Diel grazing patterns of all ontogenetic stages of Euphausia lucens and in situ predation rates on copepods in the southern Benguela upwelling region

Abstract

Diel feeding rhythms of all developmental stages of Euphausia lucens were investigated using the gut fluorescence technique. Pronounced nocturnal increase in gut pigment levels were noted in adults and juveniles, but no diel changes were recorded for the calyptopis larvae. Furcilia larvae showed diel feeding rhythms in 2 out of 3 cruises. Feeding periodicity was thus related to ontogenetic stage, which in turn was linked to vertical migratory behaviour. Adults and juveniles undergo extensive daily migrations while larval stages are largely confined to the upper layers. Gut pigment levels in adults and juveniles were influenced by ambient chlorophyll levels, being ca 10-fold higher under favourable conditions. Carnivorous feeding on copepods was examined by counting the number of copepod mandible pairs in euphausiid stomachs. Carnivory predominated in adults, with juveniles consuming relatively few copepods and larval stages being primarily herbivorous. The size of copepods consumed by adult euphausiids was estimated from the relationship between mandible width and copepod prosome length. Copepods in the size range 0.15 to 2.19 mm prosome length were consumed (mean: 0.54 to 0.86 mm). There was little relationship between euphausiid length and length of copepod consumed, although larger euphasiids contained more copepod mandibles than smaller ones (up to 8 pairs per stomach). Carnivory occurred predominantly during the day, except under conditions of low phytoplankton concentrations, when large numbers of copepods were eaten at night. A significant inverse relationship was found between gut pigment levels at night and the number of copepod mandibles per stomach, suggesting that phytoplankton concentration may be the trigger causing a switch in diet from one food source to the next. Carbon-specific ingestion rates on phytoplankton and copepod prey indicate that E. lucens is a true omnivore, with between 15 and 60% of total carbon ingested being in the form of plant material and the remainder being obtained from copepod prey. Carnivory was of prime importance when chlorophyll levels were low, but even during high phytoplankton concentrations, still contributed up to 40% of total carbon ingested. E. lucens is thus well adapted to cope with the fluctuating food environment characteristic of upwelling areas. Population ingestion rates indicate that grazing by E. lucens has a negligible impact on phytoplankton stocks in the southern Benguela region. However, predation on copepods is of greater significance and may act to limit secondary production in this area.