

 fish approaching age I; if so, they underestimate max- 

 imum size and age, and sizes-at-age. 



