Abstract
Author(s): Nobriga, Matthew L.; Smith, Wiliam E. | Abstract: Striped Bass, Morone saxatilis, has been an established member of the San Francisco Estuary’s (estuary’s) aquatic community for nearly a century and a half. As a predator, it has the potential to shape community composition through top-down control of lower trophic species, including the endangered Delta Smelt, Hypomesus transpacificus. Invasive predators can be particularly disruptive to native communities because they present novel dangers to naive populations, but, as a long-established member of the aquatic community, Striped Bass has not previously been considered to limit the Delta Smelt population. Here, we develop an argument that Striped Bass are important to controlling Delta Smelt. We support this argument by reviewing historical data which suggests that declines in Delta Smelt before the current-day monitoring program were driven by the invasion of Striped Bass into the estuary. We describe this phenomenon as the ‘phantom predator’ hypothesis in the context of an analog to the shifting baseline syndrome previously described for marine fisheries. A deeper understanding of how well studied (and rapidly changing) bottom-up drivers of the estuary food web interact with poorly understood (but also rapidly changing) controls at the top of the food web could prove very important to the conservation of other declining native fishes and possible future attempts to re-introduce captive-reared Delta Smelt to the estuary.
Highlights
By sitting at or near the top of aquatic food webs, piscivorous fishes can influence the population dynamics of co-occurring fish at multiple trophic levels via direct consumption or by changing the behavior and habitat use of potential prey (Kitchell et al 1994; Pine et al 2009)
In Part 2, we focus on catch data for age-0 Striped Bass and Delta Smelt, but provide additional context using catch data for four additional pelagic fish species that commonly occur in the region sampled by the California Department of Fish and Wildlife’s long-term monitoring programs
The phantom predator hypothesis can be summarized as follows: (1) Striped Bass were a major predator of Delta Smelt in the past, before monitoring surveys began; (2) Striped Bass predation contributed to the historical decline of Delta Smelt; (3) when Delta Smelt abundance dropped, Striped Bass switched to other prey; and (4) when Delta Smelt would temporarily rebound, Striped Bass would target Delta Smelt again until they were no longer profitable, which maintained a persistent limit on Delta Smelt production
Summary
By sitting at or near the top of aquatic food webs, piscivorous fishes can influence the population dynamics of co-occurring fish at multiple trophic levels via direct consumption or by changing the behavior and habitat use of potential prey (Kitchell et al 1994; Pine et al 2009). We summarized the distributions of Delta Smelt and the five other commonly collected pelagic fish species relative to salinity using the FMWT data divided into pre- and post-overbite clam years (Figure 5). To evaluate whether the long-term trawl surveys mischaracterize the relative abundance of Delta Smelt, we summarized the extensive published literature on Bay−Delta fish assemblages (Table 5) These studies used several classes of gear types, sampled onshore and offshore habitats, and represented many years of data collection. The only gear type in which Delta Smelt has tended to outnumber Striped Bass is Figure 5 Salinity distributions of Fall Midwater Trawl catch for six pelagic estuary fishes, summarized by pre-overbite clam invasion years (1967−1986) and post-invasion years (1987−2017). Remnant tidal marshes from San Pablo Bay to the Sac−San Joaquin river confluence
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