Biology subjects

Swalethorp, R.

Publications and source records attributed to Swalethorp, R..

8 recordsLinked to original sources

bioRxiv2026

Comparative food-web analysis of bluefin tuna spawning habitats in the eastern Indian Ocean and Gulf of Mexico

Using linear inverse ecosystem modeling as a data assimilation tool, we compare spawning grounds of Atlantic and Southern Bluefin Tuna (ABT and SBT, respectively) based on results from field campaigns in the Gulf of Mexico (GoM) and eastern Indian Ocean off northwest Australia (Argo Basin). Both regions are warm, stratified, low-nutrient waters dominated by cyanobacteria (Prochlorococcus). Despite these similarities, the Argo Basin is more productive, with [~]1.5X higher net primary production and nearly 2X higher production of top trophic levels in the model (tuna larvae, planktivorous fish, and predatory gelatinous zooplankton). Higher primary production in the Argo Basin is mainly driven by higher N2 fixation and storm mixing of new nutrients in the upper and lower euphotic zone, respectively. Increased ecosystem efficiency (secondary production of top trophic levels / primary production) results from differences in plankton food web organization. In the GoM, protistan zooplankton are the direct consumers of nearly all phytoplankton production. In contrast, higher rates of herbivory by crustaceans feeding on nanophytoplankton combines with a higher impact of appendicularians on cyanobacteria to convert plankton production into larval tuna prey more efficiently in the Argo Basin. Despite similarities in the proportions of phytoplankton production mediated by cyanobacteria and other picoplankton in both systems, food web pathways to larval tuna and other planktivorous fish are substantially shorter in the Argo Basin. Our results highlight the impact of distinct zooplankton ecological niches on ecosystem efficiency and suggest a need for better inclusion of plankton food-web structure in models simulating climate impacts on fisheries production. HIGHLIGHTSO_LIDeveloped food web models of tuna spawning habitat (Indian Ocean & Gulf of Mexico) C_LIO_LISpawning habitats in the Argo Basin and Gulf of Mexico (GoM) are both oligotrophic C_LIO_LIArgo Basin had higher net primary production in part as a result of nitrogen fixation C_LIO_LIArgo Basin had higher rates of direct herbivory by metazoan zooplankton C_LIO_LIThis resulted in greater ecosystem efficiency in the Argo Basin. C_LI

ecology↗

bioRxiv2025

Phytoplankton community structure and photophysiological strategies in the oligotrophic Indian Ocean spawning ground of Southern Bluefin Tuna

The Argo Basin, a persistently oligotrophic seascape downstream of the Indonesian Throughflow (ITF), is the only known spawning ground of Southern Bluefin Tuna (Thunnus maccoyii). Its surface waters are capped by the warm, low saline, nitrate-depleted Pacific Ocean outflow, that leads to intense water-column stratification and chronic nutrient scarcity. During the BLOOFINZ campaign (R/V Roger Revelle, January-March 2022), we combined high-resolution underway surveys with Lagrangian experiments to examine hydrography, nutrient stoichiometry, and phytoplankton photophysiology across the basin. This study was motivated by the need to: 1) identify the phytoplankton communities capable of persisting in this nutrient-impoverished region, and 2) uncover the unique photophysiological traits that underpin their survival and growth, particularly under acute nitrate limitation. Nutrient fields revealed strongly negative N*, positive Si*, and elevated Si:P ratios, indicating a system of acute nitrate scarcity but with residual phosphate and latent silicate availability. Phytoplankton communities were dominated by the cyanobacteria, Prochlorococcus, whose survival strategies included small functional absorption cross section of PSII ({sigma}PSII), enhanced PSII connectivity (p), highly elastic and reversible diel photophysiology that balanced high-light stress and nitrogen scarcity. Longer turnover times especially {tau}2 ({micro}s; PSII-PSI electron transport) seen in the region around 115{degrees} and 120{degrees}E and at 15{degrees}S, and during Cycles 3 and 4 are indicative of populations that were being stressed by Fe limitation as well. Our inference of Prochlorococcus is supported by complimentary measurements of flow cytometry, phytoplankton pigment composition and microscopy all of which confirmed the dominance of Prochlorococcus in these nutrient impoverished waters. Our observations further indicate that mesoscale eddies generated by ITF-SICC (South Indian Counter Current) interactions, can episodically shoal the nutricline, triggering transient pulses of larger eukaryotes. Collectively, these results demonstrate how acute nitrate depletion, compounded by Fe stress, structures a microbial community finely tuned to oligotrophy and capable of sustaining a short food chain involving appendicularians that directly support larval tuna. The Argo Basin therefore exemplifies how circulation-nutrient coupling shapes microbial communities and ecosystem function within a globally significant spawning habitat. Highlights1) Chronic nitrate limitation, with strongly negative N* and persistent N:P imbalance defines the Argo Basin while phosphate and silicate remain in relative excess. 2) Picocyanobacteria especially Prochlorococcus dominate the ecosystem, sustaining productivity under extreme oligotrophy through photophysiological strategies of reduced {sigma}PSII, enhanced PSII connectivity, and diel recovery from light stress. 3) Mesoscale eddies at the ITF-SICC front provide episodic relief, shoaling the nutricline and fueling transient pulses of larger phytoplankton that episodically reshape phytoplankton community structure and biomass 4) This oligotrophic-frontal seascape supports a short, efficient food web, enabling transfer of picophytoplankton to Southern Bluefin Tuna larvae via mainly appendicularians.

