The effect of seabird presence and seasonality on ground-active spider communities across temperate islands.

Seabirds influence island ecosystems through nutrient additions and physical disturbance. These influences can have opposing effects on an island's invertebrate predator populations. Spiders (order: Araneae) are an important predator in many terrestrial island ecosystems, yet little is known about how seabird presence influences spider communities at the intraisland scale, or how they respond to seasonality in seabird colony attendance.We investigated the effects of seabird presence and seasonality on ground-active spider community structure (activity-density, family-level richness, age class, and sex structure) and composition at the family-level across five short-tailed shearwater breeding islands around south-eastern Tasmania, Australia. Using 75 pitfall traps (15 per island), spiders were collected inside, near, and outside seabird colonies on each island, at five different stages of the short-tailed shearwater breeding cycle over a year. Pitfall traps were deployed for a total of 2674 days, capturing 1592 spiders from 26 families with Linyphiidae and Lycosidae the most common. Spider activity-density was generally greater inside than outside seabird colonies, while family-level richness was generally higher outside seabird colonies. For these islands, seabird breeding stage did not affect activity-densities, but there were some seasonal changes in age class and sex structures with more adult males captured during winter. Our results provide some of the first insights into the spatial and temporal influences seabirds have on spider communities. We also provide some of the first records of spider family occurrences for south-eastern Tasmanian islands, which will provide an important baseline for assessing future change.

Myctobase, a circumpolar database of mesopelagic fishes for new insights into deep pelagic prey fields.

The global importance of mesopelagic fish is increasingly recognised, but they remain poorly studied. This is particularly true in the Southern Ocean, where mesopelagic fishes are both key predators and prey, but where the remote environment makes sampling challenging. Despite this, multiple national Antarctic research programs have undertaken regional sampling of mesopelagic fish over several decades. However, data are dispersed, and sampling methodologies often differ precluding comparisons and limiting synthetic analyses. We identified potential data holders by compiling a metadata catalogue of existing survey data for Southern Ocean mesopelagic fishes. Data holders contributed 17,491 occurrence and 11,190 abundance records from 4780 net hauls from 72 different research cruises. Data span across 37 years from 1991 to 2019 and include trait-based information (length, weight, maturity). The final dataset underwent quality control processes and detailed metadata was provided for each sampling event. This dataset can be accessed through Zenodo. Myctobase will enhance research capacity by providing the broadscale baseline data necessary for observing and modelling mesopelagic fishes.

Safeguarding marine life: conservation of biodiversity and ecosystems.

Marine ecosystems and their associated biodiversity sustain life on Earth and hold intrinsic value. Critical marine ecosystem services include maintenance of global oxygen and carbon cycles, production of food and energy, and sustenance of human wellbeing. However marine ecosystems are swiftly being degraded due to the unsustainable use of marine environments and a rapidly changing climate. The fundamental challenge for the future is therefore to safeguard marine ecosystem biodiversity, function, and adaptive capacity whilst continuing to provide vital resources for the global population. Here, we use foresighting/hindcasting to consider two plausible futures towards 2030: a business-as-usual trajectory (i.e. continuation of current trends), and a more sustainable but technically achievable future in line with the UN Sustainable Development Goals. We identify key drivers that differentiate these alternative futures and use these to develop an action pathway towards the desirable, more sustainable future. Key to achieving the more sustainable future will be establishing integrative (i.e. across jurisdictions and sectors), adaptive management that supports equitable and sustainable stewardship of marine environments. Conserving marine ecosystems will require recalibrating our social, financial, and industrial relationships with the marine environment. While a sustainable future requires long-term planning and commitment beyond 2030, immediate action is needed to avoid tipping points and avert trajectories of ecosystem decline. By acting now to optimise management and protection of marine ecosystems, building upon existing technologies, and conserving the remaining biodiversity, we can create the best opportunity for a sustainable future in 2030 and beyond.

Poleward bound: adapting to climate-driven species redistribution.

