Planktivorous predation on copepods. Evaluation of mandible remains in predator guts as a quantitative estimate of predation

Size and shape of mandibles from 8 common species of copepods in Norwegian coastal waters are described, with the aim of using gut contents of copepod mandibles as a precise technique of quantifying predation rates on copepods.The carapace length of 7 of the 8 copepod species could be closely predicted from a general h e a r regression equation of carapace length versus mandible width.The exception, Temora longicornis, had lower carapace length than expected from the equation.Simi- larly, the 6 copepodite stages of Calanus finmarchicus produced a highly significant species-specific linear regression equation between carapace length and mandible width, although adult males diverged somewhat by showing a lower carapace length than predicted.A correspondence analysis showed that it was impossible to separate all species and developmental stages from each other based on linear measurements of mandible structures alone, and the skill of a well-trained person is therefore needed.The relationship between individual dry weight and mandible width was well described by a regression equation, which hence could be used to calculate the biomass of consumed copepods, based on the contents of copepod mandibles in the predator's stomach.A comprehensive evaluation of the precision and accuracy in the estimates of prey mass is given, including effects of inter-and intraspecific variability and seasonal variation.By using a power function to regress body mass (mg dry weight) from mandible width (pm), the true prey biomass in the stomach content is estimated with a precision of *20% or better, when based on more than a few mandibles.However, a considerable seasonal variation in the length/weight relationships of the prey copepods may cause considerable bias in the estimates if not compensated for KEY WORDS: Planktivorous predation.