ecology↗

bioRxiv2025

Revisiting daily growth and survival insights of Southern bluefin tuna (Thunnus maccoyii) larvae in the eastern Indian Ocean

This study analyzed the growth patterns and survival of Southern bluefin tuna (SBT, Thunnus maccoyii) larvae collected during January-February 2022 in their only known spawning area in the eastern Indian Ocean (IO). Otolith microstructure was examined to characterize both population-level and intra-population growth (OPT-optimal and DEF-deficient group), with special emphasis on the flexion process, as well as to provide insights into larval survival. SBT larvae began flexion at sizes and ages comparable to those reported in other bluefin tuna species. At the intra-population level, larvae reached flexion at the same age; however, optimal (OPT) larvae reached this stage in better physical condition, with greater length, weight, and body depth, likely increasing their chances of survival at later stages. The observed larval growth rates (0.38 mm{middle dot}d-{superscript 1}) exceeded those from a historical study in 1987 (0.33 mm{middle dot}d-{superscript 1}), likely due to a [~]2 {degrees}C increase in sea surface temperature and shifts in prey availability. Larval survival appears to depend on a selective process based on growth, in which only a small proportion of individuals (<2%) exhibited width increment in otoliths similar to those of surviving larvae, allowing for faster development and earlier access to larger prey. These findings highlight the need for expanded research on the early life stages of SBT, particularly in the context of ongoing ocean warming and climate change.

ecology↗

bioRxiv2025

Optimal Maternal Feeding Isotopic Niche: influence of breeder trophic behaviour on larval growth and survival in bluefin tuna species

Maternal effects play a fundamental role in shaping early larval growth and survival in marine fishes. This study explores the relationship between maternal trophic ecology and larval growth in bluefin tunas, with a focus on Southern Bluefin Tuna (SBT) and an expanded dataset from multiple Atlantic Bluefin Tuna populations incorporated into the General Bluefin Model (GBM). Daily growth and stable isotopes ({delta}{superscript 1}N and {delta}{superscript 1}3C) were obtained from 80 pre-flexion SBT larvae and 355 pre-flexion larvae from the GBM dataset. Results revealed a significant negative linear trend between larval age and {delta}{superscript 1}N values, consistent with the gradual attenuation of maternally inherited isotopic signatures during development. Faster growing larvae had higher growth rates showed significantly lower {delta}{superscript 1}N and {delta}{superscript 1}3C values, indicating that maternal trophic behaviour and their prey sources critically influence larval growth potential. Maternal isotopic niche breadth, inferred from larval isotope data, was markedly narrower in groups with optimal larval growth, suggesting that specialized (stenophagous) maternal feeding strategies promote enhanced offspring performance. These patterns were observed consistently in two bluefin species across seven different populations, despite geographic and temporal variability, highlighting a robust ecological link between maternal foraging behaviour and larval development. From these findings, we introduce the hypothesis of an Optimal Maternal Feeding Isotopic Niche, representing a constrained isotopic range associated with increased larval growth and survival. This framework advances our understanding of the influence of maternal trophic ecology on offspring fitness and offers valuable insights for the conservation and management of highly migratory pelagic species with complex reproductive strategies.