One of the most pronounced effects of climate change on the world's oceans is the (generally) poleward movement of species and fishery stocks in response to increasing water temperatures. In some regions, such redistributions are already causing dramatic shifts in marine socioecological systems, profoundly altering ecosystem structure and function, challenging domestic and international fisheries, and impacting on human communities. Such effects are expected to become increasingly widespread as waters continue to warm and species ranges continue to shift. Actions taken over the coming decade (2021-2030) can help us adapt to species redistributions and minimise negative impacts on ecosystems and human communities, achieving a more sustainable future in the face of ecosystem change. We describe key drivers related to climate-driven species redistributions that are likely to have a high impact and influence on whether a sustainable future is achievable by 2030. We posit two different futures-a 'business as usual' future and a technically achievable and more sustainable future, aligned with the Sustainable Development Goals. We then identify concrete actions that provide a pathway towards the more sustainable 2030 and that acknowledge and include Indigenous perspectives. Achieving this sustainable future will depend on improved monitoring and detection, and on adaptive, cooperative management to proactively respond to the challenge of species redistribution. We synthesise examples of such actions as the basis of a strategic approach to tackle this global-scale challenge for the benefit of humanity and ecosystems. Supplementary Information: The online version contains supplementary material available at 10.1007/s11160-021-09641-3.

Warming world, changing ocean: mitigation and adaptation to support resilient marine systems.

Proactive and coordinated action to mitigate and adapt to climate change will be essential for achieving the healthy, resilient, safe, sustainably harvested and biodiverse ocean that the UN Decade of Ocean Science and sustainable development goals (SDGs) seek. Ocean-based mitigation actions could contribute 12% of the emissions reductions required by 2030 to keep warming to less than 1.5 ºC but, because substantial warming is already locked in, extensive adaptation action is also needed. Here, as part of the Future Seas project, we use a "foresighting/hindcasting" technique to describe two scenarios for 2030 in the context of climate change mitigation and adaptation for ocean systems. The "business-as-usual" future is expected if current trends continue, while an alternative future could be realised if society were to effectively use available data and knowledge to push as far as possible towards achieving the UN SDGs. We identify three drivers that differentiate between these alternative futures: (i) appetite for climate action, (ii) handling extreme events, and (iii) climate interventions. Actions that could navigate towards the optimistic, sustainable and technically achievable future include:(i)proactive creation and enhancement of economic incentives for mitigation and adaptation;(ii)supporting the proliferation of local initiatives to spur a global transformation;(iii)enhancing proactive coastal adaptation management;(iv)investing in research to support adaptation to emerging risks;(v)deploying marine-based renewable energy;(vi)deploying marine-based negative emissions technologies;(vii)developing and assessing solar radiation management approaches; and(viii)deploying appropriate solar radiation management approaches to help safeguard critical ecosystems. Supplementary Information: The online version contains supplementary material available at 10.1007/s11160-021-09678-4.

Combating ecosystem collapse from the tropics to the Antarctic.

Globally, collapse of ecosystems-potentially irreversible change to ecosystem structure, composition and function-imperils biodiversity, human health and well-being. We examine the current state and recent trajectories of 19 ecosystems, spanning 58° of latitude across 7.7 M km2 , from Australia's coral reefs to terrestrial Antarctica. Pressures from global climate change and regional human impacts, occurring as chronic 'presses' and/or acute 'pulses', drive ecosystem collapse. Ecosystem responses to 5-17 pressures were categorised as four collapse profiles-abrupt, smooth, stepped and fluctuating. The manifestation of widespread ecosystem collapse is a stark warning of the necessity to take action. We present a three-step assessment and management framework (3As Pathway Awareness, Anticipation and Action) to aid strategic and effective mitigation to alleviate further degradation to help secure our future.

Decades of dietary data demonstrate regional food web structures in the Southern Ocean.

Understanding regional-scale food web structure in the Southern Ocean is critical to informing fisheries management and assessments of climate change impacts on Southern Ocean ecosystems and ecosystem services. Historically, a large component of Southern Ocean ecosystem research has focused on Antarctic krill, which provide a short, highly efficient food chain, linking primary producers to higher trophic levels. Over the last 15 years, the presence of alternative energy pathways has been identified and hypotheses on their relative importance in different regions raised. Using the largest circumpolar dietary database ever compiled, we tested these hypotheses using an empirical circumpolar comparison of food webs across the four major regions/sectors of the Southern Ocean (defined as south of 40°S) within the austral summer period. We used network analyses and generalizations of taxonomic food web structure to confirm that while Antarctic krill are dominant as the mid-trophic level for the Atlantic and East Pacific food webs (including the Scotia Arc and Western Antarctic Peninsula), mesopelagic fish and other krill species are dominant contributors to predator diets in the Indian and West Pacific regions (East Antarctica and the Ross Sea). We also highlight how tracking data and habitat modeling for mobile top predators in the Southern Ocean show that these species integrate food webs over large regional scales. Our study provides a quantitative assessment, based on field observations, of the degree of regional differentiation in Southern Ocean food webs and the relative importance of alternative energy pathways between regions.

