Describing biological parameters of several endemic Salmo species from the Caspian Sea and Persian Gulf basins

The fish fauna of Türkiye is considered highly diverse (Bayçelebi, 2020; Bayçelebi et al., 2015; Freyhof et al., 2014, 2025; Kaya et al., 2016, 2020). The Anatolian region alone encompass three biodiversity hotspots, providing diverse freshwater habitats for a highly diverse native fish fauna, including salmonids (Freyhof et al., 2025). Salmonids are vital for ecological, economic, and cultural reasons. They are integral to commercial and recreational fisheries, significantly contributing to regional economies such as the Pacific Northwest and Scandinavia. Salmonids are a nutritious, omega-3-rich food source vital to seafood production and aquaculture (Zhang et al., 2023). They also serve as ecological indicators of water system health due to their sensitivity to habitat and water quality changes (Kroglund et al., 2008). Their unique life cycles, involving both freshwater and marine phases, enhance biodiversity and nutrient cycling, as they transport marine nutrients to freshwater ecosystems during spawning (Naiman et al., 2002). Salmonids are studied for their adaptability and migration, providing insights into fish biology and conservation (Liu et al., 2016), while also supporting sustainable aquaculture and holding cultural significance in traditions, fishing, and tourism. For these reasons, Salmonidae play a particularly important part in Türkiye in terms of fish biodiversity and socio-cultural values. In particular, native brown trout populations in Türkiye’s water resources, with their considerable genetic diversity, represent an economic and ecological heritage that requires protection (Zencir Tanir & Fakioğlu, 2017).

Numerous studies investigated Salmo trutta Linnaeus, 1758, a species inhabiting Anatolia’s freshwaters, focusing on systematics, reproduction, and growth (Karataş, 1999; Kuru, 1975; Slastanenko, 1995), making this species the most extensively researched fish species in Türkiye’s waters (Alp et al., 2005; Arslan et al., 2007; Başusta et al., 2013; Gülle et al., 2007; Kaya, 2020; Kocabaş et al., 2012). Until the early 2010s, the diversity of salmonids in Anatolia was largely unknown. Subsequent systematic studies on the genus Salmo led to the description of sixteen new species (Turan & Aksu, 2021; Turan et al., 2010, 2011, 2012, 2014a, 2014b, 2017, 2021, 2022). While there is ample information on Salmo trutta, data on other members of the Salmonidae family are even more scarce. Moreover, data on their unique characteristics and population parameters remain limited, hindering the development of management strategies and conservation efforts for these species (Tarkan et al., 2008).

Seasonal variations in water regimes, along with domestic and industrial wastewater pollution and habitat destruction in both breeding and living areas, pose significant threats to the sustainability of brown trout populations (Zencir Tanir & Fakioğlu, 2017). These factors are likely to also affect the sustainability of other trout species. The impact of climate change further exacerbates these issues, leading to altered flow patterns and water temperatures in rivers and streams, which can disrupt the life cycles and migration patterns of salmonids (Ficke et al., 2007). Overfishing and illegal fishing practices contribute to the ongoing decline in populations and their respective sizes (Martinez et al., 2009; Okumuş et al., 2006), while the introduction of non-native species can lead to competition for resources and habitat, as well as the spread of diseases (Yoğurtçuoğlu et al., 2021). Additionally, the use of pesticides and chemicals in agriculture, which can seep into waterways, poses another risk to the health and survival of salmonids (Stanley & Preetha, 2016). Efforts to mitigate these threats include the implementation of stricter regulations on industrial discharges, improving wastewater treatment processes, restoring and conserving natural habitats, enforcing sustainable fishing practices, and conducting ongoing research and monitoring to better understand and manage salmonid populations (Young et al., 2018).

