Resilient California Fishes: The Legendary Threespine Stickleback

By Peter Moyle and Tom Taylor

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The fresh waters of California support a diverse, highly endemic fish fauna. Many of the fishes are on extinction trajectories. In this blog series, however, we discuss native fishes that have sufficient resiliency to keep populations large and sustainable, even in highly altered habitats. Information on these species should help us to understand why they have such resiliency in the face of large-scale changes to California waterways. This blog post is the sixth in a series that started with an account of the Sacramento Sucker.

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Figure 1. An Unarmored Threespine Stickleback photographed in San Francisquito Creek on 3 June 2024. Stickleback are easy to observe and fascinating to watch when snorkeling in habitats that contain them. Photograph by Steve Howard.

The Threespine Stickleback (Gasterosteus aculeatus) is a tiny but charismatic fish (1-2 inches long) that is well known to fish enthusiasts and researchers. It is common in streams, lakes, and estuaries along the Pacific Coast, including California (Moyle 2002). It is present as well in coastal waters in eastern North America, Asia, and Europe. The keys to its resilience include its wide distribution and its ability to live in fresh and salt water.

A close-up photograph shows six small fish arranged on a wooden measuring ruler. At left is a broad, oval-bodied mottled brown fish with both eyes positioned near the top of its flattened body and a long, translucent tail extending downward. Around it are five smaller, slender fish with silvery bodies, dark eyes, and pointed heads. One lies horizontally across the center, two angle diagonally toward the lower right, one extends upward near the center, and one rests at the far right. The ruler has dense black measurement lines and large black numerals, while the surrounding surface is warm, grain-textured wood.

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Six small fish, including one broad mottled flatfish, arranged on a wooden ruler for size comparison. — Figure 2. Fish from a trawl sample collected by the San Francisco Bay Study in September 1983, showing a single stickleback (center) along with a mix of juvenile estuarine fishes: Two Northern Anchovies, two Plainfin Midshipman, and a Starry Flounder. This illustrates that sticklebacks typically co-exist with other species of fish. Photo by Tom Taylor.

Life Histories

Threespine Sticklebacks have two basic life histories: freshwater resident and anadromous (sea-run). Both types can often be found in the same stream. In each stream, resident sticklebacks are most closely related to anadromous sticklebacks in the same stream, which presumably founded the population and can hybridize with the resident form. If a population is extirpated from a coastal stream, the stream can be recolonized by sea-run fish from other streams, which then give rise to resident fish in the fresh waters. This migration behavior has a genetic basis, which reflects their considerable ability to adapt to local conditions. This also results in variation between populations in characteristics such as the size of their spines and number of armor plates.

Importance

Threespine sticklebacks are among the most studied fishes in the world because of their ready availability to researchers and their ease of living and reproducing in aquaria. They also have a short life cycle, typically just a year, making it possible to complete experimental studies on an academic schedule. They are attractive little fish, with easily observed behavior. In fact, Dr. Niko Tinbergen won a Nobel Prize in 1973 in part for his studies on stickleback behavior. Hundreds of papers, and a number of books (including a children’s book), have been written about their biology, especially as it relates to the evolution of fish species in general.

The wide geographic distribution of sticklebacks means that there have been lots of opportunities for populations to become isolated and, as a result, to develop distinctive adaptations to diverse habitats in oceans, estuaries, streams and lakes. Many of the hundreds of freshwater populations have diverged genetically from their anadromous ancestors, so each has the potential to be described as a separate species. Fortunately, biologists have largely refrained from doing this. Dr. Michael Bell, who studied the fascinating evolution of Threespine Sticklebacks in California, has suggested characterizing the Threespine Stickleback as a ‘superspecies’ made up of ‘semispecies’ (Bell 1976).

An underwater close-up shows a small, mottled fish partially concealed among dense aquatic vegetation and tangled plant matter. The fish faces left and has a rounded head, a visible dark eye, and a pale pinkish-brown body with darker horizontal markings. Brown, green, purple, and gray strands of algae or debris fill the frame, creating a textured, camouflaged background.

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A small mottled fish hides among dense underwater algae and plant debris. — *Figure 3. Male stickleback guarding stickleback juveniles from the opening of a nest constructed from filamentous algae. Toro Creek Lagoon, San Luis Obispo County, April 2006. Photo by Steve Howard.*

Breeding behaviour

One of the most studied stickleback attributes is their breeding behavior. An important requirement for breeding male sticklebacks is the availability of aquatic plants and algal mats that can be used as material for building tunnel nests for spawning. The males guard the nests and lure females in to spawn with their famous zig-zag courtship behavior. The males then guard the fertilized eggs and larvae until the juvenile fish can manage on their own, outside the nest.

The image shows four black-and-white scientific drawings of fish arranged vertically, each facing left. The fish are depicted in progressively smaller, more streamlined forms from top to bottom. All have large round eyes, pointed snouts, visible gill covers, dorsal fins, pectoral fins, ventral fins, narrow caudal peduncles, and vertically oriented fan-shaped tail fins. The top fish has the most prominently exposed rib-like skeletal structures and internal anatomy, while the lower three show increasingly smooth, textured body surfaces with fewer visible internal details. Fine stippling and crosshatching define the scales, muscles, fins, and anatomical features against a plain white background.

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Four vertically arranged black-and-white anatomical fish illustrations, facing left, showing progressively smaller forms. — Figure 4. The various forms of Threespine Stickleback found in California showing from the top: the anadromous, or fully plated form, the resident forms that include partially plated form, low plate count form, and the unarmored form, with 0-2 plates on the front of each side. Courtesy of M.A. Bell, SUNY, Stoneybrook. From Moyle (2002).

