High occurrence of jellyfish predation by black‐browed and Campbell albatross identified by DNA metabarcoding

Abstract

Gelatinous zooplankton are a large component of the animal biomass in all marine environments, but are considered to be uncommon in the diet of most marine top predators. However, the diets of key predator groups like seabirds have conventionally been assessed from stomach content analyses, which cannot detect most gelatinous prey. As marine top predators are used to identify changes in the overall species composition of marine ecosystems, such biases in dietary assessment may impact our detection of important ecosystem regime shifts. We investigated albatross diet using DNA metabarcoding of scats to assess the prevalence of gelatinous zooplankton consumption by two albatross species, one of which is used as an indicator species for ecosystem monitoring. Black-browed and Campbell albatross scats were collected from eight breeding colonies covering the circumpolar range of these birds over two consecutive breeding seasons. Fish was the main dietary item at most sites; however, cnidarian DNA, primarily from scyphozoan jellyfish, was present in 42% of samples overall and up to 80% of samples at some sites. Jellyfish was detected during all breeding stages and consumed by adults and chicks. Trawl fishery catches of jellyfish near the Falkland Islands indicate a similar frequency of jellyfish occurrence in albatross diets in years of high and low jellyfish availability, suggesting jellyfish consumption may be selective rather than opportunistic. Warmer oceans and overfishing of finfish are predicted to favour jellyfish population increases, and we demonstrate here that dietary DNA metabarcoding enables measurements of the contribution of gelatinous zooplankton to the diet of marine predators.

Counting with DNA in metabarcoding studies: How should we convert sequence reads to dietary data?

“…Then, when converting these read counts to occurrence data, a threshold number of reads is often required for each taxon to be tallied as an occurrence. Sequencing depth can vary considerably between samples, so in practice a threshold percentage of reads is often used (e.g., 1% of food sequences McInnes, Alderman, Lea, et al., ), or sequencing depth can be rarefied to a common level (O'Rorke, Holland, Cobian, Gaughen, & Amend, ). These approaches normalize detection across samples, so that more sequences are required for an occurrence to be recorded in samples with higher read depths.…”

“…Depending on the study system and research question, secondary predation may or may not be a serious problem. However, occurrence‐based data sets are expected to over‐emphasize these detections and ruling out secondary predation in occurrence summaries may require information of RRA, examination of prey co‐occurrence or expert knowledge (Bowser, Diamond, & Addison, ; Hardy et al., ; McInnes, Alderman, Lea, et al., ).…”

“…Cross‐validation has the problem that all methods of diet determination are biased, so if there is disagreement, the correct answer may be unclear (Soininen et al., ). However, congruence between data sets is reassuring and known biases can be taken into account when making conclusions (e.g., jellyfish are digested quickly, so occurrence in faecal DNA but not stomach contents is credible; Jarman et al., ; McInnes, Alderman, Lea, et al., ). Large differences in results between methods warrant further investigation; multiple lines of independent evidence provide the strongest support for any conclusion.…”

“…Another issue is the impact of collecting data with markers that have low taxonomic resolution (McInnes, Alderman, Lea, et al., ) or collating data at higher taxonomic levels to increase certainty in taxonomic assignment (Biffi et al., ). Depending on how broad the groupings are, occurrence summaries may not be very informative as many occurrences are potentially pooled.…”

DNA Metabarcoding as a Marine Conservation and Management Tool: A Circumpolar Examination of Fishery Discards in the Diet of Threatened Albatrosses

“…The results from the 18S region are presented to show the fish component of the diet and allow calculations of the overall proportion of the population consuming discards. Further details and discussion on the proportions of each prey group for each site can be found in McInnes et al (2017b).…”