Papers by Rasmus Swalethorp
Elementa: Science of the Anthropocene, 2016
Polynyas, or recurring areas of seasonally open water surrounded by sea ice, are foci for energy ... more Polynyas, or recurring areas of seasonally open water surrounded by sea ice, are foci for energy and material transfer between the atmosphere and the polar ocean. They are also climate sensitive, with both sea ice extent and glacial melt influencing their productivity. The Amundsen Sea Polynya (ASP) is the greenest polynya in the Southern Ocean, with summertime chlorophyll a concentrations exceeding 20 µg L−1. During the Amundsen Sea Polynya International Research Expedition (ASPIRE) in austral summer 2010–11, we aimed to determine the fate of this high algal productivity. We collected water column profiles for total dissolved inorganic carbon (DIC) and nutrients, particulate and dissolved organic matter, chlorophyll a, mesozooplankton, and microbial biomass to make a carbon budget for this ecosystem. We also measured primary and secondary production, community respiration rates, vertical particle flux and fecal pellet production and grazing. With observations arranged along a gradi...
bioRxiv (Cold Spring Harbor Laboratory), Apr 10, 2024
Feeding success and maternal effects on larval size have long been hypothesized as important cont... more Feeding success and maternal effects on larval size have long been hypothesized as important contributors to interannual recruitment variability in marine fishes. This study examined the feeding ecology and influences of diet and size-at-birth on length and growth of larval rockfishes (Sebastes spp.). Prey carbon biomass and selection were calculated from gut contents, size-at-birth was estimated using otolith core size, and recent growth was derived from outer otolith increment widths. Biomass contributions of preferred prey and otolith data were integrated into Bayesian hierarchical models predicting length and growth. Larvae primarily fed on and selected for copepod nauplii and Calanoid copepodites, modulating feeding with ontogeny and in response to prey availability. Based on carbon weight, the relative contribution of Calanoid copepodites to the diet was more strongly and positively correlated with length and growth than that of nauplii. Younger larvae experienced faster growth in association with Calanoid copepodite consumption than older larvae. Positive effects of core radius suggest that initial larval size, believed to be mediated by maternal provisioning, increases the likelihood of survival, larger size and faster growth. These findings ultimately provide evidence that selective feeding and size-at-birth mediate rockfish survival in early life stages.
Identifying the factors influencing fish recruitment is critical for fishery management, and fail... more Identifying the factors influencing fish recruitment is critical for fishery management, and failure to do so can have major ecological and economic consequences. Many hypotheses over the past century have been proposed, and the recently postulated Trophic Efficiency in Early Life (TEEL) hypothesis argues that a shorter food chain length can result in more efficient energy transfer from primary producers to young fishes, thereby increasing growth rate and larval condition, reducing early-life mortality and ultimately leading to a stronger recruitment cohort. To test this hypothesis, we analyzed the trophic position (TP) through compound-specific isotopic analysis of amino acids, as well as otolith microstructure and stomach content of larval shortbelly rockfish (Sebastes jordani). Results show larval rockfish that ate lower TP prey were both heavier and faster growing. This suggests the trophic characteristics of early life diet are critical to larval survival, and provide evidence ...
Research Square (Research Square), Aug 24, 2022
Biomass fluctuations of small coastal-pelagic fishes represent perhaps the most iconic temporal 1... more Biomass fluctuations of small coastal-pelagic fishes represent perhaps the most iconic temporal 11 record of the impacts of natural climate variability on marine ecosystems 1,2,3. These fishes are 12 key constituents of the marine pelagic food web as primary feeders on plankton, prey to higher trophic level foragers such as birds, marine mammals, piscivore fishes 4,5 and valued for human consumption and industry 6. Despite over a century of research, the mechanisms governing their population volatility remain elusive 7,8. Here, we use a 45-year record of nitrogen stable isotopes measured in larvae of Northern Anchovy (Engraulis mordax) in the California Current to assess patterns in food chain length. Larval trophic efficiency associated with a shortened food chain increased for boom periods of high adult biomass, during which the ratio of large to small larvae decreased by an order of magnitude. In contrast, the ratio increased during periods of low adult biomass, likely reflecting a higher rate of early larval mortality. For the first time, we connect 21 energy transfer efficiency to early larval survival, which in turn regulates the decadal-scale 22 volatility of anchovy biomass, leading to the proposal of the Trophic Efficiency in Early Life 23 (TEEL) hypothesis. Our findings illustrate a potential for trophic indicators to inform on the 24 condition and recruitment to coastal-pelagic fish populations. 25
Oceanography
Environmental DNA (eDNA)-based ecological co-occurrence networks can provide a valuable tool for ... more Environmental DNA (eDNA)-based ecological co-occurrence networks can provide a valuable tool for fisheries and conservation management. In the past, it was nearly impossible to explore the microscopic world of larval fishes in one sampling event. Now, eDNA data and ecological co-occurrence network modeling provide windows into ecosystems that support larval fishes and upon which subsequent fisheries rely. Thus, there is great potential for eDNA methods coupled with ecological network analyses to provide a holistic understanding of community composition and species interactions and to develop indicators for fisheries and ecosystem-based management.
