Gut fluorescence technique to quantify pigment feeding in Downs herring larvae

The ingestion of chlorophyll pigments (chlorophyll a and phaeopigments) by Downs herring Clupea harengus larvae (8-13 mm) collected in the English Channel and the North Sea during winter 2014 (International Bottom Trawl Survey) was quantitatively estimated via gut fluorescence analysis, a method classically used for copepods. Our results confirmed the consumption of chlorophyll pigments either directly, or indirectly through the consumption of herbivorous copepods. Higher mean pigment ingestion rates were observed for small larvae (8-11 mm, 52 +/- 51 [SD] ng chl a eq. ind.(-1) d(-1)), whereas lower mean rates (43 +/- 48 ng chl a eq. ind.(-1) d(-1)) were measured in larger larvae (12-13 mm). This decrease in the rate of pigment ingestion coincided with an ontogenetic shift in prey preference that occurred at 12-13 mm. Chlorophyll pigment ingestion covered on average up to 18% of the daily metabolic needs of 8-13 mm herring larvae and thus constitutes a non-negligible part of the larval diet mainly during the first feeding stages. Direct ingestion of autotrophic protist prey primarily involved small larvae (8-11 mm, 73 +/- 38 to 84 +/- 34%), as it can synergistically increase digestion efficiency of other prey items (e.g. copepods). The gut fluorescence method applied to fish larvae provides a global estimate of total ingested pigments and a snapshot of their diets. As a fast and easy method, it should be deployed in future surveys to compare food intake (ingestion) in different areas, and to quantitatively assess the nutritional status of fish larvae in the field.