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Fishery Bulletin 92(3), 1994 



G + R = C-M-W, (1) 



where the components (in kilocalories) are 



G = somatic growth; 

 R = reproduction; 

 C = food consumption; 

 M = metabolism; 

 W = fecal loss. 



Assimilated ration (A) equals consumption minus 

 fecal losses. A small fraction of the assimilated en- 

 ergy is lost through nitrogenous excretion (Brett and 

 Groves, 1979) but was not estimated in this study. 

 The remaining portion of the assimilated energy was 

 assumed available for growth and metabolism. The 

 methods used to estimate each parameter are de- 

 scribed below. 



Growth 



The northern sand lance growth rate is 0.87 yr -1 in 

 grams, as determined by back calculation of length 

 at age from otolith increments (Larimer, 1992). Mul- 

 tiplied by 6.02 g, the wet weight of an average fish, 

 this is equivalent to a growth rate of 5.24 gyr -1 . Wet 

 weight was converted to dry weight using the fol- 

 lowing equation (Larimer, 1992): 



dry wt. = 0.309 wet wt. - 0.286 (r 2 =0.859). 



The growth rate (in diy grams) was 1.20 g-fish _1 -yr _1 . 

 Growth in kilocalories was calculated based on a 

 mean caloric content of 6.73 kcaldry gram -1 

 (Larimer, 1992) and was 8.08 kcal-fisrr'yr- 1 . 



Growth was also estimated on a monthly basis. 

 Reay (1972) measured monthly growth in length of 

 age 1+ A. tobianus off the coast of England. These 

 fish and A. dubius are of similar size (range: 84-138 

 mm, Reay, 1972, versus 85-138 mm, Larimer, 1992) 

 and seasonal temperatures in their habitats are simi- 

 lar (3-19C off England, Reay, 1972; 3.4-14.4°C for 

 Georges Bank, Hopkins and Garfield, 1981). There- 

 fore, I assumed that their monthly growth rates 

 would be similar. Reay ( 1972) found that A. tobianus 

 grows from April to October; however, it spawns from 

 February to March, later than the December to Feb- 

 ruary spawning of A. dubius (Bigelow and Schroeder, 

 1953; Norcross et al., 1961; Reay, 1970; Colton et al., 

 1979; Sherman et al., 1984). Nelson and Ross ( 1991) 

 found that gonadal development of A. dubius on 

 Georges Bank was in progress by September. Thus, 

 I assumed that A. dubius weight gain beginning in 

 September is devoted to gonadal rather than to so- 

 matic growth, and therefore, their somatic growing 

 season extends from April to August. 



The percentage of annual growth occurring dur- 

 ing each month of the growing season was calculated 

 from two years of monthly growth data for A. tobianus 

 as reported by Reay (1972). This monthly average 

 was multiplied by the total annual growth measured 

 for A. dubius (8.08 kcal-yr -1 , see above) to determine 

 monthly net growth in caloric content. 



Reproductive energetics 



Gonad weight and caloric content were measured to 

 estimate the portion of the northern sand lance an- 

 nual energy budget devoted to reproduction. In De- 

 cember 1990, eight fish judged to be ripe (stage III, 

 of Macer, 1966) were measured (fork length, mm), 

 and wet weighed (g). The gonads were extracted, 

 weighed and dried. The dried gonads were weighed, 

 ground to a powder with mortar and pestle, and their 

 caloric content measured with a Phillipson 

 microbomb calorimeter (Phillipson, 1964). 



Metabolism 



Metabolism was estimated for an "average" day in 

 each month by using mean monthly water tempera- 

 ture calculated from averages of the top 40 m on 

 Georges Bank (Hopkins and Garfield, 1981) and the 

 number of hours of daylight at mid-month. I assumed 

 the fish actively feed during half of the daylight 

 hours. Thus, I divided a day into three periods: a 

 nighttime resting period equivalent to the hours of 

 darkness, a feeding period that is assumed to be half 

 of the daylight hours, and a postfeeding period that 

 is the remaining half of the daylight hours. Meta- 

 bolic rates were estimated for each of these periods 

 from the rates measured during similar periods at 

 6, 12, and 18°C (Larimer, 1992). The lowest tempera- 

 ture that could be maintained in the lab was 6°C so 

 these temperatures were chosen as the best approxi- 

 mation of the annual temperature range on Georges 

 Bank (Backus and Bourne, 1987). Because there was 

 no clear relationship between metabolic rates and 

 temperature evident in the respiration experiments 

 (Larimer, 1992), the 6°C values were used for April, 

 May, and December, the 12 C values were used for 

 June, July, October, and November, and the 18 C 

 values were used for August and September. 



Assimilation efficiency 



The efficiency of energy assimilation by sand lance 

 was determined by the monthly temperature on 

 Georges Bank (Backus and Bourne, 1987) and the 

 relationship of assimilation efficiency to temperature 

 found in Larimer ( 1992): 



AE = 82.41 + 0.764 T, 



(2) 



