MEPS 172:149-168 (1998)  -  doi:10.3354/meps172149

ABSTRACT: Quantitative analyses of copepod mandibles in stomach contents of the 4 most dominant euphausiid species from the Skagerrak and the northwest coast of Norway were used to evaluate trophic interactions between copepods and krill and to develop a new method to estimate degree of carnivory in the diet. Copepods were an important food item and, based on the new method, gave a ranking in degree of carnivory mainly as Meganyctiphanes norvegica >= Thysanoessa inermis = T. longicaudata >> T. raschii. M. norvegica preying on Calanus finmarchicus had similar predation rates in both laboratory studies and field samples (average 1.2 copepods ind.^-1 h^-1, 12.7% daily ration) and showed that (1) copepod mandibles were not accumulated in the predator stomach, (2) there was no diel predation rhythm, and (3) ca 20% of copepods caught by the predator were only partly eaten, with no strong preference for the front or back part of the copepods. Carnivory was less important in the Skagerrak than in the northern area for all 3 species occurring in both areas. T. inermis from the Skagerrak seldom fed on copepods, whereas average carnivory in stations from the northern area ranged from 8 to 35%. T. raschii usually had high stomach fullness but almost never took copepod prey. C. finmarchicus was by far the most dominant prey, making up 85% of the copepod prey mass for small M. norvegica (<50 mg dry weight) and 93 to 95% for the larger M. norvegica. Similar prey species dominated the other krill species. Peak size of C. finmarchicus prey corresponded to copepodid II for small (5 to 7.5 mm carapace length) M. norvegica, copepodid III for the larger ones, with no differences between predator sexes. Similar size distributions of C. finmarchicus prey were shown for T. inermis and T. longicaudata. The high daily ration on C. finmarchicus for M. norvegica, T. inermis and T. longicaudata, in combination with the commonly high abundance for both prey and predators, indicates that krill predation may contribute significantly to the mortality of C. finmarchicus. Furthermore, because of the pronounced importance of krill for many pelagic fish, seabirds and marine mammals in high-latitude areas, krill species may be of utmost importance in transferring copepod production to food yield for man.

KEY WORDS: Meganyctiphanes · Thysanoessa · Carnivory · Ingestion · Digestion · Gut contents · Predatory impact · Pelagic food chains