Biology subjects

Malca, E.

Publications and source records attributed to Malca, E..

6 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

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↗

bioRxiv2024

Ancient DNA reveals historical demographic decline and genetic erosion in the Atlantic bluefin tuna

Overexploitation has depleted fish stocks during the past century, nonetheless its genomic consequences remain poorly understood. Characterising the spatiotemporal patterns of these consequences may provide baseline estimates of past diversity and productivity to aid management targets, help predict future dynamics, and facilitate the identification of evolutionary factors limiting fish population recovery. Here, we evaluate human impacts on the evolution of the iconic Atlantic bluefin tuna (Thunnus thynnus), one of the longest and most intensely exploited marine fishes, with a tremendous cultural and economic importance. We sequenced whole genomes from modern (n=49) and ancient (n=41) specimens dating up to 5000 years ago, uncovering several novel findings. First, we identify temporally stable patterns of population admixture, as bluefin tuna caught off Norway and in the eastern Mediterranean share a greater degree of ancestry with Gulf of Mexico bluefin tuna than western and central Mediterranean bluefin tuna. This suggests that Atlantic spawning areas are important mixing grounds for the genetic diversity of Mediterranean bluefin tuna. We model effective population size to show that Mediterranean bluefin tuna began to undergo a demographic decline by the year 1900 to an extent not observed across the previous millennia. Coinciding with this, we found that heterozygosity and nucleotide diversity was significantly lower in modern (2013-2020), than ancient (pre-1941) Mediterranean bluefin tuna, suggesting bluefin tuna underwent a genetic bottleneck. With this work we show how ancient DNA provides novel perspectives on ecological complexity with the potential to inform the management and conservation of fishes. SignificanceAchieving the aim of the current UN Ocean Decade to "protect and restore ecosystems and biodiversity" is stymied by a lack of historical knowledge on how human exploitation has impacted and therefore what should be restored. Here, we sequence DNA in ancient fish bones to evaluate the historical diversity of the Atlantic bluefin tuna; which has been of great commercial importance for centuries. We find that bluefin tuna began to undergo demographic decline by 1900, 70 years earlier than currently recognised. Correspondingly, we find modern bluefin tuna had lower levels of genetic diversity than historical ones. This suggests that human impacts on the diversity of marine fishes are likely to have begun earlier and be more complex than previously thought.

ecology↗

bioRxiv2024

Five millennia of mitochondrial introgression in Atlantic bluefin tuna identified using ancient DNA

Mitogenomic (MT) introgression between species is readily documented in marine fishes. Such introgression events may either be long-term natural phenomena or the result of human-driven shifts in spatial distributions of previously separated species. Determining the drivers behind MT introgression is stymied by the difficulty of directly observing patterns of interbreeding over long timescales. Using ancient DNA spanning five millennia, we here investigate the long-term presence of MT introgression from Pacific bluefin tuna (Thunnus orientalis) and albacore (Thunnus alalunga) into Atlantic bluefin tuna (Thunnus thynnus), a species with extensive exploitation history and observed shifts in abundance, and demographic distribution. Comparing ancient (n=130) and modern (n=78) mitogenomes of specimens covering most of the range of Atlantic bluefin tuna we detect no significant spatial or temporal population structure. This lack of spatiotemporal genomic differentiation is indicative of ongoing gene flow between populations and large effective population sizes over millennia. Moreover, we identify introgressed MT genomes in ancient specimens up to 5000 years old and find that this rate of introgression has remained similar through time. We therefore conclude that MT introgression in the Atlantic bluefin tuna is to date unaffected by anthropogenic impacts. By providing the oldest example of directly observed MT introgression in the marine environment, our results highlight the utility of ancient DNA to obtain temporal insights in the long-term persistence of such phenomena.

evolutionary biology↗

bioRxiv2020

Trade-offs between risks of predation and starvation in larvae make the shelf break an optimal spawning location for Atlantic Bluefin tuna

Atlantic Bluefin tuna (ABT) (Thunnus thynnus) travel long distances to spawn in oligotrophic regions of the Gulf of Mexico. To estimate regional larval ABT mortality, we developed a spatially-explicit, Lagrangian, individual-based model that simulates dispersal, growth, and mortality within realistic predator and prey fields during the spawning periods from 1993-2012. Modelled larval ABT experience high mortality in the first week of feeding with an average mortality rate of 0.53 {+/-} 0.26 d-1 prior to postflexion. Survival ranged from 0.12%-0.32% suggesting that recruitment may vary by a factor of 2.7 due to early life stage mortality alone. Starvation is the dominant source of mortality driven by the early critical period; however, survival is ultimately limited by predation on older individuals. As a result, first-feeding larvae survive better in the more food-rich areas on the shelf, while larger larvae survive better in the open ocean with fewer predators, making the shelf break an optimal spawning area. Our findings support the hypothesis that ABT spawn in oligotrophic regions to minimize predation on their larvae. Ocean modeling tools presented in this study may help facilitate an ecosystem-based management approach to improve future stock assessment models by better resolving the stock-recruitment relationship.

ecology↗

bioRxiv2020

Plankton food webs of the Gulf of Mexico spawning grounds of Atlantic Bluefin tuna

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) that increases trophic transfer from phytoplankton to larval fish.

ecology↗