Marine Ecology Progress Series
The recovery of endangered predators has the potential to influence the ecosystems they inhabit. ... more The recovery of endangered predators has the potential to influence the ecosystems they inhabit. After suffering severe population declines due to fishing pressure, giant sea bassStereolepis gigasin southern California, USA, are beginning to recover. As large-bodied predators often associated with the kelp forest and rocky reef environments of southern California and Baja California, Mexico, the local recovery of this species could influence trophic dynamics in these systems. Here we leverage stable isotope and gut content analysis to describe the trophic ecology of adult giant sea bass. We found that they are generalist predators, with larger individuals relying more heavily on macroalgae-derived carbon, occupying a larger trophic niche, and obtaining higher trophic positions. Using these results, we speculate about the relationship between giant sea bass and kelp forest ecosystems, a vulnerable yet key habitat, including the impact of the return of these predators, as well as how ...
Biomass fluctuations of small coastal-pelagic fishes represent perhaps the most iconic temporal r... more Biomass fluctuations of small coastal-pelagic fishes represent perhaps the most iconic temporal record of the impacts of natural climate variability on marine ecosystems1,2,3. These fishes are key constituents of the marine pelagic food web as primary feeders on plankton, prey to higher trophic level foragers such as birds, marine mammals, piscivore fishes4,5 and valued for human consumption and industry6. Despite over a century of research, the mechanisms governing their population volatility remain elusive7,8. Here, we use a 45-year record of nitrogen stable isotopes measured in larvae of Northern Anchovy (Engraulis mordax) in the California Current to assess patterns in food chain length. Larval trophic efficiency associated with a shortened food chain increased for boom periods of high adult biomass, during which the ratio of large to small larvae decreased by an order of magnitude. In contrast, the ratio increased during periods of low adult biomass, likely reflecting a higher ...
Journal of Plankton Research
Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds ... more Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds in the North Atlantic to spawn in oligotrophic waters of the Gulf of Mexico (GoM). Stock recruitment is strongly affected by interannual variability in the physical features associated with ABT larvae, but the nutrient sources and food-web structure of preferred habitat, the edges of anticyclonic loop eddies, are unknown. Here, we describe the goals, physical context, design and major findings of an end-to-end process study conducted during peak ABT spawning in May 2017 and 2018. Mesoscale features in the oceanic GoM were surveyed for larvae, and five multi-day Lagrangian experiments measured hydrography and nutrients; plankton biomass and composition from bacteria to zooplankton and fish larvae; phytoplankton nutrient uptake, productivity and taxon-specific growth rates; micro- and mesozooplankton grazing; particle export; and ABT larval feeding and growth rates. We provide a general in...
