McGOWAN: SPAWNING OF NORTHERN ANCHOVY 



1979. Ichthyoplankton and zooplankton samples 

 were collected from a 5 m Boston Whaler with a 1 

 m diameter, cylinder-cone net of 0.308 mm mesh 

 nylon with a 0.200 mm mesh cod end. The net was 

 attached to a sled which kept the lower rim of the 

 net 10 cm above the bottom and which had a tow- 

 bridle that did not obstruct the mouth of the net. 

 A frame attached to the transom permitted the sled 

 to be launched and retrieved over the stern while 

 underway. The sled was lowered to the bottom while 

 underway at 1-2 kn, towed at the bottom for 1 min, 

 and then retrieved at a constant rate and constant 

 wire angle. Tow time, excluding that spent lower- 

 ing the net to the bottom, was approximately 6 min. 

 The gear was effective because it often caught an- 

 chovies and herring longer than 30 mm, a size not 

 usually captured in towed gear (Clarke 1983) or in 

 a plankton purse seine (Murphy and Clutter 1972). 



A calibrated flowmeter suspended off-center in 

 the mouth of the net measured the amount of water 

 filtered during the tow. Volumes calculated from the 

 flowmeter readings were similar to a hypothetical 

 volume calculated from net mouth area and tow 

 distance: approximately 300 m 3 /tow. 



Microzooplankton was collected with a 0.5 m diam- 

 eter net with 0.080 mm mesh, which was towed just 

 submerged at the surface for 2 min during the ich- 

 thyoplankton tow. Because the flowmeter in this net 

 frequently malfunctioned, hypothetical volumes 

 calculated from mouth diameter and tow length (ap- 

 proximately 25 m 3 /tow) were used to standardize 

 catches of microzooplankton. The net probably did 

 not filter as much water as calculated so 

 microzooplankton were underestimated. All samples 

 were preserved with 2% formaldehyde in seawater 

 buffered with sodium borate. 



Water turbidity was measured with a Secchi disk 

 (Tyler 1968). Water samples for salinity and tem- 

 perature measurements were taken with a Van 

 Dorn water sampler from 1 m below the surface and 

 from 1 m above the bottom. The temperature was 

 measured to 0.1°C with a laboratory thermometer, 

 and salinity was measured to 0.5°/oo with a temper- 

 ature-compensated refractometer. 



Laboratory Procedures 



Northern anchovy eggs were easily recognized 

 and distinguished from other regional pelagic fish 

 eggs by their oval shape and their size, approxi- 

 mately 0.75 mm x 1.25 mm. Eggs were not as- 

 signed to stages, but some of the embryos were 

 developed enough to be identified as those of north- 

 ern anchovies. Northern anchovy eggs were counted 



under a dissecting microscope; at the same time, fish 

 larvae were picked from the samples. The northern 

 anchovy can be separated from other similar look- 

 ing larvae by its myomere count (43-47), its gut 

 length, and its median fin positions (Miller and Lea 

 1972; McGowan and Berry 1984). 



All northern anchovy larvae <10 mm long were 

 measured to the nearest 0.1 mm using an ocular 

 micrometer. Longer larvae were measured to 1 mm 

 using vernier calipers or a plastic ruler graduated 

 in millimeters. The distance from the tip of the snout 

 to the tip of the notochord was measured in pre- 

 flexion larvae, standard length in larger specimens. 



Zooplankton were subsampled from a 500 mL 

 pharmaceutical beaker by stirring and taking an ali- 

 quot with a 1 mL or 2 mL Stempel pipet. Zooplank- 

 ton were identified to major taxonomic group under 

 the dissecting microscope using standard references 

 such as Smith (1977). All holoplanktonic, meroplank- 

 tonic, and nektonic invertebrates were considered 

 to be zooplankton if they were suitably sized prey 

 for adult anchovies. Isopods were included; adult 

 shrimp and gelatinous invertebrates were not. 

 Plankton was allowed to settle in water in a grad- 

 uated cylinder to estimate zooplankton volume. 



Microzooplankton were subsampled from a stirred 

 beaker with a pipet. A settling chamber and inverted 

 compound microscope with movable stage were used 

 to count microzooplankton (0.050-0.200 mm 

 diameter) at 100 x magnification. Dinoflagellates 

 known to be eaten by anchovy larvae were counted 

 as microzooplankton. 



Precision Estimates 



The precision of the microzooplankton counts was 

 estimated by the method of Lund et al. (1958). If 

 the counts are treated as a Poisson variable then 

 the 95% confidence limits for a single count are 



Upper limit = X + 2.42 + 1.96(Z + 1.5) 1/2 

 Lower limit = X + 1.42 - 1.96(X + 0.5) 1/2 . 



The limits are approximately ±20% if 100 organ- 

 isms are counted. Confidence intervals for micro- 

 zooplankton counts in this study range from + 50% 

 at the lowest count (5) to + 9% at the highest count 

 (659). 



The precision of the zooplankton subsampling esti- 

 mates was evaluated by taking triplicate subsam- 

 ples, with replacement, from 10 randomly selected 

 samples. The mean coefficient of variation (standard 

 deviation divided by the mean) of the triplicates was 

 0.29. 



881 



