Can the Continuous Plankton Recorder (CPR) be used to infer sardine diet?

Knowledge on the impact of climate variability in the diet of planktivorous fish is limited by the laborious work involved in stomach content analysis, impractical for large scale studies. Routine measurements of plankton such as the Continuous Plankton Recorder (CPR) survey provide valuable information of the temporal variation of phyto- and zooplankton prey availability for higher trophic levels. Sardines are a world-wide distributed and commercially important planktivorous fish, at the basis of the pelagic marine food web. Being predominantly non-selective filter-feeders, their diets closely correspond to the water plankton species and a significant relationship was recently found between Sardina pilchardus feeding intensity and remotely sensed chlorophyll alpha . Data of sardine stomach prey composition and CPR were obtained during 2003 for the same location off the west coast of Portugal, an area characterised by strong seasonality of plankton abundance and composition, mainly governed by upwelling events. Phyto- and zooplankton prey in sardine stomachs were identified to the lowest possible taxa and their numerical and volumetric abundance was registered, as well as their contribution to the prey carbon content. The seasonal variation of the abundance and composition of sardine diet was then compared to the abundance and composition of the water plankton obtained with the CPR at the same time and for the same area where the fish were collected, in order to evaluate if CPR data can be used to proxy sardine prey availability and diet composition at large temporal scales.

Sardine cycles, krill declines, and locust plagues: revisiting ‘wasp-waist’ food webs

‘Wasp-waist’ systems are dominated by a mid trophic-level species that is thought to exert top-down control on its food and bottom-up control on its predators. Sardines, anchovy, and Antarctic krill are suggested examples, and here we use locusts to explore whether the wasp-waist concept also applies on land. These examples also display the traits of mobile aggregations and dietary diversity, which help to reduce the foraging footprint from their large, localised biomasses. This suggests that top-down control on their food operates at local aggregation scales and not at wider scales suggested by the original definition of wasp-waist. With this modification, the wasp-waist framework can cross-fertilise marine and terrestrial approaches, revealing how seemingly disparate but economically important systems operate.