Globally consistent reef size spectra integrating fishes and invertebrates.

The frequency distribution of individual body sizes in animal communities (i.e. the size spectrum) provides powerful insights for understanding the energy flux through food webs. However, studies of size spectra in rocky and coral reef communities typically focus only on fishes or invertebrates due to taxonomic and data constraints, and consequently ignore energy pathways involving the full range of macroscopic consumer taxa. We analyse size spectra with co-located fish and mobile macroinvertebrate data from 3369 reef sites worldwide, specifically focusing on how the addition of invertebrate data alters patterns. The inclusion of invertebrates steepens the size spectrum, more so in temperate regions, resulting in a consistent size spectrum slope across latitudes, and bringing slopes closer to theoretical expectations based on energy flow through the system. These results highlight the importance of understanding contributions of both invertebrates and fishes to reef food webs worldwide.

Functional traits explain trophic allometries of cephalopods.

Individual body size strongly influences the trophic role of marine organisms and the structure and function of marine ecosystems. Quantifying trophic position-individual body size relationships (trophic allometries) underpins the development of size-structured ecosystem models to predict abundance and the transfer of energy through ecosystems. Trophic allometries are well studied for fishes but remain relatively unexplored for cephalopods. Cephalopods are important components of coastal, oceanic and deep-sea ecosystems, and they play a key role in the transfer of biomass from low trophic positions to higher predators. It is therefore important to resolve cephalopod trophic allometries to accurately represent them within size-structured ecosystem models. We assessed the trophic positions of cephalopods in an oceanic pelagic (0-500 m) community (sampled by trawling in a cold-core eddy in the western Tasman Sea), comprising 22 species from 12 families, using bulk tissue stable isotope analysis and amino acid compound-specific stable isotope analysis. We assessed whether ontogenetic trophic position shifts were evident at the species-level and tested for the best predictor of community-level trophic allometry among body size, taxonomy and functional grouping (informed by fin and mantle morphology). Individuals in this cephalopod community spanned two trophic positions and fell into three functional groups on an activity level gradient: low, medium and high. The relationship between trophic position and ontogeny varied among species, with the most marked differences evident between species from different functional groups. Activity-level-based functional group and individual body size are best explained by cephalopod trophic positions (marginal R2  = 0.43). Our results suggest that the morphological traits used to infer activity level, such as fin-to-mantle length ratio, fin musculature and mantle musculature are strong predictors of cephalopod trophic allometries. Contrary to established theory, not all cephalopods are voracious predators. Low activity level cephalopods have a distinct feeding mode, with low trophic positions and little-to-no ontogenetic increases. Given the important role of cephalopods in marine ecosystems, distinct feeding modes could have important consequences for energy pathways and ecosystem structure and function. These findings will facilitate trait-based and other model estimates of cephalopod abundance in the changing global ocean.

Finding mesopelagic prey in a changing Southern Ocean.

Mesopelagic fish and squid occupy ocean depths extending below the photic zone and their vertical migrations represent a massive pathway moving energy and carbon through the water column. Their spatio-temporal distribution is however, difficult to map across remote regions particularly the vast Southern Ocean. This represents a key gap in understanding biogeochemical processes, marine ecosystem structure, and how changing ocean conditions will affect marine predators, which depend upon mesopelagic prey. We infer mesopelagic prey vertical distribution and relative abundance in the Indian sector of the Southern Ocean (20° to 130°E) with a novel approach using predator-derived indices. Fourteen years of southern elephant seal tracking and dive data, from the open ocean between the Antarctic Polar Front and the southern Antarctic Circumpolar Current front, clearly show that the vertical distribution of mesopelagic prey is influenced by the physical hydrographic processes that structure their habitat. Mesopelagic prey have a more restricted vertical migration and higher relative abundance closer to the surface where Circumpolar Deep Water rises to shallower depths. Combining these observations with a future projection of Southern Ocean conditions we show that changes in the coupling of surface and deep waters will potentially redistribute mesopelagic prey. These changes are small overall, but show important spatial variability: prey will increase in relative abundance to the east of the Kerguelen Plateau but decrease to the west. The consequences for deep-diving specialists such as elephant seals and whales over this time scale will likely be minor, but the changes in mesoscale vertical energy flow have implications for predators that forage within the mesopelagic zone as well as the broader pelagic ecosystem.

