Delta Fisheries Predator Prey Relationships Who eats whom?

If Fish Could Talk...

In this first installment of the “If Fish Could Talk” segment in, California native and retired Department of Fish and Game Fishery Biologist  Dennis P. Lee discusses some of the science and management behind nonnative Delta predators like striped and largemouth bass, and their relationship to native species.

Delta Fisheries Predator- Prey Relationships – Who eats whom?

By Dennis P. Lee, Fishery Scientist

Prior to the late 19th century, the Sacramento-San Joaquin Delta was comprised of a maze of interconnected waterways, sloughs, and islands formed by the meeting of the Sacramento and San Joaquin rivers.  Historically water flowed in and out of the Delta in relation to river inflow and tidal influences.  However, early agriculture led to levee construction and eventually, the Delta was transformed into an engineered water movement system where the flow is altered by levee construction, water control structures, and diversions.  Instead of natural tidal marshes, wetlands, and abundant stands of mixed vegetation, the waterways and sloughs are now lined with riprap rock, overhanging black berry bushes, narrow stands of native California Tule, and shallow waterweed beds comprised of introduced aquatic species such as Brazilian pondweed Egeria densa.

The US Geological Survey reports that about 83% of the water exported from the Delta is used for agricultural and the remaining goes to urban and domestic supplies.  How much water is actually exported from the Delta is often debated.  Jerry Meral, Deputy Secretary of the California Natural Resources Agency in charge of the Bay Delta Conservation Planning Program, indicated that about 5 million acre feet of water is exported annually.  Total water production from the Sacramento and San Joaquin rivers combined can range from 7 to as much as 42 million acre feet annually depending on the type of water year.  In addition, the amount of water that eventually reaches Delta has increased as greater demands are placed on existing supplies.

Not only has the character and habitats of the Delta changed but so has the fish fauna.  Historically, the Sacramento and San Joaquin rivers, and California waters in general, did not support the variety of fish species compared to geologically older East Coast and Southeastern rivers.  In the late 1800’s, the introduction of fish from the other parts of the county was considered good management.  Early anglers and fish culturists encouraged the notion of moving and introducing fish into new waters, generally to develop commercial and sport fisheries.  Many of the early California fish introductions did not survive to produce self-sustaining populations, while others have become established throughout state waters.  In the case of many species, some people are still surprised when they are told that a particular fish is not native to California.

The first introduction of a nonnative fish to California was the goldfish, Carassius auratus.  The species was noted in the East Coast in the 1600’s, coming from Japan via Europe.  The first California specimens were noted in 1862 and most likely originated from pet releases.  The common carp Cyprinus carpio was not introduced to California until 1879 as a replacement for the “worthless and unpalatable fish of the warm waters of the great valleys in the interior of the state”.

One species that has experienced lasting success is striped bass Morone saxitallis.  In 1879, about 135 Striped bass were brought from the Navesink River in New Jersey, and planted in the Carquinez Straits near Martinez.  This was follow by an introduction in 1882 of 300 striped bass from the Shrewsbury River, also in New Jersey.  The fish were planted into Suisun Bay near Army Point.  The introduction was successful and within ten years, a commercial gill net fishery flourished in the Sacramento-San Joaquin rivers.  Later, in 1935 the commercial fishery was curtailed in favor of the sport fishery.

Another nonnative species that has gained popularity as a sport species is the largemouth bass Micropterus salmonids.  Although there are no early records of largemouth bass being intentionally stocked in the Delta, “black bass”, most likely smallmouth bass Micropterus dolomieu, were introduced into the Feather River in 1891.  Additional introductions followed and as early as 1910, many of the rivers and sloughs of the Sacramento and San Joaquin rivers were reported as “teeming” with largemouth bass M. salmoides.  By the 1950’s, sport fishing for largemouth bass in the Delta was popular and by the 1970’s became even more so because of the popularity of black bass tournaments.

The Sacramento and San Joaquin rivers historically provided spawning and nursery habitats for large runs of salmon and steelhead.  As early as the 1860’s, the Sacramento and San Joaquin rivers supported commercial salmon fisheries.  However, gold mining with its dredging and water diversion in second half of the 1800’s, followed by dam construction and development on all large rivers in the mid-1900’s, blocked runs and eliminated important spawning and nursery habitats that led to major reductions of Central Valley anadromous fish runs.

