FEEDING ECOLOGY AND GROWTH ENERGETICS OF LARVAL 

 NORTHERN ANCHOVY, ENGRAULIS MORDAX 



Gail H. Theilackeri 



ABSTRACT 



The relation between prey consumption and gross growth efficiency was determined for first-feeding 

 northern anchovy, Engraulis mordax, fed rotifers for 2 weeks. Larval length- and weight-specific daily 

 consumption, given as prey numbers, dry weight, and caloric value, were less for the 2/mL rotifer diet 

 and gut residence time was longer, resulting in a higher gross growth efficiency (0.46) than the 25/mL 

 rotifer diet (0.37). Daily percent increases in dry weight for northern anchovy fed rotifers were 15% 

 at the low density and 21% at the high density. Northern anchovy grew the most, 23% per day, when 

 fed 2/mL copepods, but their length-specific weight was less than those fed on the rotifer diets. Equa- 

 tions for the rotifer and copepod diets are given for calculating growth in length and weight, size-specific 

 stomach contents related to feeding period, and daily food consumption based on empirically determined 

 gastric evacuation rates. Respiration was measured directly and indirectly, by using a starvation analysis 

 to measure the caloric equivalent of metabolism; results from both methods agreed. A power equation 

 was used to express metabolism as a function of dry weight. Estimates of gross growth efficiencies showed 

 that larval northern anchovy may exhibit a high growth rate or a high efficiency, but not both at the 

 same time. Information also is given on increase in size of prey selected as northern anchovy larvae grow. 



Measurements of gross-growth efficiency (calories 

 of growth/calories consumed) of fishes are a good 

 indicator of the adequacy of their diet and state of 

 health (Brett and Groves 1979). Generally a favor- 

 able environment for larval fish growth can be in- 

 ferred by using information on growth efficiencies 

 as related to prey densities. A high-growth efficiency 

 is the result of efficient assimilation of food energy 

 for growth, with relatively little energy lost as feces 

 or used in respiration. 



A wealth of information is available on larval 

 northern anchovy, Engraulis mordax. Research has 

 been directed toward understanding the factors that 

 affect their survival, yet no information exists on 

 the growth efficiency of larval northern anchovy. 



Larval northern anchovy have been cultured in the 

 laboratory, and their growth and survival on wild 

 plankton (Kramer and Zweifel 1970; O'Connell and 

 Raymond 1970) and on cultured foods (Lasker et al. 

 1970; Theilacker and McMaster 1971; Hunter 1976) 

 have been described and compared with their 

 growth in the field (Methot and Kramer 1979). In- 

 cubation times, yolk absorption and the onset of 

 feeding (Lasker 1964; Lasker et al. 1970), feeding 

 success and swimming behavior (Hunter 1972) and 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La 

 Jolla, CA 92038. 



their ability to withstand starvation (Hunter 1976; 

 Theilacker and Dorsey 1980) have been studied. 

 Here I describe how variations in prey density af- 

 fect consumption, growth, and gross growth effi- 

 ciencies and compare my results to those on other 

 larval fishes. 



MATERIALS AND METHODS 



The rationale for my experimental design was to 

 avoid known problems that affect interpretation of 

 results of energetic studies. Mainly I determined gut 

 contents directly, which gives a measure of in- 

 dividual variability, rather than by controlling prey 

 level in the tank and estimating feeding by differ- 

 ence in prey numbers over time. To obtain more 

 precise fresh dry weight values for these small lar- 

 vae, I grouped them by size class to increase the 

 measured weight. I converted the number of prey 

 eaten to width-specific and species-specific fresh dry 

 weights and caloric values, thus precluding problems 

 inherent with feeding rate estimations that use pre- 

 served sample weights and/or average prey weights. 

 Because gastric evacuation depends on feeding 

 rates, I conducted the evacuation experiments with 

 actively feeding larvae. In addition, I compared and 

 validated traditional oxygen uptake measurements 

 with a starvation analysis used to estimate the 

 caloric equivalent of metabolism. 



Manuscript accepted February 1987. 

 FISHERY BULLETIN: VOL. 85, NO. 2, 1987. 



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