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Introduction

Horseshoe Lake is located near the golf course in Bidwell Park, Chico, California. It was constructed about 1930 as a reservoir to provide storage of water diverted by gravitational flow from Big Chico Creek for irrigating the golf course. Its original purpose was abandoned after a few years when it became apparent that maintaining the diversion flume cost more than pumping water. Horseshoe Lake has remained as a shallow, turbid pond, fed by winter and spring runoff, gradually losing water between wet seasons, but never going completely dry. Horseshoe Lake water is usually turbid and quite green due to a dense bloom of bluegreen bacteria of the genera Microcystis and Coelosphaerium. This water bloom is undoubtedly a result of the large quantity of nutrients entering the lake in the feces of the domestic ducks and geese.

The food web of Horseshoe Lake, while simple compared to most natural ecosystems, is still complex. Energy enters the lake by photosynthesis of three groups:

  1. Phytoplankton (single celled algae, diatoms, and bluegreen bacteria) can be used directly by zooplankton which also use bacterioplankton from the detritus chain. While relatively little energy may move this way, the planktonic chain is vital to the fish community since zooplankton provide food essential to the fry of most species.
  2. The abundant submerged plant (Najas graminea) forms the base of a grazing food chain, being eaten by crayfish, snails and the amphipod, Hyalella azteca.
  3. Bluegreen bacteria, such as Microcystis and Coelosphaerium, produce huge amounts of biomass which mostly contributes to the detritus chain since their colonies are too large for zooplankton to handle.
Energy in detritus is not lost however; the sediments are mined by Oligochaete worms and Chironomid larvae. From this complex base, energy passes through the food web according to the feeding habits of the component fishes. While the majority of the energy is dissipated as metabolic heat within the food web, some leaves the lake in the form of fish caught by egrets, herons, and people.

Many species of fish have been introduced into the lake in its 60+ year lifespan, but only the golden shiner and a handful of centrarchids have established significant breeding populations. In the last few years, several thousand 1-3 lb. channel catfish have been planted annually into the lake for a kid's fishing day ("Hooked on Fishing").

Although all the Horseshoe Lake fish are exotic to California, they evolved more,or less together in the eastern US, so have had time for evolutionary adjustment to one another and consequently form a reasonably stable community.

For a number of years, biology classes from CSU, Chico have been monitoring physical and biological features of the lake. While techniques have varied slightly over the years, they have all been similar: Physical/chemical measurements were made with electronic instruments, invertebrates and fish were captured with plankton nets, dipnets and beach seines, and water samples were examined microscopically. Fish collected were identified to species and measured to the nearest millimeter. Subsamples of each species were weighed to the nearest gram on an electronic balance. Stomach contents were examined from a few Horseshoe Lake fish and this data was supplemented with information from the literature to create a probable food web for the lake.

Some data from these investigations are presented here to provide information for interested area residents and park visitors.

Physical/Chemical Parameters of Horseshoe Lake
Date DIssolved
Oxygen
(ppm)
Conductivity
µmhos/cm
pH Visibility
(cm)
surface bottom surface bottom surface bottom
10/15/90 7.3 5.1 115 116 7.7 7.45 5
10/14/91 8.3 5.7 115 116 7.65 -- --
10/25/93 9.6 3.4 105 100 7 6.3 35
10/25/95 8.1 5.5 -- 110 6.7 6.35 --
11/04/96 9.3 5.6 86 87 6.9 6.7 17

Horseshoe Lake Fish

Golden Shiner Notemigonus crysoleucas
Minnow Family (CYPRINIDAE)

The golden shiner was introduced to California as a bait minnow. It has become widely distributed by anglers who transported minnows from one area to another. Golden shiners are silvery with a hint of gold and often lose scales when handled. They feed on zooplankton, filamentous algae, and some insects. Golden shiners seldom exceed 25 cm (10 in.) in length. Populations of golden shiners in Horseshoe Lake have varied tremendously during the years we have studied it.

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Bluegill Sunfish Lepomis macrochirus
Sunfish & Black Bass Family (CENTRARCHIDAE)

The bluegill sunfish was native to the eastern US but has been introduced around the world as a pan fish It is now the most abundant fish in many warm water lakes and ponds in California, including Horseshoe Lake, California Park Lake, and the Teichert Borrow Pits. Young-of-the-year bluegills school in open water, feeding on zooplankton, but larger ones work weed beds for invertebrates. While bluegills can get up to dinner plate size in some waters, their population is so dense in Horseshoe Lake that they seldom get over 13 cm (5 inches).

