CalCOFI Reports, Vol. 27, 1986 - California Cooperative Oceanic ...

SCHMITT: SELECTIVITY AND FEEDING OF NORTHERN ANCHOVY LARVAECalCOFI Rep., Vol. XXVII, 1986where m = number of prey types = 5ri = proportion of prey type i consumedpi = proportion of prey type i available.pi was assumed to be constant for the duration ofeach experiment; i.e., the initial prey concentrationwas assumed to be not significantly depleted by larvaefeeding during the experiment. ri was determined fromgut content analyses.I calculated the alpha index for each of the five preycategories, for each larva that fed during experiments.The values so obtained were not suitable for parametricstatistical analyses, because the variances remainedheterogeneous after arc sine transformation. Therefore,I examined the median values and the quartilesabove and below the median. The observed values arecompared with the value expected if feeding was randomon all categories of prey (a = l/m = 0.20).In order to compare feeding rates between sizes oflarvae at the target concentrations, I fitted feeding-ratedata for each IO-mm size class of larvae to a log-linearmultiple regression model:In (rate + 1) = In (a) + bl (SL) + b2 (concentration)I backtransformed the resulting estimates, taking intoaccount the variance of the regression, and calculated95% confidence limits (Beauchamp and Olson 1973).I calculated the metabolic and growth requirementsusing the following values and assumptions:1. Larvae fed at the estimated mean feeding ratesthroughout a 12-hr feeding day.2. Daily minimum metabolic requirement (24 hr)was calculated from the following model presented byTheilacker’ for anchovy larvae at 15.5”- 16°C:Q = 4.269 w0.697where Q = metabolic rate in pl 02/dayW = fresh dry weight in mgand converted to pg prey material using the followingfactors: 0.00463 cal/pl O2 and 4.9 x IOp3 cal/pg, forcopepod prey (Theilacker and Kimball 1984).3. Assimilation efficiency was 70% for larvae of allsizes, a rough extrapolation from Theilacker’s results.24. Weight-specific growth rates of larvae in the seawere taken from Methot (1981). The surface temperaturesreported in that study (generally representative ofanchovy spawning areas) ranged from 13” to 19”C,with the mode at approximately 15”- 16°C.‘Theilacker, G.H. MS. Feeding ecology and growth energetics of larval northernanchovy, Engraulis mordax.’Ibid.5. Fresh dry weights were calculated from the formulain Hunter (1976) for 10-30-mm larvae, and fromunpublished data of Hunter for 40-mm larvae.RESULTS AND DISCUSSIONThe target and actual larval lengths and prey concentrationsare given in Table 1, along with the feedingincidence (% of larvae with one or more prey in gut).Because IO-mm larvae did not feed at 10 prey/liter, Itested them at 100 and 1000 prey/liter. Feeding incidencetended to increase with both larval length andprey concentration; similar results have been reportedfor other species of fish larvae (Houde and Schekter1980).Swimming ActivityThe mean swimming speed, measured as bodylengthdsec (bl/s), in each experiment is shown in Table2, along with t-tests of the equality of means beforefeeding (after 1-3 days of food deprivation) and duringfeeding. For IO-mm larvae, mean speed was alwaysless than 1 bl/s, and showed no changes with the additionof food. My results contrast with those of aprevious study in which anchovy larvae from 4-8 daysold (< 6 mm SL) swam slower when inside a densepatch of food cells (up to 260 cells/ml) than when outsidesuch a patch (Hunter and Thomas 1974). The differencein these results could be because the “patches”created by adding food in my experiments were 3 to 4orders of magnitude less dense than those used byHunter and Thomas.For 20-40-mm larvae, mean swimming speedranged from 0.6 to 3.3 bl/s and tended to increase duringfeeding (9 of 11 t-tests with p < .lo). However,there was no difference in the relative increase in speedduring feeding between high and low prey concentrations;Le., larvae did not swim faster while feedingon high prey concentrations (Table 2).Prey SelectivityThe mean proportion of various prey types consumedby various sizes of larvae at high and low concentrationsis compared to the mean available sizecomposition in Figure 1. The 10-mm larvae consumedprimarily rotifers (the “others” category), althoughthey consumed some nauplii and copepods at 1000prey/liter. The 20-40-mm larvae tended to consume ahigher proportion of nauplii at low concentrations thanat high concentrations, and higher proportions ofmedium and large copepods at high concentrationsthan at low concentrations. Medium copepods (100-300 pm wide) formed the largest proportion (50%-80%) of prey consumed, both by numbers and byweight for 20-40-mm larvae at both concentrations.156