EUPHAUSIID PREDATION ON COPEPODS IN COASTAL WATERS OF THE NORTHEAST ATLANTIC

Quantitative analyses of copepod mandibles in stomach contents of the 4 most dominant euphausiid species from the Skagerrak and the northwes t coast of Norway were used to evaluate trophic interactions between c opepods and krill and to develop a new method to estimate degree of ca rnivory in the diet. Copepods were an important food item and, based o n the new method, gave a ranking in degree of carnivory mainly as Mega nyctiphanes norvegica greater than or equal to Thysanoessa inermis = T . longicaudata much greater than T. raschii. M. norvegica preying on C alanus finmarchicus had similar predation rates in both laboratory stu dies and field samples (average 1.2 copepods ind.(-1) h(-1), 12.7% dai ly 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, w ith no strong preference for the front or back part of the copepods, C arnivory was less important in the Skagerrak than in the northern area for all 3 species occurring in both areas. T. inermis from the Skager rak seldom fed on copepods, whereas average carnivory in stations from the northern area ranged from 8 to 35%. T. raschii usually had high s tomach fullness but almost never took copepod prey. C. finmarchicus wa s by far the most dominant prey, making up 85% of the copepod prey mas s for small hi. norvegica (<50 mg dry weight) and 93 to 95% for the la rger hi. norvegica. Similar prey species dominated the other krill spe cies. Peak size of C. finmarchicus prey corresponded to copepodid II f or 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 a nd T. longicaudata. The high daily ration on C. finmarchicus for M. no rvegica, T. inermis and T. longicaudata, in combination with the commo nly high abundance for both prey and predators, indicates that krill p redation may contribute significantly to the mortality of C. finmarchi cus. Furthermore, because of the pronounced importance of krill for ma ny pelagic fish, seabirds and marine mammals in high-latitude areas, k rill species may be of utmost importance in transferring copepod produ ction to food yield for man.