California sticklebacks

In California, all three types of sticklebacks are present. All stickleback populations are subject to predation by native snakes, frogs, egrets, and small fish (e.g., Prickly Sculpin). The number of plates and the size of the spines reflect the likelihood of being eaten by a predator. The armored form faces a wide variety of predators in both estuaries and rivers, so it has a robust, armored body and stout spines. The resident forms have less armor and smaller spines, presumably to reduce the bioenergetic costs of growing armor and spines. The unarmored fish likely avoid these costs, because highly variable environments can exert stronger selection pressures than just predation. No doubt other environmental factors are involved as well, such as abundance of suitable prey and water quality. Nevertheless, the spines and armoring of sticklebacks reflect that predation is an important factor in their short lives.

When abundant, they can be an important part of the diets of other aquatic organisms, from dragonfly larvae to herons. For example, Striped Bass, a non-native species in California, has been found to feed on sticklebacks in outflows from shallow ponds being drained into channels in Suisun Marsh. When a freshly caught adult bass from the outflow is subjected to ‘gastric lavage’, some individuals may have eaten 100 or more sticklebacks; flushed out of the bass guts, some still alive.

An underwater side view of a small, slender fish swimming toward the left. Its body is mottled blue, silver, and gray with shimmering scales, while the snout and lower front portion are bright orange. The fish has a prominent blue-ringed eye, translucent fins, and a narrow, tapered tail. It is surrounded by suspended particles and positioned against a dark, textured background of rocks and algae.

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Small iridescent blue-and-silver fish with an orange snout swimming left over a rocky, algae-covered seabed. — Figure 5. Unarmored Threespine Stickleback were reintroduced in 2014 after the local San Francisquito Creek population was considered extirpated in 2005. This is a male in full spawning coloration. This population appears to be doing well. June 2024. Photograph by Steve Howard.

Unarmored Threespine Sticklebacks (G.a. williamsoni) are of special interest because populations occur or occurred in streams in coastal southern California and Baja California, where many of the streams have been highly altered, drained, and polluted. The unarmored form was listed as Endangered by the federal government in 1970, and by the state in 1971, so considerable attention has been paid to protecting their few remaining stream habitats and to transplanting them to new or restored habitats.

A group of small, silvery fish swim over a gravel-covered bottom. The fish have elongated bodies, translucent fins, dark eyes, and faint vertical bands or mottled markings along their sides. Several fish face right, while others face left or angle toward the viewer. The water appears clear, and the multicolored gravel includes gray, tan, brown, and dark stones.

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A group of small silvery fish swimming above a multicolored gravel bottom. — Figure 6. Stickleback are often found in large schools. These Unarmored Threespine Stickleback cruise over the sandy substrate in the Santa Clara River near confluence with San Francisquito Creek in Ventura County 3 June 2024. Photo taken by Steve Howard.

This includes an unusual population of Unarmored Threespine Stickleback that is found in a small stream, Shay Creek, in the San Bernardino Mountains; the stream flows into Baldwin Lake, a usually dry placer lake (elevation 2055 meters). The closest relatives are in the Santa Ana River at much lower elevations. This population most likely was introduced, although there are no records of that happening (Turba et al. 2022). Fortunately, this distinctive population is being managed as a population of G.a. williamsoni. In contrast, elsewhere in California, resident sticklebacks have been widely introduced into streams and lakes of the eastern Sierra Nevada, including the Owens River. They were apparently present in water that was pumped into trucks that transported domesticated Rainbow Trout from fish hatcheries to streams in the 1950s, to benefit anglers.

Conclusion

The Threespine Stickleback is obviously a species that is highly resilient and can rapidly adapt to changing conditions. It is a nice fish to have around, especially in its native habitats; it is easy to observe with well-documented behavior. They are an interesting and attractive native fish. Even the unarmored form, listed as Endangered in Southern California, demonstrates resilience. While it was once abundant in its native streams, only a few isolated populations survived the urbanization of the Los Angeles region. These survivors, like other populations, respond readily to habitat restoration projects and translocation to ‘new waters.’ Indeed, habitat conservation in our coastal streams, lakes, and estuaries benefits many stickleback populations. Thanks to their resilience, they can bounce back quickly after a major event such as a large flood or a pesticide spill. In fact, the abundance of Threespine Sticklebacks is a good indicator of habitat quality and diversity.

We thank Steve Howard for providing us with the photographs we used in this blog.

About the Authors

Peter Moyle is a Distinguished Professor Emeritus at the University of California, Davis, and is Associate Director of the Center for Watershed Sciences.

Tom Taylor has had a long career as an agency and consulting biologist, often working with native fishes. He has also taken underwater photos of many of the fishes, which will illustrate the blogs.

Further Reading

Bell, M.A., 1976. Evolution of phenotypic diversity in Gasterosteus aculeatus superspecies on the Pacific coast of North America. Systematic Zoology, 25(3): 211-227.

Moyle, P.B. 2002. Inland Fishes of California, Revised and Expanded. Berkeley: University of California Press.

Turba, R., Richmond, J. Q., Fitz-Gibbon, S., Morselli, M., Fisher, R. N., Swift, C. C., Ruiz-Campos, G., Backlin, A. R., Dellith, C., & Jacobs, D. K. (2022). Genetic structure and historic demography of endangered unarmoured threespine stickleback at southern latitudes signals a potential new management approach. Molecular Ecology, 31, 6515–6530. https://doi.org/10.1111/mec.1