Expanded consumer niche widths may signal an early response to spatial protection.

Marine management interventions are increasingly being implemented with the explicit goal of rebuilding ocean ecosystems, but early responses may begin with alterations in ecological interactions preceding detectable changes in population-level characteristics. To establish a baseline from which to monitor the effects of spatial protection on reef fish trophic ecology and track future ecosystem-level changes, we quantified temperate reef fish densities, size, biomass, diets and isotopic signatures at nine sites nested within two fished and one five-year old marine protected area (MPA) on the northwest coast of Canada. We calculated rockfish (Sebastes spp.) community and species-specific niche breadth for fished and protected areas based on δ13C and δ15N values. We found that rockfish community niche width was greater inside the MPA relative to adjacent fished reefs due to an expanded nitrogen range, possibly reflecting early changes in trophic interactions following five years of spatial protection. Our data also demonstrated that the MPA had a positive effect on the δ15N signature of rockfish (i.e., trophic position), but the effect of rockfish length on its own was not well-supported. In addition, we found a positive interaction between rockfish length and δ15N signature, such that δ15N signatures of rockfish caught within the MPA increased more rapidly with body size than those caught in fished areas. Differences in rockfish size structure and biomass among fished and unfished areas were not clearly evident. Species of rockfish and lingcod varied in trophic and size responses, indicating that life-history traits play an important role in predicting MPA effects. These results may suggest early changes in trophic behavior of slow-growing rockfish due to predation risk by faster growing higher trophic level predators such as lingcod inside MPAs established on temperate reefs. Consequently, spatial protection may restore both the trophic and behavioral roles of previously fished consumers earlier and in measurable ways sooner than observable changes in abundance and size.

Trophodynamics of Southern Ocean pteropods on the southern Kerguelen Plateau.

Pteropods are a group of small marine gastropods that are highly sensitive to multiple stressors associated with climate change. Their trophic ecology is not well studied, with most research having focused primarily on the effects of ocean acidification on their fragile, aragonite shells. Stable isotopes analysis coupled with isotope-based Bayesian niche metrics is useful for characterizing the trophic structure of biological assemblages. These approaches have not been implemented for pteropod assemblages. We used isotope-based Bayesian niche metrics to investigate the trophic relationships of three co-occurring pteropod species, with distinct feeding behaviors, sampled from the Southern Kerguelen Plateau area in the Indian Sector of the Southern Ocean-a biologically and economically important but poorly studied region. Two of these species were gymnosomes (shell-less pteropods), which are traditionally regarded as specialist predators on other pteropods, and the third species was a thecosome (shelled pteropod), which are typically generalist omnivores. For each species, we aimed to understand (a) variability and overlap among isotopic niches; and (b) whether there was a relationship between body size and trophic position. Observed isotopic niche areas were broadest for gymnosomes, especially Clione limacina antarctica, whose observed isotopic niche area was wider than expected on both δ13C and δ15N value axes. We also found that trophic position significantly increased with increasing body length for Spongiobranchaea australis. We found no indication of a dietary shift toward increased trophic position with increasing body size for Clio pyramidata f. sulcata. Trophic positions ranged from 2.8 to 3.5, revealing an assemblage composed of both primary and secondary consumer behaviors. This study provides a comprehensive comparative analysis on trophodynamics in Southern Ocean pteropod species, and supports previous studies using gut content, fatty acid and stable isotope analyses. Combined, our results illustrate differences in intraspecific trophic behavior that may be attributed to differential feeding strategies at species level.

Can lipid removal affect interpretation of resource partitioning from stable isotopes in Southern Ocean pteropods?