Global Change Biology, 2021
The 2014–2016 Northeast Pacific marine heatwave (MHW) induced the warmest 3‐year period on record... more The 2014–2016 Northeast Pacific marine heatwave (MHW) induced the warmest 3‐year period on record in the California Current Ecosystem. We tested whether larval fish assemblage structure, phenology, and diversity dynamics were comparable to past warming events from 1951 to 2013. First, we hypothesized, based on past observations of biological effect of warming, that mesopelagic species with southern distributions relative to southern California and Pacific sardine Sardinops sagax (a coastal pelagic species) would increase during the MHW while northern mesopelagics and northern anchovy Engraulis mordax (coastal pelagic) abundances would decline. Similar to past warming, southern mesopelagics increased and northern mesopelagics decreased. Unexpectedly, however, a common southern mesopelagic, Mexican lampfish Triphoturus mexicanus, was approximately three times more abundant than the previous annual high. Furthermore, whereas sardine abundance did not increase, larval anchovy abundance rose to near‐record highs in summer 2016. Second, we hypothesized that fishes would spawn earlier during the MHW. Fishes did not spawn in an earlier season within a year, but five of six southern mesopelagic taxa spawned earlier than typical within winter and spring. Third, we predicted that species richness would increase moderately due to an influx of southern and exodus of northern species. Richness, however, was very high in all seasons and the highest ever during the summer as multiple species with primarily southern distributions were recorded spawning for the first time in southern California. The richness of northern species was also unexpectedly high during the MHW. Northern species likely persisted in the study area because in addition to the warm water, pockets of cold water were consistently present. If, as predicted, conditions similar to the MHW become more common as oceans warm, this unique and largely unexpected combination of fishes may reflect future biological conditions.
Journal of Plankton Research, 2021
We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in t... more We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in the larval habitat of Atlantic bluefin tuna (ABT) in the oceanic Gulf of Mexico (GoM) during the peak spawning month of May. Euphotic-zone biomass ranged from 101 to 513 mg C m−2 during the day and 216 to 798 mg C m−2 at night. Grazing varied from 0.1 to 1.0 mg Chla m−2 d−1, averaging 1–3% of phytoplankton Chla consumed d−1. Carnivorous taxa dominated the biomass of > 1-mm zooplankton (78% day; 60% night), while only 13% of smaller zooplankton were carnivores. δ15N enrichment between small and large sizes indicates a 0.5–0.6 trophic-step difference. Although characteristics of GoM zooplankton are generally similar to those of remote oligotrophic subtropical regions, zooplankton stocks in the ABT larval habitat are disproportionately high relative to primary production, compared with HOT and BATS averages. Growth-grazing balances for phytoplankton were resolved with a statistically insi...
The study site was located in the Disko Bay off Qeqertarsuaq, western Greenland. Due to land-conn... more The study site was located in the Disko Bay off Qeqertarsuaq, western Greenland. Due to land-connected sea ice coverage during winter, 2 sampling sites were combined. At the first site in winter (21 February to 23 March 2008), sampling was conducted through a hole in the ice at ca. 65 to 160 m depth approximately 0.5 nautical mile (n mile) south of Qeqertarsuaq (69° 14' N, 53° 29' W). In spring and summer (9 April to 18 July), sampling was done at a monitoring station 1 n mile south from Qeqertarsuaq (69° 14' N, 53° 23' W) at 300 m depth. Sampling was carried out between 10:00 and 17:00 h. During sampling from the ice, mesozooplankton was collected using a modified WP-2 net (45 µm) equipped with a closing mechanism (Hydrobios). Samples were collected in 3 depth strata (0-50, 50-100, and 100-150 m). During ship-based sampling, mesozooplankton was collected with a multinet (50 µm) equipped with a flow meter (Multinet, Hydrobios type midi), and 2 additional depth strata (150-200m and 200-250 m) were included. In addition to the seasonal study one diurnal investigation with sampling every 6 h was conducted from 29 April at 12:00 h to 30 April 30 at 12:00 h. Samples were immediately preserved in buffered formalin (5% final concentration) for later analyses. Biomass values of the different copepod species were calculated based on measurements of prosome length, and length/weight relationships. Two regressions for Calanus spp. were established for biomass calculations: one applicable prior to and during the phytoplankton bloom until 4 May, and another from 9 May onwards.
