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Phase II: Limnological and Fisheries Investigation of Lake Pleasant
Project Update - October 2003

Prepared by:
Eric Kohagen and Scott Bryan
Arizona Game and Fish Department
Research Branch
602.789.3664
[email protected]

Background

In 1968, increasing water demands in the Southwest prompted Congress to authorize the construction of the Central Arizona Project (CAP) for delivery of water from the Colorado River to central and southern Arizona. Lake Pleasant was the rational choice for storage of the new water source because of its proximity to the Phoenix metropolitan area. However, storage capacity of Lake Pleasant was not large enough to meet CAP needs. As a result, construction of New Waddell Dam was proposed to increase the size of the reservoir (nearly three-fold). Prior to construction, both public and government entities raised concerns that the new dam would negatively impact the fishery. Therefore, a multi-phased research project was initiated to evaluate the effects of enlarging the reservoir on the fish communities in Lake Pleasant. Phase I of the study (1987-1989) was conducted to determine baseline data on the fisheries and limnology of the lake. We are currently conducting Phase II of the study. It is a five-year project that began in November of 2000, and will identify and address changes that have occurred in the reservoir since the construction of New Waddell Dam. The primary components of the study include an evaluation of angler success and satisfaction, fish population dynamics, and water quality.

Creel Survey

Creel surveys are used to determine such factors as angler success satisfaction and preference, fishing pressure, and catch and harvest rates. Angler surveys were conducted on 71 random days throughout the year. Based on angler pressure, surveys were divided among the three main access points: 23 were conducted at Ten-lane Ramp, 23 at Castle Creek Ramp, and 11 at Pleasant Harbor North Ramp. The remaining 14 surveys targeted shore anglers (throughout the year) and boaters accessing the Agua Fria River during the eagle closure.

Summary of Angler Survey Results
· Estimates of fishing pressure from 2002 have decreased from 2001 estimates, but still indicate that Lake Pleasant is one of the most popular fishing lakes in Arizona with 148,590 anglers fishing for over 528,000 hours.
· The primary reason for the decline in fishing pressure is that the average angler day in 2002 was nearly 38 minutes less than that in 2001, which may be attributed to drought conditions and poor fishing success.
· Anglers caught over 151,000 fish in 2002, a 56% decrease from 2001, and overall harvest rates remained at 27%. Largemouth bass harvest rates were 15% in 2001 and 14% in 2002.
· Angler satisfaction remained low in 2002, with 56% of anglers rating their experience as “poor”. Not surprisingly, satisfaction was positively related to angler catch rates.
· Although most anglers still prefer largemouth bass, white bass catch rates were highest, followed by sunfish. Largemouth bass catch rates decreased significantly between 2001 (0.33 fish/hour) and 2002 (0.24 fish/hour).

Fisheries Surveys

Fish were collected from random locations using gill netting and electrofishing gear. Sampling is conducted biannually during spring and fall, using both gear types. The purpose of this sampling is to describe the status of fish populations in Lake Pleasant.