Fish biology demonstrates unlimited growth, resulting in continual changes in both length and weight (Longo et al., 2021). Fish exhibit continuous growth, with changes in length and weight influenced by water resource conditions (Longo et al., 2021). Understanding the length–weight relationship is vital for assessing fish stocks and life cycles in different habitats (Froese et al., 2011; Petrakis & Stergiou, 1995). The slope parameter (i.e. b values) in these relationships, which vary by region, sex, and habitat, are key for population monitoring (Froese & Pauly, 2023), with an optimal range of 2.5–3.5 for salmonids (Froese, 2006). The condition factor (CF) further aids in evaluating population health (Anene, 2005), while growth patterns, whether allometric or isometric, serve as indicators of population status (Froese, 2006; Tesch, 1971).

While the current literature reveals a significant amount of recent research on the biological and ecological aspects of fish stocks, many of Türkiye’s endemic salmonid species still suffer from a lack of data. Although there are several studies on the distribution of the numerous species in the family Salmonidae in Türkiye, there remains only anecdotal information on their biological parameters (Mazlum & Turan, 2018). In FishBase, for instance, there are still no detailed data available for the studied species in this research: Salmo araxensis, S. ardahanensis, S. baliki, S. fahrettini, S. munzuricus, and S. murathani, such as maximum length or weight (Froese & Pauly, 2023). This indicates that there remains a pressing need for biological data on members of the Salmonidae family, particularly regarding their length–weight relationships, growth types (GT), and CFs. The results of such research would not only aid in developing management strategies for this family but also contribute to a database for future studies. Therefore, it is hoped that the findings of this study will offer valuable data to support both conservation and management efforts. Hence, the main aim of this study is to identify lacking biological information for members of the Salmonidae family in Türkiye. For this, this study investigates six species of the Salmo genus and different water resources from both the Caspian Sea and the Persian Gulf basins in Türkiye.

Sampling 136 specimens from S. araxensis, S. ardahanensis, S. baliki, S. fahrettini, S. munzuricus, and S. murathani (Figure 2), we found that the maximum length and weight recorded were 31.8 cm for S. munzuricus (from the Persian Gulf basin) and 451.60 g for S. baliki (also from the Persian Gulf basin), respectively. In contrast, the minimum length and weight were observed in S. araxensis from the Caspian Sea basin, with a length of 6.9 cm and a weight of 3.56 g (Table 2). The average length L_mean across all populations ranged from 14.73 cm in S. araxensis to 23.60 cm in S. ardahanensis. The maximum length and weight were found to be L_max = 31.8 cm (S. munzuricus from the Persian Gulf basin) and W_max = 451.60 g (S. baliki from the Persian Gulf basin), and the minimum length and weight were found to be L_max = 6.9 cm and W_max = 3.56 g (S. araxensis from the Caspian Sea basin) (see Table 2). The L_mean of all the populations varied between 14.73 cm (S. araxensis) and 23.60 cm (S. ardahanensis).

Figure 2

The salmonids examined in this study: (A) Salmo baliki, FFR 3242, holotype, 212 mm SL, male, Sinek Stream; (B) S. murathani, FFR 3228, paratype, 241 mm SL, female, Arpaçay Stream; (C) S. araxensis, FFR 3224, holotype, 259 mm SL, male, Kırkpınar Stream; (D) S. fahrettini, FFR03231, holotype, 232 mm SL, male, Stream Ömertepesuyu; (E) S. munzuricus, FFR 3226, 240 mm SL, male, stream Alakoçlu; (F) S. ardahanensis, FFR 3239, holotype, 222 mm SL, male, Toros Stream.

The r² values indicated a high correlation (between 0.97 and 0.99; for all populations as >0.97). The constant a ranged between 0.0033 and 0.0104 for the populations of all six species, while the constant b was found to be above 3.00 for all populations, revealing the highest b parameter as 3.4440 for S. baliki and the lowest value for S. ardahanensis (b = 3.0032). The mean CF ranged between 0.0998 (± 0.001) (for the species S. araxensis and S. murathani) and 1.010 (± 0.019) (for S. munzuricus) (see Table 3). The t-test found that while five of the Salmo populations showed a positive allometric growth pattern, S. ardahanensis expressed negative allometric growth patterns (Table 3).

