THEILACKER: FEEDING AND GROWTH OF NORTHERN ANCHOVY 



The timing of the second prey addition was not 

 critical for determining gut clearance rates at high 

 prey densities. But when the timing was not cor- 

 rect for the tests that used low prey concentrations, 

 deciphering the meaning of the gut contents was 

 problematical. Results from most of these low- 

 density tests could not be used. 



A series of evacuation experiments also was con- 

 ducted with nonfeeding northern anchovy that were 

 removed from their food source, rotifers, to filtered 

 seawater. 



To reduce the incidence of injury during transfer, 

 I constructed a cylindrical, clear plastic container 

 (15 mm high and 7 mm diameter) with handle and 

 a removable bottom grooved to fit the circumference 

 of the cylinder. Larvae to be transferred were sur- 

 rounded by the cylinder, and then the bottom was 

 fitted onto the cylinder. The container with fish was 

 transferred and lowered into the test tank. Remov- 

 ing the bottom and slowly raising the cylinder re- 

 leased the fish. Prey that were transferred into the 

 experimental tank with the larvae were removed by 

 an air-lift pump that slowly recirculated water and 

 was screened to prevent the removal of larvae 

 (O'Connell and Paloma 1981). 



Growth Efficiency 



To determine growth efficiencies, I used the in- 

 formation on growth (Table 4), daily food consump- 

 tion estimated from the general equations (Table 3) 

 and evacuation rates of 1.15 hours for the high- 

 density rotifer diet and 1.5 hours for the low-density 

 diet. Gross growth efficiency was estimated based 

 on dry weight and on caloric estimates. It is the ratio 

 of growth to ingestion. To estimate assimilation ef- 

 ficiency, I used the information on weight-specific 

 metabolic rates and simply combined the energy of 

 metabolism and growth and divided it by the energy 



consumed. Assimilated energy lost as feces and 

 urine was not accounted for. 



RESULTS 



Feeding 



Sizes of prey fed to larval northern anchovy in 

 these experiments ranged from 50 /um for Gymno- 

 dinium, to 74-195 ^m for rotifers and copepod 

 nauplii, and 147-221 f^m for copepodites. In contrast 

 to larvae fed the rotifer diets, fish offered the cope- 

 pod diet did not switch from eating Gymnodinium 

 at first feeding on day 3 to eating larger prey on 

 day 5. All fish fed copepods and sampled on day 5 

 contained Gymnodinium in their gut, and 96% of 

 these guts were full of Gymnodinium (Table 6). By 

 day 7, the number of copepod-fed northern anchovy 

 eating Gymnodinium had decreased to 80%, with 

 10% full. These data reveal that young northern an- 

 chovy were unsuccessful at capturing Tigriopus 

 nauplii at 1/mL until 7-8 days of age, and because 

 of this behavior and the failure of the copepod 

 culture on day 9, 1 was unable to quantify daily con- 

 sumption for northern anchovy fed copepods. 



Northern anchovy reared on the low-density 

 (2/mL) and high-density (25/mL) rotifer diets ate 

 only a few Gymnodinium cells after day 4. Between 

 rotifer treatments, incidence of feeding on Gymno- 

 dinium after day 4 was higher for fish fed the low- 

 density diet (Table 6). On day 5, northern anchovy 

 concentrated on eating rotifers in the 75-150 /^m size 

 range; between 84 and 97% of the rotifers eaten by 

 larvae sampled from both rotifer treatments were 

 <150 Mm width (Table 7). Only 7% of the rotifers 

 available to these larvae were smaller than 150 /^m 

 width. Hence on day 5 larvae were selecting small 

 rotifers in higher proportions than were available; 

 in the two treatments, the apparent density of roti- 



Table 6.— Presence of Gymnodinium in guts of larval northern anchovy related to 



age of larvae and to diet. 



'Apparent density of rotifers <150 ^im = 2/mL. 



^Apparent density of rotifers <150 jjm = 0.1/mL. 



^Apparent density of nauplii < 150 ^jm = 1/mL. 



••Percent of larvae having Gymnodinium cells In guts with rotifers or copepods. 



^Percent of larvae having guts full vtrith Gymnodinium cells. 



217 