Summary of Fisheries Survey Results

· Sixteen species were collected during the first two years of Phase II, including common carp, red shiner, threadfin shad, channel catfish, flathead catfish, green sunfish, bluegill, redear sunfish, hybrid sunfish, largemouth bass, white bass, striped bass, golden shiner, white crappie, black crappie, and tilapia.
· Threadfin shad, white bass, sunfish, and largemouth bass were most prevalent in our samples. Tilapia, flathead catfish, and golden and red shiners were rarely collected.
· Catch per Unit Effort in Fall 2002 and Spring 2003 was greatly influenced by an extremely high catch rate of threadfin shad.
· Sunfish catch rates increased significantly in Fall 2002 and Spring 2003 and is likely due to a large 2001 year-class.
· Largemouth bass size structure is within the range of a balanced population. White bass size structure indices indicate the population is dominated by an abundance of very large fish.
· Mean length of striped bass remained unchanged throughout the study. Although there appeared to be a strong 2001 year-class, there has been no evidence of reproduction in the reservoir. All gill netted striped bass in May 2003 exhibited discolored ovaries which is consistent with egg reabsorbtion due to a missed spawn.
· Condition (plumpness) of the largemouth bass, striped bass, and crappie are below the optimal level for most length categories. However, weight of white bass is within the optimal range for most length categories, indicating that white bass out-compete the other species for the limited threadfin shad resource.
· A preliminary investigation of the stomachs of 76 largemouth bass (mean length= 10 inches) and 33 white bass (mean length = 14 inches) were examined to determine diet of each species. Largemouth bass diets were comprised of crayfish (48%), sunfish (37%), threadfin shad (10%), and unidentified fish (5%). White bass diets consisted almost entirely of threadfin shad (92%) with crayfish making up the remaining 8%.
· Sixty largemouth bass were analyzed for Largemouth Bass Virus and all tested negative for the virus.

Water Quality

Monthly water quality sampling was conducted for temperature, dissolved oxygen (DO), pH, conductivity, chlorophyll-a, and water clarity. Quarterly sampling for metals and nutrients was conducted by the Central Arizona Project Environmental Department (CAP).

Summary of Water Quality Surveys

· Nutrient data indicates that Lake Pleasant is a mesotrophic reservoir (intermediate inputs of nutrients) during spring months (February-June) and an oligotrophic reservoir (low inputs of nutrients) throughout the rest of the year.
· Positive relationships between chlorophyll levels and Agua Fria inflow indicate that the river inflow may have more influence over productivity than the canal, even though the majority of water enters the lake through the canal.

Forecast

Bass fishing should improve over the next few years due to one important variable: precipitation. Precipitation leads to the runoff of organic compounds (i.e. phosphorus and nitrogen) that support the growth and abundance of algae, zooplankton, and all other organisms down the line. If in fact the Agua Fria has more influence over the nutrients and productivity of the lake, years with greater runoff will produce greater benefits for the fishery. Due to high flows from the Agua Fria River in the spring of 2001, Lake Pleasant was loaded with nutrients. These nutrients made it possible for threadfin shad, sunfish, and largemouth bass to achieve strong year classes that year. After a one-year lag period, catches of both shad and sunfish increased dramatically in 2002 and 2003. A high flow event occurred again in the spring of 2003 and could produce a similar effect. If shad are a limiting factor for bass growth, more shad equates to more available prey and faster growth. Furthermore, when shad are not accessible, bass could shift to sunfish, a more traditional prey item that has not been as readily available in years past. With more available and accessible prey, expect larger bass to be yielded from Lake Pleasant in the years to come.

Continued Research

Research during the next year will include a continuation of the creel survey to look at trends in angler catch and satisfaction data. We will also continue to sample using gill nets and electrofishing to evaluate yearly trends and sample water quality and chemistry in the reservoir and observe how these variables change with different environmental factors and water level fluctuations. We will also continue to age and analyze diets of the primary sportfish to enhance the data we now collect. We intend to expand our understanding of the striper population by experimenting with various gear types in order to increase catches and launching an intensive study of the striper population that will begin in January 2004. Lastly, the impact of leaving Lake Pleasant Dam intact when New Waddell Dam was completed will be assessed.
At the conclusion of the study, we will compare results collected in Phase II with those in Phase I to determine how the fishery and water quality has changed since the construction of New Waddell Dam.
 
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Eric Kohagn study

I just wanted to say Eric that we appreciate all the hard work you guy's do on our water systems. I found the Lake Pleasant study very enlightening. Again great job.
 

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Fishery Study

I would like to thank Eric and everyone else who is participating in this effort to improve and maintain the quality of this great fishery. I remember giving Eric my phone number and email address for the gill netting and electronic shocking to keep tabs on the fish in the spring and fall. No one has called though and I'm still available if you need my help in the future. Thank you Wayne Sobberi
 
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