Spatial Variability in the Fish Assemblage of a Large River–Reservoir Ecosystem

Abstract

Physical and hydrologic connectivity among river-reservoir ecosystem (RRE) segments, including the river, riverreservoir interface (RRI), and reservoir, requires consideration of management at larger spatial scales. Multiple objectives (e.g., ecosystem function, sport fisheries, and maximizing fish diversity) seem well suited to this broad spatial scale; however, data limitations regarding the distribution of taxa within RREs currently hamper such an approach. We quantified the fish assemblage of the Trinity River-Lake Livingston ecosystem in Texas, USA, using a stratified approach to determine the contribution of individual segments to the overall fish assemblage. In general, fish assemblages varied along a continuum from the river to the reservoir; however, fish assemblages in several river backwaters did not follow this gradient, thus adding diversity to the overall RRE. Variability in fish assemblages among ecosystem segments was the result of both species composition and relative abundance. Species richness (N = 58 total) was highest in the river segment (N = 49) followed by the RRI (N = 46) and was much lower in the reservoir (N = 36). In contrast, average catch per standardized unit of sampling effort (all species combined) was greatest in the RRI, about 25% lower in the reservoir, and about 50% lower in the river. Across the ecosystem, 26 species (45%) were considered common, while 32 species (55%) were considered rare. Notably, species of conservation need were not always most abundant in river habitats, and species supporting recreational fisheries were not always most dominant in the reservoir. These findings suggest that the RRE can serve as an appropriate spatial scale for both conservation and recreational fishery management actions.Regulation of rivers through impoundment provides many benefits to humans (e.g., water supply, electricity, and recreation); however, alteration of these ecosystems has been detrimental to many native fish species, especially fluvial specialists (e.g.,

Can spatial and temporal differences in fish assemblage structure inform conservation of an endangered sucker in a large western reservoir?

“…Similarly, fish abundance is often higher at inflow areas (Mueller and Horn 2004; Lin et al 2019; Pennock et al 2021), potentially as fish track abundant food resources or seek complex habitat. Alternatively (but not exclusively), fishes that are adapted to lotic habitats, such as Flannelmouth Sucker, might seek out inflow areas within reservoirs (da Silva et al 2015; Smith et al 2023). This is not always the case, as rheophilic species can be observed at high abundances away from inflow areas (e.g., Bullhead Minnow Pimephales vigilax ; Smith et al 2023).…”

“…However, conversion of lotic to lentic habitat after river impoundment can lead to habitat loss and can promote introduced species that threaten native aquatic organisms. River regulation generally reduces the abundance and diversity of native riverine species (Poff et al 2007; Lintermans 2012; Mims and Olden 2013), but river–reservoir ecosystems can provide beneficial habitats for some native species in altered riverscapes (Smith et al 2023). This is likely because habitats within reservoirs are highly dynamic, both temporally and spatially, and some habitats mimic those of large rivers, such as backwaters and deltas (Buckmeier et al 2014; Volke et al 2015; Albrecht et al 2018), which are often lacking or limited in regulated rivers.…”

“…Reservoirs are novel aquatic ecosystems resulting from fragmentation, lotic‐to‐lentic habitat conversion, and often the creation of non‐coevolved fish assemblages. Native riverine species using reservoirs are typically generalists with the ability to capitalize on lentic habitats, such as pools or floodplain backwaters (Agostinho et al 2016; Smith et al 2023). Reservoirs act as introduction points for both desirable and undesirable fishes (Martinez et al 1994; Havel et al 2005; Johnson et al 2008).…”

“…To better manage native species in reservoirs, we need a more complete understanding of factors limiting populations, such as the relative impact of introduced fishes versus habitat or food availability. This information is critical if we are to achieve both recreational fisheries management and conservation objectives for native species, as recently called for by several authors (Cowx et al 2010; Smith et al 2023).…”

Old Reservoirs, Old and New Challenges: Introduction to the 2021 Reservoir Symposium

“…Although much maligned for their effect on native riverine species, reservoirs are part of larger ecosystems that can harbor a variety of fish species both in the reservoir itself and in the river upstream (Smith et al. 2022, this special section). However, the future of some reservoir fisheries is uncertain as invasive species continue to be introduced into aquatic ecosystems throughout the world.…”