ecology↗

bioRxiv2025

Microbial food web dynamics in tropical waters of the bluefin tuna spawning region off northwestern Australia

Whereas recruitment success for many fisheries depends on coincident timing of larvae with abundance peaks of their prey, less can be more in the tropical/subtropical spawning areas of bluefin tunas if lower but steady food resources are offset by reduced larval vulnerability to pelagic predators. To understand larval habitat characteristics for Southern Bluefin Tuna (SBT), we quantified microbial community carbon flows based on growth and grazing rates from depth profiles of dilution incubations and carbon biomass assessments from microscopy and flow cytometry (FCM) during their peak spawning off NW Australia (Indian Ocean) in February 2022. Two Chla-based estimates of phytoplankton production gave differing offsets due to cycling or mixotrophy, exceeding 14C net community production on average (677 {+/-} 98 versus 447 {+/-} 43 mg C m-2 d-1). Productivity was higher than in the Gulf of Mexico spawning area for Atlantic Bluefin Tuna but less than similar studies of oceanic upwelling regions. Microzooplankton grazing averaged 482 {+/-} 63 mg C m- 2 d-1 (71 {+/-} 13% of production). Two measurement variables for Prochlorococcus gave average production and grazing rates of 282 {+/-} 36 and 248 {+/-} 32 mg C m-2 d-1 (86 {+/-} 6% grazed). Prochlorococcus comprised almost half of production and grazing fluxes in the upper (0-25 m) euphotic zone where SBT larvae reside. Prochlorococcus declined and eukaryotic phytoplankton and heterotrophic bacteria increased in relative importance in the lower euphotic zone. These results describe relatively classic open-ocean oligotrophic conditions as the food web base for nutritional flows to SBT larvae.

ecology↗

bioRxiv2024

Diet and Size-at-Birth Affect Larval Rockfish Condition and Survival

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.

ecology↗

bioRxiv2023

Trophic Efficiency Facilitates Larval Shortbelly Rockfish (Sebastes jordani) Development

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 in support of the TEEL hypothesis.

ecology↗

bioRxiv2019

Errors associated with compound specific δ15N analysis of amino acids in preserved fish samples purified by high pressure liquid chromatography

Compound specific isotopic analysis of amino acids (CSIA-AA) is increasingly used in ecological and biogeochemical studies tracking the origin and fate of nitrogen (N). Its advantages include the potential for resolving finer-scale trophic dynamics than possible with standard bulk SIA and for reconstructing historical changes in the food webs of consumers from analyses of specimens in preserved sample archives. For the latter, assessing the effects of chemical preservatives on {delta}15NAA has been inconclusive because the conventional CSIA approach for derivatized AAs by gas chromatography - combustion - isotope ratio mass spectrometry (GC-C-IRMS) has analytical errors (0.4 - 1.0 {per thousand}) in the range expected from chemical preservation. Here, we show improved analytical precision (0.15 {+/-} 0.08 {per thousand}) for 11 underivatized AA standards analyzed by high pressure liquid chromatography followed by offline elemental analysis - IRMS (HPLC/EA-IRMS), an approach originally developed by Broek and McCarthy (2014). Using this method, we report the first high-precision tests of preservation effects on {delta}15NAA in Northern Anchovy (Engraulis mordax) kept 1[1/2]-year in ethanol and up to 27-years in formaldehyde. We found minimal methodological induced fractionation for 8 AAs, and preservation effects on {delta}15N were similar regardless of duration and preservative used. Although some of the AAs differed significantly from frozen control samples (average +1.0 {+/-} 0.8 {per thousand}), changes in {delta}15N in the source AA Phenylalanine and trophic position estimates were statistically insignificant. Our results are encouraging for resolving fine-scale natural variability using HPLC/EA-IRMS on chemically preserved specimens and for ultimately reconstructing biogeochemical records and trophic dynamics over long time scales.

ecology↗