Does egg production represent adult female copepod growth? A call to account for body weight changes

Abstract

An almost universal assumption in determining growth in copepods is that, over short periods, an individual adult female's net growth is equal to the amount of material expelled as eggs. This assumption relies upon adult body mass being in steady-state between the start and end of the same period. We explore different situations where this assumption is violated. Initially, concepts of how adult body weight and egg output are coupled over time are addressed. Using a refined concept of growth, we show that using typical 24 h incubation methods to measure egg output in sac spawners or broadcasters that produce clutches of eggs with a periodicity of >1 d may give correct mean population growth rates, but erroneous individual rates (including maximum and minimum individual growth, and measurements of individual variability such as coefficient of variation). Measurements derived from laboratory and field studies are then used to explore errors associated with the steady-state assumption. Decoupling of egg production from assimilation, and non-steady-state body weight in large lipid-storing higher-latitude species are relatively well documented, yet growth estimates allowing for such changes have almost never been made. Errors are not limited only to such species, however, and changing adult body weights can occur in small temperate and tropical species too. Body weight can increase or decrease whether or not eggs are exuded over the same period. The errors that can arise if we assume that the output of eggs by females equals their net growth rate are large and variable; in our compilation they range from -208% (i.e. egg output being 9.7% of body carbon weight per day, but adult carbon weight simultaneously declining by 13.7% d-1) to +71% (i.e. egg output being 1.5% of body carbon weight per day, and adult carbon weight simultaneously increasing by 4.3% d-1). Using measurement of the natural variability in adult body weights, we determined that in order to be able to discriminate significant changes in body weight of 1 and 10% respectively, >1000 and <100 replicates are necessary, if applying typical sacrificial weighing methods. If we are to make accurate estimates of growth in adult copepods, then changes in body weight are of fundamental importance. We make initial recommendations for tackling these problems and reducing errors in the future.