The discovery of 16 new Salmo species in Anatolia (Turan & Aksu, 2021; Turan et al., 2010, 2011, 2012, 2014a, 2014b, 2017, 2021, 2022) highlights the region’s high degree of biodiversity. However, significant knowledge gaps remain, particularly regarding other members of the Salmonidae family. While studies on the distribution of these species exist, crucial data on their unique traits and population parameters are limited, hindering the development of effective management and conservation strategies (Tarkan et al., 2008). Though S. trutta has been relatively well studied, there is still a lack of information on other species, preventing a comprehensive understanding of emerging threats such as habitat degradation, climate change, and human impact.

Prior to this study, no data existed on length–weight relationships and CFs of the species of the genus Salmo. Our study fills this gap by providing information on length–weight relationships and CFs of Salmo species found in the Caspian and Persian Gulf basins, which may serve as the groundwork for further research on the fish populations. This study provides new information about the growth performance, length–weight relationships, and CFs of the endemic trout species inhabiting these two basins in Anatolia.

Despite employing traditional methods and working with a limited dataset, this study provides valuable biological data for six species from the Salmonidae family. This contribution is important as it is the very first study related to these species’ biological parameters in both of these river basins. Despite previous research done in Türkiye, some results were obtained regarding the main trends in fish stock fluctuation; however, several endemic salmonid species do not have any biological database yet. Although distribution studies exist, detailed biological parameters are still scarce, such as maximum size and weight (Mazlum & Turan, 2018). Further species, such as S. araxensis, S. ardahanensis, S. baliki, S. fahrettini, S. munzuricus, and S. murathani, still lack critical data in resources like FishBase (Froese & Pauly, 2023). This gap in data hampers effective population monitoring and the formulation of management strategies, hence a need for more comprehensive studies to support the conservation effort.

The individuals collected in this study exhibited total lengths ranging from 6.9 cm to 31.8 cm and body weights varying from 3.56 g to 451.60 g. Moreover, S. munzuricus had the greatest length (L_max = 31.8 cm), while S. fahrettini recorded the shortest (L_max = 26.7 cm) among the species studied. The heaviest weight was noted in S. baliki (W_max = 451.60 g). The average length of the fish populations was calculated to range from L_mean = 14.73 ± 1.772 cm (S. araxensis) and 23.60 ± 0.680 cm (S. ardahanensis). A recent study by Mazlum and Turan (2018) provided a database with information on various Salmo species. According to their findings, new maximum length records were established for S. abanticus (45.1 cm), S. chilo (34.4 cm), and S. caspius (29.9 cm) (Fishbase, 2023). This current study contributes new maximum length data for six Salmo species. The collected individuals of S. munzuricus showed the highest maximum length of the studied species, with total lengths ranging from 12.7 cm to 31.8 cm and a L_mean of 21.36 ± 0.805 cm. Salmo munzuricus total weight ranged from 27.10 g to 441.19 g, with a W_mean of 143.01 ± 16.787 g. Conversely, S. araxensis, which had the smallest length sample, had a total length ranging from 6.9 cm to 29.5 cm, with L_mean = 14.73 ± 1.772 cm. This species’ total weight ranged from 3.56 g to 334.40 g, with W_mean = 73.34 ± 24.428 g.

