Euphausiacea
Ingestion

Realistic estimates of daily ingestion and grazing impact can only be obtained from a combination of in situ gut content, and ship-board experimental studies. Laboratory experiments addressing these should be avoided, per se, because of problems both with extrapolation to the field and containment artifacts. Per capita ingestion (I) can be calculated as the product of average gut content (G) and gut evacuation rate (K): I = K.G

Average gut contents (G) must be estimated at frequent and regular intervals throughout the day so that daily feeding rhythms reflecting DVM can be determined (Moloney and Gibbons, 1996). Indirect estimates of the amount of phytoplankton eaten by zooplankton (as chlorophyll a) can be determined by gut pigment flourescence (Mackas and Bohrer, 1976; Dagg and Walser, 1987), experimentally corrected for pigment destruction (Perissinotto and Pakhomov, 1996). The carapaces of all specimens used in gut pigment analysis should be inspected under a microscope for the presence of epizoophytes, which can otherwise distort results (McClatchie et al., 1990). Carnivory can be determined by immuno-analysis of gut contents (Theilacker et al., 1993) or by microscopic examination of stomach contents (Gibbons et al., 1991b; Karlson and Bamstedt, 1994).

Gut passage time (K) is temperature dependant (Dam and Peterson, 1988) and experiments to determine K should be conducted on board ship in filtered seawater with a suspension of inert charcoal particles (Perissinotto and Pakhomov, 1996).

Errors in converting individual ingestion to population ingestion result partly from asynchronous behaviour (Gibbons, 1993) and can be minimised by using large samples (Moloney and Gibbons, 1996). In calculating grazing impact, it is important to remember that feeding rate should be expressed against production and not standing biomass. Ambient production should be calculated across the depth strata in which the euphausiids graze, and not proportions thereof.