With fewer numbers of salmon and steelhead coupled with the listing of certain species and runs as endangered, threatened, or as a species of special concern, along with our inability to correct environmental water management problems, more attention has been directed at non-water related topics.  What was considered a minor problem in the past, an issue such as predation by nonnative fish on native species has today become the focus of attention and even lawsuits.

So what exactly do we mean when we talk about predator- prey relationships?  Predation is a biological interaction where a predator (the one doing the eating) feeds on its prey (the one that is eaten) and the relationship between the two is referred to as the predator-prey relationship.  So why is it important to understand predator- prey relationships?  After all, most Delta anglers already know that a chartreuse spinnerbait is effective for black bass, and a hair-raiser or Hopkins jig will usually catch stripers. A recent lawsuit brought by the Coalition for a Sustainable Delta, a nonprofit coalition made up of San Joaquin Valley water agencies, alleges that the enforcement of the sport fishing regulations enacted by the Fish and Game Commission maintains an elevated striped bass population.  They contend that increased numbers of striped bass increase predation on fish species that are listed under the Endangered Species Act, such as winter-run Chinook salmon and Delta smelt.  The Act prohibits any government agency, entity, or individual from "taking" a federally protected species without prior authorization.  The plaintiffs contend that since Delta smelt and specific runs of salmon are listed by the Federal Government, the Commission’s action in setting bag and size limits and allowing striped bass to prey on listed species (e.g., take) is a violation of the Act.  The Coalition claims on their website, “Predation by non-native striped bass is the leading cause of mortality among migrating juvenile salmon.”  Is the plaintiffs’ contention correct – Do Delta striped bass sport fishing regulations result in more striped bass predation on listed species?  Alternatively, will eliminating angling regulations result in less predation on salmon and smelt?  Moreover, probably most important to anglers, will the changes affect my fishing success? To answer these questions we must first examine the claim that striped bass eat juvenile winter run Chinook salmon and Delta smelt.  Delta striped bass food habitats have been extensively studied and one early study is found in the Department of Fish and Game’s 1966 Fish Bulletin136: Ecological Studies of The Sacramento-San Joaquin Delta, Part II: Fishes of The Delta, compiled by Jerry L. Turner and Don, W. Kelly.  The bulletin included an article by Don E. Stevens, “Food habitats of striped bass, Roccus saxatilis, in the Sacramento-San Joaquin Delta”.  This research was done at a time when neither Delta smelt or any runs of Chinook salmon were listed as protected species and we presume were more abundant that today. 

Stevens reported that his study was based on an analysis of stomach contents of 8,628 striped bass from eight types of Delta environments.  The stomachs were collected from September 1963 through August 1964.  Stevens reported that the mysid shrimp, Neomysis awatschensis, and the amphipods, Corophium stimpsoni and Corophium spinicorne, were the most important foods of young bass.  As bass grew their diet shifted to forage fishes, primarily small striped bass and threadfin shad, Dorosoma petenens, another nonnative species.  The composition of the diet varied by season and area and there was evidence that mysid shrimp were the preferred food of young bass.

Steven’s 1966 paper also reported that both king salmon Oncorhynchus tshawytscha and pond smelt (Delta smelt) Hypomesus transpacificus were found in the stomachs of all sizes of striped bass but the percentages were low.  Stevens did not report which specific salmon run (spring, fall, late-fall, or winter) was eaten and the pond smelt were most likely Delta smelt.  The taxonomy of the two smelt species is confusing and Pond smelt H. olidus, presently called Wakasagi H. nipponensis, are very difficult to distinguish from Delta smelt.  Wakasagi were first introduced into California 1959 but were not reported from the Delta until after the species was introduced into several Central Valley reservoirs. 

John Thomas, another DFG Fishery Biologist, reported that juvenile bass consumed quantities of small salmon in the spring and summer in the Sacramento River above the Delta in Fish and Game Quarterly (Vol. 53 No. (1) 1966) –“The diet of juvenile and adult striped bass, Roccus saxatilis, in the Sacramento-San Joaquin River”.  It was suggested that the availability of salmon to striped bass might have been the result of greater clarity and smaller width of the river.  Migrating juvenile salmon are also more concentrated in the relatively narrow river channel as compared to the Delta channels, and are most available to striped bass during the spring out migration period.