The effects of temperature and food was examined for Calanus finmarchicus and C. glacialis during... more The effects of temperature and food was examined for Calanus finmarchicus and C. glacialis during 3 phases of the phytoplankton spring bloom in Disko Bay, western Greenland. The 2 species were collected during pre-bloom, bloom, and post-bloom and exposed to temperatures from 0 to 10°C, combined with deficient or excess food. Fecal pellet and egg production were measured as indices for grazing and secondary production, respectively. Furthermore, changes in body carbon, nitrogen, and lipid content were measured. C. glacialis sampled before the bloom and incubated with excess food exhibited high specific egg production at temperatures between 0 and 2.5°C. Higher temperatures did not increase egg production considerably, whereas egg production for C. finmarchicus more than tripled between 2.5 and 5°C. Starved C. glacialis produced eggs at all temperatures stimulated by increasing temperatures, whereas starved C. finmarchicus needed temperatures above 5°C to produce eggs fueled by their lipid stores. Few C. finmarchicus had mature gonads at the initiation of the pre-bloom and bloom experiment, and egg production of C. finmarchicus therefore only increased as the ratio of individuals with mature gonads increased. During the bloom, both C. glacialis and C. finmarchicus used the high food availability for egg production, while refueling or exhausting their lipid stores, respectively. Finally, during the post-bloom experiment, production was low by C. finmarchicus, whereas C. glacialis had terminated production. Our results suggest that a future warmer ocean will reduce the advantage of early spawning by C. glacialis and that C. finmarchicus will become increasingly prevalent.
The study site was located in the Disko Bay off Qeqertarsuaq, western Greenland. Due to land-conn... more The study site was located in the Disko Bay off Qeqertarsuaq, western Greenland. Due to land-connected sea ice coverage during winter, 2 sampling sites were combined. At the first site in winter (21 February to 23 March 2008), sampling was conducted through a hole in the ice at ca. 65 to 160 m depth approximately 0.5 nautical mile (n mile) south of Qeqertarsuaq (69° 14' N, 53° 29' W). In spring and summer (9 April to 18 July), sampling was done at a monitoring station 1 n mile south from Qeqertarsuaq (69° 14' N, 53° 23' W) at 300 m depth. Sampling was carried out between 10:00 and 17:00 h. During sampling from the ice, mesozooplankton was collected using a modified WP-2 net (45 µm) equipped with a closing mechanism (Hydrobios). Samples were collected in 3 depth strata (0-50, 50-100, and 100-150 m). During ship-based sampling, mesozooplankton was collected with a multinet (50 µm) equipped with a flow meter (Multinet, Hydrobios type midi), and 2 additional depth strata (150-200m and 200-250 m) were included. In addition to the seasonal study one diurnal investigation with sampling every 6 h was conducted from 29 April at 12:00 h to 30 April 30 at 12:00 h. Samples were immediately preserved in buffered formalin (5% final concentration) for later analyses. Biomass values of the different copepod species were calculated based on measurements of prosome length, and length/weight relationships. Two regressions for Calanus spp. were established for biomass calculations: one applicable prior to and during the phytoplankton bloom until 4 May, and another from 9 May onwards.
bioRxiv, 2020
We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian... more We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic Bluefin Tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show that the food web is dominated by the microbial loop (>80% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). In contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process <4% of net primary production in the mixed layer. Nevertheless, ABT larvae feed preferentially on calanoid copepods and other suspension-feeding zooplankton that in turn derive much of their nutrition from diatoms and mixotrophic flagellates. This allows ABT larvae to maintain a comparatively low trophic level (∼4.0 for pre-flexion larvae; ∼4.2 for post-flexion larvae)...
Title: Investigation of the trophic ecology and flexibility of larvae sardine and anchovy, its re... more Title: Investigation of the trophic ecology and flexibility of larvae sardine and anchovy, its relationship to growth, feeding opportunities and recruitment success, and implementation in stock assessment. Principle Investigator: Andrew R. Thompson Principle Investigator Institution: NOAA NMFS SWFSC, Fisheries Research Division (FRD) Research priorities: Our project addresses the following FATE research priorities: 1. Studies that develop indices of environmental and oceanographic indicators, test hypotheses, or evaluate analytical tools to investigate specific mechanisms driving interactions between fisheries and climate and environmental drivers of managed species. Vital rates and parameters of interest include: recruitment, growth, distribution, trophic ecology, natural mortality, health, and habitat availability. Such studies should directly improve stock assessments via explanation and/or prediction of variability in the above population processes. Of particular interest are st...
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Papers by Rasmus Swalethorp