RATIONALE: Stable isotope analysis (SIA) is a powerful tool to estimate dietary links between polar zooplankton. However, the presence of highly variable 12 C-rich lipids may skew estimations as they are depleted in 13 C relative to proteins and carbohydrates, consequently masking carbon signals from food sources. Lipid effects on pteropod-specific values requires examining, since accounting for lipids is rarely conducted among the few existing pteropod-related SIA studies. It is currently unclear whether lipid correction is necessary prior to SIA of pteropods. METHODS: Whole bodies of three species of pteropods (Clio pyramidata f. sulcata, Clione limacina antarctica, and Spongiobranchaea australis) sampled from the Southern Ocean were lipid-extracted chemically to test the effects on δ13 C and δ15 N values (n = 38 individuals in total). We determined the average change in δ13 C values for each treatment, and compared this offset with those from published normalization models. We tested lipid correction effects on isotopic niche dispersion metrics to compare interpretations surrounding food web dynamics. RESULTS: Pteropods with lipids removed had δ13 C values up to 4.5‰ higher than bulk samples. However, lipid extraction also produced higher δ15 N values than bulk samples. Isotopic niche overlaps between untreated pteropods and their potential food sources were significantly different from overlaps generated between lipid-corrected pteropods and their potential food sources. Data converted using several published normalization models did not reveal significant differences among various calculated niche metrics, including standard ellipse and total area. CONCLUSIONS: We recommend accounting for lipids via chemical extraction or mathematical normalization before applying SIA to calculate ecological niche metrics, particularly for organisms with moderate to high lipid content such as polar pteropods. Failure to account for lipids may result in misinterpretations of niche dimensions and overlap and, consequently, trophic interactions.

The winter pack-ice zone provides a sheltered but food-poor habitat for larval Antarctic krill.

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protection from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.

From Bacteria to Whales: Using Functional Size Spectra to Model Marine Ecosystems.

Size-based ecosystem modeling is emerging as a powerful way to assess ecosystem-level impacts of human- and environment-driven changes from individual-level processes. These models have evolved as mechanistic explanations for observed regular patterns of abundance across the marine size spectrum hypothesized to hold from bacteria to whales. Fifty years since the first size spectrum measurements, we ask how far have we come? Although recent modeling studies capture an impressive range of sizes, complexity, and real-world applications, ecosystem coverage is still only partial. We describe how this can be overcome by unifying functional traits with size spectra (which we call functional size spectra) and highlight the key knowledge gaps that need to be filled to model ecosystems from bacteria to whales.

The paradox of inverted biomass pyramids in kelp forest fish communities.

Theory predicts that bottom-heavy biomass pyramids or 'stacks' should predominate in real-world communities if trophic-level increases with body size (mean predator-to-prey mass ratio (PPMR) more than 1). However, recent research suggests that inverted biomass pyramids (IBPs) characterize relatively pristine reef fish communities. Here, we estimated the slope of a kelp forest fish community biomass spectrum from underwater visual surveys. The observed biomass spectrum slope is strongly positive, reflecting an IBP. This is incongruous with theory because this steep positive slope would only be expected if trophic position decreased with increasing body size (consumer-to-resource mass ratio, less than 1). We then used δ(15)N signatures of fish muscle tissue to quantify the relationship between trophic position and body size and instead detected strong evidence for the opposite, with PPMR ≈ 1650 (50% credible interval 280-12 000). The natural history of kelp forest reef fishes suggests that this paradox could arise from energetic subsidies in the form of movement of mobile consumers across habitats, and from seasonally pulsed production inputs at small body sizes. There were four to five times more biomass at large body sizes (1-2 kg) than would be expected in a closed steady-state community providing a measure of the magnitude of subsidies.

Productivity and fishing pressure drive variability in fish parasite assemblages of the Line Islands, equatorial Pacific.

Variability in primary productivity and fishing pressure can shape the abundance, species composition, and diversity of marine life. Though parasites comprise nearly half of marine species, their responses to these important forces remain little explored. We quantified parasite assemblages at two spatial scales, across a gradient in productivity and fishing pressure that spans six coral islands of the Line Islands archipelago and within the largest Line Island, Kiritimati, which experiences a west-to-east gradient in fishing pressure and upwelling-driven productivity. In the across-islands data set, we found that increasing productivity was correlated with increased parasite abundance overall, but that the effects of productivity differed among parasite groups. Trophically transmitted parasites increased in abundance with increasing productivity, but directly transmitted parasites did not exhibit significant changes. This probably arises because productivity has stronger effects on the abundance of the planktonic crustaceans and herbivorous snails that serve as the intermediate hosts of trophically transmitted parasites than on the higher-trophic level fishes that are the sole hosts of directly transmitted parasites. We also found that specialist parasites increased in response to increasing productivity, while generalists did not, possibly because specialist parasites tend to be more strongly limited by host availability than are generalist parasites. After the effect of productivity was controlled for, fishing was correlated with decreases in the abundance of trophically transmitted parasites, while directly transmitted parasites appeared to track host density; we observed increases in the abundance of parasites using hosts that experienced fishing-driven compensatory increases in abundance. The within-island data set confirmed these patterns for the combined effects of productivity and fishing on parasite abundance, suggesting that our conclusions are robust across a span of spatial scales. Overall, these results indicate that there are strong and variable effects of anthropogenic and natural drivers on parasite abundance and taxonomic richness. These effects are likely to be mediated by parasite traits, particularly by parasite transmission strategies.