Note: The apparent smaller numbers shown in the histogram for 1997 reflect sampling rather than abundance. (See Numbers Chart below.)

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Largemouth Bass Micropterus salmoides
Sunfish & Black Bass Family (CENTRARCHIDAE)

The largemouth bass was introduced to California as a warm water game fish and has been widely distributed within the state. Largemouths are exclusively carnivorous, eating anything that will fit into their mouths as long as it tries to get away. While they are the top aquatic predator in Horseshoe, they are still subject to predation by large interface predators such as egrets, herons, and anglers. Largemouths can grow quite large, up to about 75 cm (30 in.) in length with weights over 10 kg (22 lb.).

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White Crappie Pomoxis annularis
Sunfish & Black Bass Family (CENTRARCHIDAE)

White crappies are common in Horseshoe Lake. They are thin, deep-bodied fish, with large delicate mouths. They have long, fine gill rakers which enable them to filter out zooplankton as a major source of food, but larger ones also take small fish.

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Black Crappie Pomoxis nigromaculatus
Sunfish & Black Bass Family (CENTRARCHIDAE)

Black crappies have also been introduced to the lake, but are less common. A black crappie's dorsal fin, measured along the base, is about the same length as the distance from the back of its eye to the front of its dorsal fin. A white crappie's dorsal fin is distinctly shorter. Although very large crappies have been reported, neither species commonly grows larger than about 35 cm (14 in.).

Several other species of fish are found in Horseshoe Lake but are much less Common.

CENTRARCHIDAE Sunfish & Black Bass Family
Green sunfish, Lepomis cyanellus, resemble bluegills, but are thicker, not as deep, and have a much larger mouth. They prefer low gradient streams with intermittent pools and have become one of the dominant fish in Little Chico Creek through town to the detriment of native fish which they out-compete or eat. They are relatively uncommon in Horseshoe Lake. Green Sunfish seldom get over 15 cm (6 in.) in length.

ICTALURIDAE Catfish Family

Channel catfish, Ictalurus punctatus, have been introduced many times into Horseshoe Lake. They don't seem to reproduce there, probably because of lack of "caves" (muskrat burrows, undercut banks) where big males can set up territory and induce females to spawn. Annual planting in association with "Hooked on Fishing" keeps a substantial population of adults in the lake. The catfish family was native to eastern North America, but many of its members have been widely introduced as game fish and aquaculture subjects. The channel catfish is the most widely farmed warm-water fish in North America. It can grow quite large, up to 1 meter (39 in.) in length.

Both the brown bullhead, Ictalurus nebulosus, and black bullhead, Ictalurus melas, have been collected in Horseshoe Lake, but neither is common there. Brown and black bullheads are heavier bodied than channel catfish and have square tails. Black bullheads are dark dorsally and usually yellow ventrally with a sharp separation between colors. The membranes of their fins, particularly the anal fin, are darker than the rays. Brown bullheads are blotchy gray-brown in color, countershaded like most fish, but without the distinct line separating colors, and usually without a distinctly yellow belly. Their fin rays and membranes are about the same shade. Neither bullhead gets very large; fish longer than 20 to 30 cm (8 to 12 in.) are unusual.

POECILIIDAE Livebearer Family

The mosquitofish, Gambusia affinis, native to the American Southeast, has been introduced around the world for mosquito control. Despite being smaller than 50 mm (2.5 in.), it is a voracious predator on small invertebrates. Mosquitofish will be found near the surface in dense vegetation or shallow water. Lack of habitat coupled with abundant predators keeps it's population low in Horseshoe Lake.

OTHERS

Common Carp Cyprinus carpio , Goldfish Carassius auritus, and Smallmouth Bass Micropterus dolomieui, have been occasionally observed but do not seem to reproduce in the lake. They probably represent release of pet fish or specimens caught in the nearby creek.

Summary of the Horseshoe Lake Fish Community

Relative biomass and numbers of dominant fish Species in Horseshoe Lake on October 20, 1997.