Ontogenetic Development of Growth Rates in Larval Pacific Herrings, Clupea harengus pallasi, Measured with RNA–DNA Ratios in the Strait of Georgia, British Columbia

Abstract

During the spring of 1986, a cohort of Pacific herring, Clupea harengus pallasi, larvae was sampled for 36 d in the Strait of Georgia to measure growth rates using RNA–DNA ratios for individual larvae. Concurrent with the field study, a population of herring larvae was starved from hatching in captivity for comparison with those caught in the field. The mean RNA–DNA ratio at hatching for the starved larvae was close to 2 but it quickly rose to 3.4 by age 4 d. The mean RNA–DNA ratio subsequently dropped back to 2 and below by age 8 d, presumably reflecting the exhaustion of the endogenous food supply of the yolk sac. The mean RNA–DNA ratio at the calculated point-of-no-return was 2.06 which was very similar to the zero protein growth rate or what we define as the "critical ratio." Herring larvae from the field generally showed an increase in the RNA–DNA ratio over the 36 d from approximately 2 to 7 although the first 18 d showed more variation than the latter. There was no evidence of mass starvation ("critical period") for the 1986 year class but there was a noticeable drop in the growth rate during the change to exogenous feeding. We suggest that starvation probably only directly affected the developing larvae during a window of about 11 d. Frequency distributions of the RNA–DNA ratios are shown for larvae over time.