Understanding The Effects Of Gizzard Shad In Roosevelt Lake Arizona

Roosevelt Lake is the upper most and largest in a series of four reservoirs on the Salt River. The Bureau of Reclamation constructed Roosevelt Dam in 1911 forming Roosevelt Lake. At full capacity it is approximately 22 miles long with 91 miles of shoreline; its maximum depth is over 300 feet and it can store 1,653,043 acre feet of water. Roosevelt Lake is also home to a variety of game fish including largemouth and smallmouth bass, black crappie, yellow bass, bluegill, channel and flathead catfish and carp. Threadfin shad, gizzard shad, and bluegill are the predominant forage fish.

The land around Roosevelt Lake is managed by the U.S. Forest Service, more specifically the Tonto National Forest. The aquatic species within the lake come under the authority of Arizona Game And Fish Department (Department). As the Region VI Fisheries Program Manger, I have had the pleasure to manage Roosevelt Lake for the Department since 2009. One of the Department’s sportfish management objectives are to provide a diversity of sportfish to anglers, however, no management plan is currently developed for Roosevelt Lake.

Roosevelt Lake is estimated to experience over 98,000 angler use days per year and is also considered to be one of the top bass fishing lakes in Western North America. For this reason, Roosevelt Lake is a popular tournament lake, holding multiple bass fishing tournaments every week for most of the year. Over the past 30 years, Roosevelt Lake has experienced numerous social, political, and biological changes.

In October 2011, my program once again extensively surveyed Roosevelt Lake. This was part of a reoccurring study to monitor and evaluate the fishery and fish community structure and to detect changes over time in the fish community with priorities set for largemouth bass and gizzard shad. Priorities were set on largemouth bass due to the popularity of recreational angling for this species and for Gizzard shad due to their recent inhabitance in the Roosevelt Lake. The information obtained provides my program with data to aid in the development of recommendations to protect and improve the sport fishery of Roosevelt Lake and provide the baseline for the development of a management plan for Roosevelt Lake.

The findings of this study were significant. In 2011, gizzard shad made up the highest percentage of fish captured during the survey comprising 27.7% of the catch. As relatively new inhabitants of Roosevelt Lake, gizzard shad are flourishing. Their numbers have quickly grown over the last few years to become the most prevalent species (Figure 1) in the lake. Comparing Gizzard shad population size structure from 2008, 2009, and 2011, data visually indicates a shift in gizzard shad population structure from one population primarily dominated by fish in the 7-12in total length to one where the majority of the population is comprised of fish that are 13in total length and greater (Figure 2). The data suggests one of two possible scenarios: either there was either a really poor year class in 2010 or predators are finally starting to utilize gizzard shad as a prey base

The effects of gizzard shad in Roosevelt Lake, though not fully understood, are now becoming evident. Gizzard shad are documented as having an impact on sportfisheries back east through competition with other forage and game species at the larval stage and a reduction as a food resource for largemouth bass as they soon outgrow largemouth bass mouth gapes. A fish researcher in the southeast developed an index used to determine the availability of gizzard shad as prey for largemouth bass in a body of water. His research designates 8 inches, was on average the upper limit for gizzard shad to be available as prey for an average sized (16 in), largemouth bass. Our data indicates 95% of the gizzard shad in Roosevelt Lake are too large o eat for the average largemouth bass. Comparing rates of prey vulnerability in Roosevelt Lake from 2008, 2009, and 2011, we show a decline from 34 to 24 to 5, which mimics the shift in size structure to a population dominated by large, >8in gizzard shad.

So what does this mean to the fisherman? Additional data collected indicated a severe lack of preferred and trophy sized bluegill in all years surveyed and a notable decline in the body condition of bluegill (Figure 3). This could be a cause and effect relationship due to the introduction of gizzard shad. Bluegill and gizzard shad spawning events are dependent on temperature and both peak between 66-71oF. If gizzard shad appear first depleting the zooplankton densities bluegill suffer reduced growth. Couple this with the fact that gizzard shad are providing little prey for largemouth bass and this may cause bass to feed more heavily on bluegill, thereby reducing the larger size classes in the population. Additionally, documented bluegill weights show a significant decrease since 2008 (Figure 4). This is most likely cause by direct food source competition with gizzard shad.

Similar to the bluegill, largemouth bass have shown a significant decline in relative weights in Roosevelt Lake (Figure 5). This weight decline corroborates reports from tournament anglers stating that overall bag weights have declined over the past few years. This decline in body condition may be a result of poor levels of prey availability of gizzard shad in combination with the lack of habitat structure in the lake. Roosevelt Lake is known to be limited in habitat as it contains very little aquatic vegetation, cover, and structure; the essentials of good fish habitat. This is indicative of many of our desert reservoirs in Arizona, as they have steep slopes to deep water and spring/summer water level declines reducing the ability for submersed aquatic vegetation to become establish. This limits cover to areas where large woody debris and rocky edge structure exist. In lakes void of diverse habitats, largemouth bass find their prey by searching, pursuing, and following, all of which a bass must do more of to obtain a meal in habitat limited waters. The more energy it takes a fish to find a meal; the slower they grow. Providing habitat structure in Roosevelt Lake may assist in providing more bass a concentrated prey base and the opportunity to ambush and attack prey more efficiently which in turn should result in increasing fatter, larger bass.

Although the effects of gizzard shad on sportfish and zooplankton have been studied extensively, most of the work has been done in the Midwest and East; poor equivalents to Arizona’s arid climates. Additionally, little data exist on the interactions between gizzard shad and forage species important to bass and their effects to game fish in Roosevelt and Apache Lake. In spring of 2012, gizzard shad were detected in Apache Lake for the first time. What is now needed is an intensive research project studying the effects of introduced gizzard shad on game fish species in Roosevelt and Apache Lake. Information gained through this research will be invaluable to future management of Arizona fisheries.

However, gizzard shad are not the only factor negatively affecting Arizona’s fishery resources. In the coming issues, I will discuss some of additional factors and threats to Arizona’s fisheries including Golden Algae, Largemouth Bass Virus, and the Importance of Fish Habitat in Arizona. Only by understanding and prioritizing these factors can we begin to proactively manage toward continued success of fisheries in Arizona.

By Chris Cantrell, Region VI Fisheries Program Manager, Arizona Game and Fish Department.