Effects of fishing and environmental parameters on the commercial bony fish assemblage in the southern Caspian Sea

The Caspian Sea (CS), the largest lake in the world, is a unique ecosystem known for its rich and diverse aquatic fauna (Karpinsky et al. 2005). Since the 1990s, intensive fishing and invasive species are major direct or indirect threats to the structural and functional organization of this unique ecosystem. Direct effects are manifested as a spatial and temporal gradient in the abundance of target species, and habitat destruction (Roohi et al. 2010; Pourang et al. 2016; Tavakoli et al. 2019; Fazli et al. 2013Fazli et al. 2020Fazli et al. 2021), while indirect effects as changes in community structure or differential effects on functional groups (Roohi et al. 2010; Pourang et al. 2016) in the CS. On the other hand, one of the main sources of impact on marine resources is climate variability and regime shifts (Johnson & Welch 2009). Beyraghdar Kashkooli et al. (2017) documented that the benthopelagic fish stocks respond to climatic events and it seems that the CS confronts the regime shift, emphasizing the need for more research and further supporting evidence.

Knowledge of fish habitat requirements and the interactions between habitat and population dynamics is essential for the management and conservation of aquatic resources (Mouton et al. 2007; Hermosilla et al. 2011). Since single-species management strategies do not fully account for ecological interactions and environmental factors (Garcia 2003), ecosystem-based fishery management is therefore a practical management approach to the conservation and restoration of fish stocks (Thrush & Dayton 2010).

Kutum (Rutilus kutum), carp (Cyprinus carpio), roach (Rutilus rutilus), Caspian vimba (Vimba vimba), shemaya (Alburnus chalcoides), barbus (Luciobarbus capito), bream (Abramis brama), asp (Leuciscus aspius), mullets (Chelon aurata and C. saliens), pikeperch (Sander lucioperca), salmon (Salmo caspius) and pike (Esox lucius) are commercially important among 159 bony fish species that occur in the CS (Naseka & Bogutskaya 2009). Several studies have been conducted to assess the stocks of the most valuable commercial bony fishes such as kutum (Fazli et al. 2012) and golden gray mullet (Fazli et al. 2013) in the southern CS. In addition, previous studies have revealed that environmental parameters can affect the habitat preferences of R. kutum (Haghi et al. 2013Haghi et al. 2015). Despite the commercial importance of bony fish, no study has been carried out on the species composition of these fish assemblages in relation to environmental parameters in the CS. This information can help fisheries managers to adopt a sustainable approach to fisheries management (Katsanevakis et al. 2009). The current study attempts to assess the status of bony fish species composition in the southern CS between 1996 and 2017. Therefore, the objectives of this study were (i) to examine the relationships between the community structure of commercial bony fish and environmental parameters, and (ii) to analyze the Abundance Biomass Comparison (ABC).

The ABC curves and W statistics suggest that commercial bony fish communities were environmentally stressed in the southern CS from 1996 to 2003 (negative scores of the W index). The ABC variations were mainly due to changes in the relative abundance and biomass of benthic communities such as polychaete species (Warwick & Clarke 1994). Most of the species involved in the present study, such as R. kutum, C. carpio, V. vimba, L. aspius, A. brama, L. capito, are benthopelagic and other species are pelagic or neritic species (Froese & Pauly 2019). According to Yemane et al. (2005) and Blanchard et al. (2004), ABC plots of demersal fish may reflect changes in the relative abundance of large and small species in assemblages. Roohi et al. (2010) reported that the Caspian macrobenthos composition changed after the invasion of Mnemiopsis leidyi (Ctenophora), which took place in the 1990s, and the macrobenthic community structure shifted from the dominant filter-feeding group (crustaceans) to deposit-feeders such as Neries diversicolor (Polychaete) and oligochaetes. Therefore, the negative scores of the W index could be related to changes in the macrobenthos community in the 1990s.

The decreasing trend of H′ suggests that the bony fish assemblage has been exposed to increasing stress over the last 22 years (Fig. 6). The ABC curves and the W index were helpful in determining the environmental stress (Mise et al. 2018), however, more conclusions were derived from analyzing the correlations between fish diversity and environmental conditions and anthropogenic activities.

In recent decades, due to the increasing impact of anthropogenic activities on marine biota, the United Nations has intensified efforts to ensure the conservation of marine biodiversity (Escobar-Toledo et al. 2015). As a result, it has been suggested that strategies for marine resources management should be based on the ecosystem context, including protection of habitats, non-target species, and populations of commercially valuable species (Pikitch et al. 2004). In the case of the CS, commercially most important fish stocks, such as two main pelagic species i.e. kilka (Fazli et al. 2020) and sturgeons (Khodorevskaya et al. 2009Khodorevskaya et al. 2014; Qiwei 2010; Tavakoli et al. 2019; Fazli et al. 2021), collapsed due to overfishing and ecosystem destruction.

The present study provides baseline information on the commercial bony fish populations in the CS. Of the 13 commercial bony fish species, only two species R. kutum and C. aurata accounted for about 90% of the total catch. Similar results were reported by Sulymanova et al. (2015) from the Absheron Gulfs of the CS using different fishing methods (set net). They reported that of the 33 species, 12 species belonged to commercial demersal fish species, of which R. rutilus and C. aurata were the most abundant ones.

NPUE and CPUE of bony fish showed a decreasing trend from 1996 to 2017 (Fig. 3), and their community structure was strongly correlated with year, PR, SSL, SST, GTA, and freshwater discharges. These environmental variables significantly correlated with the distribution of the three most migratory anadromous fish species (R. kutum, C. carpio, and R. rutilus) and two exotic catadromous species of mullets (C. aurata and C. saliens) in this study (Table 4). Beyraghdar Kashkooli et al. (2017) documented that stocks of two benthopelagic fishes (R. kutum and C. aurata) respond to climatic events and the CS seems to confront the regime shift and reported that the Siberian High (SH) triggered the SST regime shift in the late 1990s. This suggests that changes in the global climate indices are followed by changes in the regional (SST) and local indices (RD and EV) in recent decades. In addition, three factors (year, SSL, IRD) explained 86.7% of the total variation of H′. It appears that due to the current downward trend in SSL, NPUE, river discharge, spawning ground deterioration, limited habitats and increasing SST, the downward trend in biodiversity will continue in the future as most anadromous bony species from the CS spawn in the rivers.

Based on the ABC curves and W statistics obtained in the present study, it can be concluded that the fish assemblage in the southern CS was environmentally stressed between 1996 and 2003. The abundance indices of bony fish show a gradient, and their community structure is strongly correlated with environmental variability. The environmental parameters, namely PR, SST, SSL, and freshwater discharge, significantly affect the multispecies community, H′, and individual fish species. H′ shows a decreasing trend, suggesting that Iranian commercial fish species have been increasingly stressed over the past 22 years. Considering the current environmental conditions (decreasing trend in SSL river discharges and increasing SST) and anthropogenic activities (spawning ground deterioration and overfishing), it appears that the decreasing trend in H′ will continue in the future.