The detailed measurements of Salmo species in the study provide significant insights into the ecological roles and status of these salmonids in their respective habitats. The range in total lengths and weights observed among the species, such as the notable sizes of S. munzuricus and S. baliki, underscores the diversity within the genus, indicating varied ecological niches and roles in the food web (Musseau et al., 2015). Larger species like S. munzuricus likely occupy higher trophic levels and serve as key predators, exerting control over prey populations and thus influencing the overall biodiversity and balance in their ecosystems (Verep et al., 2016). The variation in average lengths and weights, as indicated by the differences between species like S. araxensis and S. ardahanensis, reflects the species-specific adaptations and ecological strategies (Sánchez-Hernández et al., 2012), being crucial for maintaining ecosystem resilience and functionality. Larger and heavier individuals, however, are typically more energetically efficient, contributing more significantly to the reproductive output and genetic diversity of populations (Garant et al., 2001). Furthermore, the establishment of new maximum length records for species such as S. abanticus and S. chilo, as reported by Mazlum and Turan (2018), highlights the dynamic nature of these populations and possibly indicates changes in habitat conditions or resource availability.

Hence, this report marks the first time that length–weight relationships for S. araxensis, S. ardahanensis, S. baliki, S. fahrettini, S. munzuricus, and S. murathani have been documented from the freshwaters of Türkiye. In this study, it was found that the b constant was greater than 3.00 for all the populations examined. Specifically, S. baliki, located in the Murat River of the Persian Gulf basin, exhibited the highest b value (3.444 ± 0.015) among all the Salmo species from both the Caspian Sea and Persian Gulf basins. Conversely, the lowest b value was recorded for S. ardahanensis in the Toros stream, part of the Kura River in the Caspian Sea basin, with a value of 3.0032 ± 0.023. Both biological and physical factors can significantly influence the length–weight relationships of fish populations. These factors include sex, seasonal variations, habitat characteristics, stomach fullness, and reproductive periods (Froese et al., 2011). Additionally, environmental factors such as maturity level, habitat health, stress, and availability of nutrients are crucial in determining the overall health of fish populations (Liang & Cai, 2020).

The findings from our study suggest that the Salmo populations are currently in a good state within their natural habitats, as indicated by their b constants being greater than 3 and their high CFs. However, it is important to recognise that Salmo species require high-quality water conditions for optimal health. Therefore, it is essential to protect and maintain the aquatic habitats of these Salmo species to preserve their population health and ensure their long-term survival. The CF of fish populations is a crucial indicator of their overall health and vitality, reflecting the physical condition of the fish and serving as a key metric for evaluating their welfare (Gücü et al., 2018). CF values are particularly informative in assessing the well-being of fish populations. In this study, the mean CF values of the populations were found to be high, ranging from 0.998 (± 0.001) to 1.010 (± 0.019). The highest mean CF was recorded in the S. munzuricus population from the Persian Gulf basin. On an individual level, the maximum CF was observed in S. munzuricus, while the minimum CF was noted at 0.838 in the S. araxensis population. Based on the results indicating high mean CF values in salmonid populations, it can be inferred that these populations are generally in good health and well-nourished. The higher CFs observed suggest that the salmonids are thriving in their environment, likely to benefit from adequate food availability, optimal habitat conditions, and effective management practices. This positive trend in CF is a promising indicator of the robust health and ecological well-being of the salmonid populations under study. Given the relationship between CF and reproductive activities, regular monitoring of the population’s condition is critically important for the sustainability of fish stocks (Ferrer-Maza et al., 2016). Such continuous observation helps in understanding and managing the health and viability of these populations, ensuring their long-term conservation and management.

Our study has effectively filled the gap in data concerning the length-weight relationships and CFs of six endemic Salmonidae species in the studied region. We advocate for further research on the Salmonidae, focusing on developing strategies to protect their welfare. This should include exploring the effects of climate warming on fish behaviour, potentially employing biotelemetry methods to gain deeper insights. Additionally, future research could involve stable isotope analysis, using non-lethal techniques similar to those employed by Kurtul et al. (2023). Such methods would not only be less invasive but also provide valuable information about the ecological niches of Salmonidae and contribute to a more comprehensive understanding of their ecological roles and requirements, facilitating more effective conservation and management strategies.