Citation: Akgun, Y.; Akoglu, E. Randall’s Threadfin Bream (Nemipterus randalli, Russell 1986) Poses a Potential Threat to the Northeastern Mediterranean Sea Food Web. Fishes 2023, 8, 402. https://doi.org/10.3390/ fishes8080402 Academic Editor: Eric T. Schultz Received: 30 June 2023 Revised: 30 July 2023 Accepted: 31 July 2023 Published: 3 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fishes Article Randall’s Threadfin Bream (Nemipterus randalli, Russell 1986) Poses a Potential Threat to the Northeastern Mediterranean Sea Food Web Yagmur Akgun 1 and Ekin Akoglu 2, * 1 Department of Biological Sciences, Middle East Technical University, 06800 Ankara, Türkiye; akgun.yagmur@metu.edu.tr 2 Institute of Marine Sciences, Middle East Technical University, 33731 Erdemli, Türkiye * Correspondence: eakoglu@metu.edu.tr Abstract: The eastern Mediterranean Sea is one of the most invaded marine ecosystems due to the introduction of Lessepsian species, which migrated from the Red Sea to the Mediterranean Sea following the construction of the Suez Canal. Some of these species may initially appear to be beneficial for fisheries by providing additional income sources for fishers; however, this usually occurs at the expense of negatively impacted native species and, thus, the ecosystem, which leads to greater economic losses for the fisheries in the long term. Therefore, this study aims to quantify the impact of N. randalli, which is one of the Lessepsian species with increasing commercial importance for the fisheries, on the food web dynamics in a coastal ecosystem in the northeastern Mediterranean Sea using a mass-balance food web modelling approach by capitalising on field data obtained from trawl samplings conducted within the scope of the study. Results showed that the ecosystem was in a developmental stage and experienced an autotrophic succession. The keystone fish group with a structuring role in the food web was sea breams and porgies. Although N. randalli had positive impacts on certain commercially exploited indigenous demersal fish species by mitigating the negative impact of another Lessepsian species, i.e., Saurida undosquamis (Richardson, 1848), in the food web, it had a negative impact on the keystone group of the food web, i.e., sea breams and porgies. Therefore, N. randalli poses a potential threat to the ecosystem’s structure, and the interactions of N. randalli with other species in the food web may instigate an ecosystem reorganisation in the future. We suggest targeted fisheries exploitation and incentives for the fishery of N. randalli as management strategies to mitigate its negative impacts. However, the mitigating role of N. randalli in regulating the negative impacts of S. undosquamis could be adversely affected by its increasing exploitation; therefore, future modelling studies should consider scenario simulations to test such effects. Keywords: alien fish species; Ecopath; food web modelling; ecological impact; Randall’s threadfin bream Key Contribution: Randall’s threadfin bream has a mitigating role against negative impacts of other Lessepsian fish species, whereas it could be a potential instigator of drastic reorganisations in the northeastern Mediterranean Sea food web. 1. Introduction Species that move or are introduced beyond their past or present distribution and are capable of surviving and reproducing in their new environment are called alien species, and alien species that threaten the biological diversity in their new environment are called invasive alien species [1]. The Levantine Sea is considered one of the marine regions most impacted by biological invasions, with an alien-to-native species richness ratio of 0.69 [2], and was tremendously affected by two anthropogenic stressors other than fisheries: the constructions of the Suez Canal and the Aswan Dam. Both have a crucial role in the species migration from the Red Sea to the Mediterranean Sea, which is known as the Lessepsian Fishes 2023, 8, 402. https://doi.org/10.3390/fishes8080402 https://www.mdpi.com/journal/fishes