Unintended cultivation, shifting baselines, and conflict between objectives for fisheries and conservation.

The effects of fisheries on marine ecosystems, and their capacity to drive shifts in ecosystem states, have been widely documented. Less well appreciated is that some commercially valuable species respond positively to fishing-induced ecosystem change and can become important fisheries resources in modified ecosystems. Thus, the ecological effects of one fishery can unintentionally increase the abundance and productivity of other fished species (i.e., cultivate). We reviewed examples of this effect in the peer-reviewed literature. We found 2 underlying ecosystem drivers of the effect: trophic release of prey species when predators are overfished and habitat change. Key ecological, social, and economic conditions required for one fishery to unintentionally cultivate another include strong top-down control of prey by predators, the value of the new fishery, and the capacity of fishers to adapt to a new fishery. These unintended cultivation effects imply strong trade-offs between short-term fishery success and conservation efforts to restore ecosystems toward baseline conditions because goals for fisheries and conservation may be incompatible. Conflicts are likely to be exacerbated if fisheries baselines shift relative to conservation baselines and there is investment in the new fishery. However, in the long-term, restoration toward ecosystem baselines may often benefit both fishery and conservation goals. Unintended cultivation can be identified and predicted using a combination of time-series data, dietary studies, models of food webs, and socioeconomic data. Identifying unintended cultivation is necessary for management to set compatible goals for fisheries and conservation.

Ecosystem ecology: size-based constraints on the pyramids of life.

Biomass distribution and energy flow in ecosystems are traditionally described with trophic pyramids, and increasingly with size spectra, particularly in aquatic ecosystems. Here, we show that these methods are equivalent and interchangeable representations of the same information. Although pyramids are visually intuitive, explicitly linking them to size spectra connects pyramids to metabolic and size-based theory, and illuminates size-based constraints on pyramid shape. We show that bottom-heavy pyramids should predominate in the real world, whereas top-heavy pyramids indicate overestimation of predator abundance or energy subsidies. Making the link to ecological pyramids establishes size spectra as a central concept in ecosystem ecology, and provides a powerful framework both for understanding baseline expectations of community structure and for evaluating future scenarios under climate change and exploitation.

Studying seabird diet through genetic analysis of faeces: a case study on macaroni penguins (Eudyptes chrysolophus).

BACKGROUND: Determination of seabird diet usually relies on the analysis of stomach-content remains obtained through stomach flushing; this technique is both invasive and logistically difficult. We evaluate the usefulness of DNA-based faecal analysis in a dietary study on chick-rearing macaroni penguins (Eudyptes chrysolophus) at Heard Island. Conventional stomach-content data was also collected, allowing comparison of the approaches. METHODOLOGY/PRINCIPAL FINDINGS: Prey-specific PCR tests were used to detect dietary DNA in faecal samples and amplified prey DNA was cloned and sequenced. Of the 88 faecal samples collected, 39 contained detectable DNA from one or more of the prey groups targeted with PCR tests. Euphausiid DNA was most commonly detected in the early (guard) stage of chick-rearing, and detection of DNA from the myctophid fish Krefftichthys anderssoni and amphipods became more common in samples collected in the later (crèche) stage. These trends followed those observed in the penguins' stomach contents. In euphausiid-specific clone libraries the proportion of sequences from the two dominant euphausiid prey species (Euphausia vallentini and Thysanoessa macrura) changed over the sampling period; again, this reflected the trend in the stomach content data. Analysis of prey sequences in universal clone libraries revealed a higher diversity of fish prey than identified in the stomachs, but non-fish prey were not well represented. CONCLUSIONS/SIGNIFICANCE: The present study is one of the first to examine the full breadth of a predator's diet using DNA-based faecal analysis. We discuss methodological difficulties encountered and suggest possible refinements. Overall, the ability of the DNA-based approach to detect temporal variation in the diet of macaroni penguins indicates this non-invasive method will be generally useful for monitoring population-level dietary trends in seabirds.