More recently, Matthew Nobriga and Fred Feyrer, California
Department of Water Resource Biologists, published a paper in 2007: “Shallow-Water Piscivore-Prey Dynamics in California's Sacramento-San Joaquin Delta” in San Francisco Estuary and Watershed Science, 5(2).  They examined the distribution and feeding ecology of striped bass, largemouth bass, M. salmoides, and native Sacramento pikeminnow, Ptychocheilus grandis, at several near shore sites in the Delta.  Based on their research, they concluded that largemouth bass have the biggest impact on nearshore fishes, including native fishes, and preyed on a greater diversity of native fishes than either striped bass or pikeminnow.  Largemouth bass also became predominantly piscivorous (eats fish) at smaller sizes than Sacramento pikeminnow, and ate more fish species during summer and fall regardless of size.  No Delta smelt were reported in any of the stomachs of the three predators, and Chinook salmon were reported found in only one of the largemouth bass stomachs (.004% of 245), and 2 of the striped bass stomachs (.01% of 217).  Additional non-identifiable fish remains were also found in the stomachs.

Based on the results of these and other studies, we can conclude that both striped and largemouth bass prey upon Chinook salmon and occasionally Delta smelt.  The total amount is unknown but most likely predation occurs at times and locations where salmon and smelt are most abundant or most vulnerable to predation.  Our conclusion: although salmon and smelt are not major prey species, they are occasionally eaten, which is probably not a big revelation to most Delta bass anglers.  Nevertheless, the question remains - Do sport fishing regulations result in more nonnative fish predation on listed species, and would the elimination of angling regulations result in less predation?  The assumption is based on the notion that without regulations, anglers would catch more bass and there would be fewer predators in the population. 

Fishery scientists and biologists have long understood that it is very difficult, if not impossible to eliminate a fish population by angling, especially in large lakes and reservoirs, or a system like the Delta.  However, the structure of the population can be altered through angling regulations and is a basis for much fishery management.  Generally, regulations are designed to protect a particular population and allow an acceptable harvest of a certain group or size of fish.  This is typically done through size, bag, season, or gear limits. 

Most angling regulation studies conclude that protective bag and minimum size regulations protect the smaller sized fish while allowing limited harvest of larger individuals.  This rationale is actually the basis for farm pond management throughout the nation: California’s 12-inch minimum size, the 5-fish bag limit on largemouth bass fisheries in the Delta and most reservoirs, and size and bag limits on Delta striped bass.  The effectiveness of a size limit regulation on black bass was studied many years ago at Merle Collins Reservoir.  Under the pre-size limit regulation, the population was dominated with smaller, slower growing fish, while larger, faster growing fish were harvested by anglers.  Without any regulations, the population was dominated by small, undesirably sized bass that were not harvested by anglers.  With a minimum size and reduced bag limit, smaller fish were protected and when they exceeded the minimum size, an allowable percentage of the population was harvested by anglers.  Many of the same conclusions relate to striped bass fishery management.  Years ago, overharvest was considered a major management problem.  Today, the issue has largely disappeared because of the popularity of catch-and-release fishing and health warnings dealing with fish consumption. 

Several years ago, the issue of de-regulation was the topic of a conference held in Oregon.  I attended and spoke at the conference.  Like the Delta, at issue were size and bag limits on smallmouth bass and walleye in the Columbia River, and predation by nonnative fish on native species, especially Chinook salmon.  Information from Columbia River fish food habit studies was similar to that reported for the Delta; some juvenile Chinook salmon were eaten by the nonnative species but they were not a major food item.  More importantly, ecologists and fishery scientists in attendance agreed that eliminating regulations on nonnative sport species would alter the population structure and result in larger numbers of smaller sized fish that were the most effective predators.  Based on this information and conclusions, the issue of de-regulation was dropped by the regulating agencies.    

Finally, will de-regulation affect my fishing success?  I suspect it may over the long term but the effect will not be noticed immediately.  The number of fish harvested from the Delta has most likely dropped in recent years because of a greater catch-and-release philosophy among angers.  Although numbers of striped bass are not reported as great as years ago, the largemouth bass fishery continues to thrive.  In addition, the current health warnings on the consumption of fish from the Delta do not add to most anglers’ desire to keep and eat Delta fish.  Some anglers do still keep and eat fish from the Delta, but I suspect they are a minority.  Illegal take of under sized fish and certain species, such as sturgeon, continues to be a problem and will persist in spite of regulations or de-regulation.

Nonetheless, in spite of an abundance of good science, the topic of predator- prey relationships still comes up as special interest groups attempt to find ways to circumvent and divert attention from the real issues.  How we as a society move and use water will continue to be an important and much-discussed topic.  A little research on the subject can help every angler and interested individual become more familiar with the issues and the science, which will lead to better and more informed decisions.