Predation on Fishes in the Sacramento-San Joaquin Delta: Current Knowledge and Future Directions

Gary D Grossman

doi:10.15447/sfews.2016v14iss2art8

Abstract

doi: http://dx.doi.org/10.15447/sfews.2016v14iss2art8 The Sacramento–San Joaquin Delta (Delta) is a heterogeneous, highly modified aquatic system.I reviewed relevant predator–prey theory, and described extant data on predator–prey relationships of Delta fishes. I ranked predator consumption rates as occasional, moderate, and common, based on frequency-of-occurrence data, and evaluated the frequency, and hypothesized the effects of predation on native and invasive species. I identified 32different predator categories and 41 different prey categories. Most predators were occasional consumers of individual prey species, although I also observed moderate and common consumption of some prey types. My analysis yielded few generalizations regarding predator–prey interactions for Delta fishes; most predators consumed a variety of both native and invasive fishes. The only evidence for predator specialization on either native or invasive fishes occurred in Prickly Sculpin which, when it consumed fishes, ate mostly native species. BothStriped and Largemouth Bass exhibited wide dietary breadth, preying upon 32 and 28 categories of fish prey respectively. Sacramento Pikeminnow, a native predator, also displayed wide dietary breadth of piscine prey, with 14 different prey categories consumed. Data for reptilian, avian, and mammalian predators were sparse; however, these predators may be significant fish predators in altered habitats or when hatchery salmonids are released. The database for predators and their fish prey was not strong, and I recommend long-term dietary studies combined with prey availability and behavioral and experimental studies to establish predator preferences and antipredator behaviors, rather than just consumption.The behavioral effects of contaminants on prey species also warrant further examination. Although it has been suggested that a reduction in the StripedBass population be implemented to reduce predation mortality of Chinook Salmon, the large number of salmon predators in the Delta make it unlikely that this effort will significantly affect salmon mortality.

Highlights

The Sacramento–San Joaquin Delta is the eastern portion of the largest estuarine system on the West Coast of the Americas (San Francisco Estuary), encompassing an area of 3,238 km2 (Whipple et al 2012)
3. biological characteristics, most of which have produced declines in habitat quality for native species. These decreases in habitat quality in concert with the effects of introduced species, are linked to major declines in formerly abundant native species such as Chinook Salmon (Oncorhynchus tshawytscha), Steelhead (Oncorhynchus mykiss), Delta Smelt and Longfin Smelt (Hypomesus transpacificus and Spirinchus thaleichthys, respectively), the extinction of the Thicktail Chub (Gila crassicauda), and the local extinction of the Sacramento Perch (Archoplites interruptus)
It is clear that native and invasive (I use the term invasive because it accurately represents the fact that these species have invaded a habitat) prey species run a gauntlet of potential predators including invasive predatory fishes (Striped Bass, centrarchids, percids and ictalurids), native and introduced snakes and amphibians, mammals (North American river otter Lontra canadensis), and birds (Draulans 1988)

Summary

INTRODUCTION

The Sacramento–San Joaquin Delta (the Delta) is the eastern portion of the largest estuarine system on the West Coast of the Americas (San Francisco Estuary), encompassing an area of 3,238 km (Whipple et al 2012). Predator densities in the treatment section tripled after the first removal, which underscores the importance of potential compensatory responses by other predatory species when a single predator species is removed This phenomenon is the biggest weakness of the “predator removal” management approach (e.g., Striped Bass predation limits Chinook Salmon populations), because in the Delta there are literally 10 to 20 other predatory species that could potentially increase in abundance (and prey consumption) if a dominant predator is removed. This may have occurred when the Northern. Moderate: Sacramento Pikeminnow consuming Longfin Smelt, Striped Bass consuming Sacramento Splittail, Largemouth Bass consuming Prickly Sculpin; and

Common

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F F F NNN FNNNF

Findings

CONCLUSIONS