Variations in the Demersal Fishes of the Northeastern Mediterranean Over the Last 40 Years in Relation to Environmental Factors

Date

2025-7-04

Author

Kurt, Merve

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The Mediterranean Sea has undergone substantial ecological transformations driven by climate change, overfishing, and the continuous introduction of non-indigenous species (NIS) through the Suez Canal. These stressors often interact, resulting in complex ecological responses. The Northeastern Mediterranean, is among the regions most heavily impacted by these factors, suffers from a scarcity of long-term datasets, making it challenging to interpret the trajectory of these changes. However, the scarcity of long-term datasets in this area makes it challenging to interpret the trajectory of these ecological changes. In this study, datasets collected by METU-IMS since the 1980s (including both CTD measurements and fisheries survey data) were integrated with newly collected data to investigate faunal changes in one of the fastest-warming and most NIS-impacted region of the Mediterranean. Rather than attributing these changes to a single factor, the aim of this study was to address them from multiple perspectives. The objectives of this study were to (i) identify fish assemblages and their temporal changes, (ii) disentangle the environmental drivers underlying biomass variability in an exploited demersal species, through an approach that separates ecological signals from the confounding effects of long-term fishing pressure, and (iii) elucidate the response of the bathymetric distribution of a native fish species to environmental changes, given their expected sensitivity to climate-driven variability. To address the latter two objectives, the red mullet (Mullus barbatus) served as a case study. As a native and heavily exploited species in the region since the 1940s, it was used in both the analysis of biomass fluctuations and the evaluation of bathymetric distribution changes. Non-metric multidimensional scaling (NMDS) and cluster analysis constructed using the trawl data collected during the spring and autumn seasons between 1983–1984 and 2022 (0–100 m depths) revealed three major zones in the region. The shallow zone, referred to as Por's "Lessepsian Province," has been consistently dominated by NIS biomass, with their contribution reaching up to 97% by 2022. In contrast, the deeper zone, which extends to circalittoral area, exhibited a relatively stable community structure with dominance of native biomass, where M. barbatus remained a significant contributor to biomass over the years. However, NIS biomass showed a notable increase during the spring season. The western zone, characterized by Posidonia oceanica meadows, partially overlaps with a marine protected area and was also dominated by native species. However, the highest proportional increase in NIS numbers to total species count was recorded in this zone. Notably, NIS such as Parupeneus forsskali, which are scarcely found in the other parts of the study area, have emerged as indicator species in the western zone. Both the depth and location of the sampling stations significantly influenced community composition. To explore fluctuations in M. barbatus biomass over the years, official landing statistics since 1968 and trawl survey data collected since 2007 (0–150 m depths) were incorporated into the stochastic surplus production model in continuous time (SPiCT). The model was used primarily to derive residuals, which served as inputs for the main analyses conducted in this study. To assess environmental influences, three large-scale climate indices were considered: (i)North Atlantic Oscillation (NAO), (ii) East Atlantic Pattern (EA), and (iii) East Atlantic/Western Russia Pattern (EA/WR), each of which can simultaneously impact multiple environmental parameters. Biomass variations that could not be explained by fisheries models, were analyzed using wavelet coherence analysis with these climate indices to explore covariance between environmental influences and biomass trends. This analysis revealed a significant coherence between these and the climate indices. Notably, the EA index consistently influenced M. barbatus biomass from 2000 to 2015, while the effects of NAO and EA/WR were particularly strong in the 1980s and early 1990s. Interestingly, all indices showed a combined influence in the early 1990s, coinciding with an increase in M. barbatus biomass. This period also overlapped with major ecological shifts observed in the Mediterranean, offering a broader perspective on the interaction between climatic variability and ecosystem changes. Although the influence of NAO is generally known to diminish towards the Eastern Mediterranean, a significant link detected in this study during the 1990s emphasizes its transient relationship with fish biomass. Furthermore, this study highlights the importance of the previously underexplored association between EA and EA/WR indices and biomass fluctuations of demersal fish. Climate-induced warming is expected to modify bathymetric habitat structure in marine ecosystems, potentially forcing species to shift deeper in the water column in search of suitable thermal conditions. To assess whether such shifts have occurred in the Northeastern Mediterranean, the bathymetric distribution of the maximum biomass of M. barbatus was examined for the period 2010 to 2023. The results revealed that the depth of maximum M. barbatus biomass followed a clear seasonal cycle in the study area, being shallowest in late winter/early spring and deepest in autumn. Beyond this seasonal cycle, a long-term deepening trend was observed, with the maximum biomass depth during the study period. Concurrently, the water column displayed significant thermal changes. The temperature model indicated a non-linear warming trend, particularly after 2016. The results showed that the thermocline exhibited a gradual deepening and prolonged persistence between 2010 and 2023. This deepening, specifically in September, showed a strong temporal correspondence with the observed long-term deepening of the maximum M. barbatus biomass depth. The Northeastern Mediterranean plays a crucial role in anticipating the future state of the Mediterranean Basin, particularly as climate change and the spread of NIS continue to accelerate at an unprecedented rate. This research provides valuable insights into faunal dynamics in this critical region and emphasizes the need for sustained monitoring and integrative approaches to predict and mitigate future ecological transformations in this rapidly changing ecosystem.

Subject Keywords

Northeastern Mediterranean, Non-indigenous Species, Large-Scale Climate Indices, Demersal Trawl, Climate Change

URI

https://hdl.handle.net/11511/117407

Collections

Graduate School of Marine Sciences, Thesis