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 Norwe gian coastal waters are described, with the aim of using gut contents of copepod mandibles as a precise technique of quantifying predation r ates on copepods. The carapace length of 7 of the 8 copepod species co uld be closely predicted from a general linear regression equation of carapace length versus mandible width. The exception, Temora longicomi s, had lower carapace length than expected from the equation. Similarl y, the 6 copepodite stages of Calanus finmarchicus produced a highly s ignificant species-specific linear regression equation between carapac e length and mandible width, although adult males diverged somewhat by showing a lower carapace length than predicted. A correspondence anal ysis showed that it was impossible to separate all species and develop mental 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 w idth was well described by a regression equation, which hence could be used to calculate the biomass of consumed copepods, based on the cont ents of copepod mandibles in the predator's stomach. A comprehensive e valuation of the precision and accuracy in the estimates of prey mass is given, including effects of inter- and intraspecific variability an d seasonal variation. By using a power function to regress body mass ( mg dry weight) from mandible width (mum), the true prey biomass in the stomach content is estimated with a precision of +/- 20% or better, w hen based on more than a few mandibles. However, d considerable season al variation in the length/weight relationships of the prey copepods m ay cause considerable bias in the estimates if not compensated for.