DURBIN ET AL: MENHADEN AGE, GROWTH. AND CHEMICAL COMPOSITION 



haden population. Such a reduction in size-at- 

 age could result from a number of factors, in- 

 cluding poor growth during age only, followed 

 by normal growth rate; an overall decline in the 

 mean growth rate of all age groups; or a shift in 

 the relative proportions of different spawning 

 groups within the population (see June 1965; 

 Nicholson 1972), where faster growing individ- 

 uals have declined and been replaced by slower 

 growing individuals. 



Growth During 1976 



Instantaneous Daily Growth Rate 



Mean instantaneous daily growth rates of 

 menhaden caught in Narragansett Bay during 

 1976 were estimated from the seasonal increase 

 in mean size of the fish. Such estimates, based on 

 successive samples from a population, assume 

 that the fish were initially of similar size and 

 that there was no significant influx of new fish, 

 with different growth histories, into the region 

 during the study period; these conditions are 

 difficult to meet with a free-ranging fish such as 

 the menhaden. However, we have evidence that 

 these conditions were met, at least for a 1-mo 

 period during the study. First, back-calculated 

 fork lengths at the most recent annulus indicated 

 that menhaden caught in Narragansett Bay 

 were of similar length at the start of the 1976 

 growing season (Fig. 1). Second, daily observa- 

 tions by the menhaden spotter pilots suggest that 

 our samples collected between 3 August and 1 

 September were derived from a single group of 

 menhaden. Many large schools were observed 

 moving into Narragansett Bay during the week 

 of 26 July. No significant additional movement of 

 schools into or out of the bay was observed until 7 

 September, when large schools were again seen 

 entering the bay. Uniformity of the back- 

 calculated fork lengths of the menhaden 

 sampled during this period (Fig. 1) supports the 

 fishermen's opinion that the same group of fish 

 was being sampled. The influx of new fish into 

 the area, observed by the commercial fishermen 

 on 7 September, was accompanied by an abrupt 

 shift in the mean and variance of back-calculated 

 fork lengths of age 3 menhaden on 7-8 Septem- 

 ber, presumably because of the mixing of new 

 arrivals with those already present (Fig. 1). 



Daily growth rates of age groups 2 and 3, the 

 most abundant age groups in the samples, were 

 estimated for the period 3 August- 1 September 



270 

 250 

 230 

 210 

 190 

 170 

 ^ 150 



* 270 



O 



U. 250 



E 

 E 



I 

 h- 



o 





< 



UJ 



230 

 210 

 19 

 170 

 150 



Age 3 



• x fork length at capture 



o x back-calculated fork length 



----Slope = 0.002l23 



Slope = OOOI854 



9 



n% //* 



§ ni 



_L 



Age 2 



~ m x fork length at capture 

 o x back-calculated fork 

 ----Slope = 0002244 

 Slope = 0.002l79 



J //- 



// 



f*T 



J^ 



10 20 30 10 20 30 

 JUNE JULY 



- 1 ' 4 ' // "L //"L 



10 20 30 10 7 4 



AUG SEPT OCT NOV 



Figure 1.— Mean fork length ±95% confidence limits of At- 

 lantic menhaden collected from Narragansett Bay during 

 1976. Curves depict the instantaneous daily growth in length 

 (Table 4, Equations (9)-(12)). 



from 1) rates of increase in mean fork length and 

 wet weight during this period (Figs. 1, 2) and 2) 

 growth rate of the scale margin beyond the 1976 

 annulus (Fig. 3). 



en 



380 

 340 

 300 

 260 

 220 

 180 

 140 

 3 100 

 "o3 340 



3 



300 

 IX 



260 

 220 



180 

 140 

 100 



Age 3 



---Slope = 00093l5 

 -Slope = 0005888 



-I L- 



J L, I//-L//- 



Age 2 



--Slope = OOI0324 

 Slope =0007133 



itf'\»\ 



10 20 30 10 20 30 

 JUNE JULY 



10 7 4 



10 20 30 

 AUG SEPT OCT NOV 



Figure 2.— Mean wet weight ±95% confidence limits of At- 

 lantic menhaden collected from Narragansett Bay during 

 1976. Curves depict the instantaneous daily growth in wet 

 weight (Table 4, Equations (13H16)). 



137 



