By Peter Moyle
*this is a repost of a blog originally published in June 2020.
Crayfish, crawdads, crawfish: whatever you call them, they are everywhere in California’s waters and are as tasty as their lobster relatives. They are especially familiar to anglers who peer into the maw of a bass or pikeminnow or flush their stomachs to see what prey caused the bulging belly. Crawdads are familiar to kids wading in streams, who dare each other to catch one without being pinched. River otters love them as food too. I have watched otters dive in Putah Creek and repeatedly come up with one. With each capture, the otter rolls on its back and crunches the crayfish down. The otters appear to be smiling with satisfaction, smacking their lips. People eating crayfish have the same general appearance.
Crayfish are so integrated into California’s aquatic ecosystems that they might be considered as native if you didn’t know their history. But most are the result of introductions as food for people or as forage or bait for game fish. And most California crayfish live in novel ecosystems. These ecosystems have a biota that is a mixture of native and non-native species living in habitats that are highly altered by the continuous actions of people. Crayfish therefore fit right in, feeding on organic matter, algae, dead fish or anything else they can process, and then being eaten themselves by native predators such as otters, herons and pikeminnows, or by non-native predators such as centrarchid (sunfish family) basses and bullfrogs. However, this integration comes at a cost, especially in less-altered waterways. Non-native crayfish, regardless of species, can (a) displace fish and native crayfish from cover, making them more vulnerable to predation, (b) reduce aquatic plant densities, making water clearer, (c) compete with fish for aquatic invertebrates as food, especially snails, and (d) displace native crayfish from their habitats. How much of all this they do in California ecosystems, however, is not well understood.
The situation with crayfish in California is actually complex because there are three native species and three non-native species. Their status ranges from extinct, to endangered, to being abundant enough to be sustainably harvested. Confusion is further generated by the tendency of many fish biologists (like me), when sampling aquatic habitats, to just refer to crayfish captured as “crayfish” with no reference to species.
Sooty crayfish. The extinct species is the sooty crayfish (Pacifastacus nigrescens) which was found only in a few streams in San Francisco. They disappeared after only a few specimens were collected in the 19th century, presumably victims of rapid urbanization. The story of sooty crayfish is a mystery because if these crayfish were present in the Bay area streams, why weren’t they found throughout streams of the Central Valley, as non-native species are today? But there is no evidence, archeological or otherwise, that crayfish were ever present in most areas that support non-native crayfish today.
Pilose crayfish. An apparent victim of poor field notes is the pilose crayfish (P. gambelii). The species was described from specimens collected in “California” (the complete locality information on the type specimen) by Charles Girard in 1852. While the pilose crayfish is widely distributed across western states, it is not found in California, indicating Girard made a mistake in noting the vague locality (Larson and Olden 2011).
Shasta crayfish. This crayfish (P. fortis) is native to cold, spring-fed streams and rivers of the Pit River system, including Fall River and Hat Creek in northeastern California. When graduate students from my lab were studying this crayfish in the 1980s and 90s, I was impressed with how different they were in their behavior from other crayfish: they were not aggressive. If there was bucket of live Shasta crayfish being measured, I could stick my hand in the bucket to grab one without looking. Representatives of any other crayfish would grab my fingers or hand with a big claw and hold on until shaken off. Shasta crayfish did not appear to have the well-developed defensive mechanism of waving their large front claws around while backing into cover. Unfortunately, this lack of aggression has made them an endangered species. Larger, much more aggressive signal crayfish have invaded their habitat. Shasta crayfish now persist only where signal crayfish have not invaded or have been removed.
Klamath signal crayfish. This crayfish (P. leniusculus klamathensis) is native to the Klamath River, where it is abundant and widespread. It is possible that crayfish in the Eel River also belong to this subspecies and are native (Riegal 1959). The Klamath signal crayfish is considered to be a subspecies because it can be distinguished by both morphology and genetics from the widespread Columbia signal crayfish. The subspecies itself was introduced into Lake Tahoe and the Truckee River in 1895 where it is thriving, although it has hybridized with non-native Colombia river signal crayfish, so is no longer a distinct taxon in the eastern Sierra Nevada. A 1970 study (Abrahamsson et al. 1970) estimated that Lake Tahoe supported over 5.5. million crayfish; a more recent estimate is over 220 million crayfish, which dominate benthic production and affect everything from water clarity to native fish abundance (S. Chandra, unpublished data). Because these crayfish are resistant to crayfish fungal plague (Aphanomyces astaci), which had destroyed the crayfish fisheries in Sweden, Sweden stocked many of their lakes with crayfish from Lake Tahoe. While crayfish fisheries have been tried in Lake Tahoe, they have not been sustained (S. Chandra, personal communication). It would be an interesting experiment to subsidize a fishery for 10-20 years to see if such a fishery could have a positive effect on the ecology of the lake, including native fish, fisheries, and water chemistry.
These crayfish are now integrated into the ecosystems of Sierra Nevada streams and lakes, for better or worse. Light (2003) found that populations of signal crayfish in Sierra Nevada streams were regulated in part by flow, similar to trout and native fish populations. However, presence of a downstream reservoir that served as a refuge was necessary for recolonization following extreme flow events. Light (2005) also investigated the effects of signal crayfish on the biology of native Paiute sculpin in a small Sierra Nevada stream. While she found some evidence of competition, effects were minor, demonstrating coexistence was possible; presumably flow was more important to regulating crayfish populations than other factors.
Columbia signal crayfish. Wide-spread the huge Columbia River basin, this crayfish (P. l. lenuisculus) was introduced into California in the early 1900s by fisheries agencies. Generally, when biologists talk about the signal crayfish, this is the form they are referring to, assuming the two subspecies do not have any ecological differences. The signal crayfish was introduced into Europe as a large, edible crayfish. Unfortunately, the introduced crayfish carried crayfish plague, a disease that pretty much wiped out the native European crayfish (Astacus astacus). This meant that in countries like Sweden, where crayfish are a traditional winter holiday food, crayfish had to be imported. For a while (1970s and 80s), a fishery for signal crayfish in the Delta helped to satisfy the demand. At its peak, the fishery involved an average of 32 boats. But the fishery has apparently disappeared or become small, presumably because cheaper crayfish from other countries or from aquaculture operations have entered the market. However, it is likely environmental change also played a role. Today, signal crayfish appear to be uncommon in the Delta, replaced by red swamp crayfish. When the Delta fishery existed, the town of Iselton held an annual crawfish festival. When the fishery collapsed (for unknown reasons), the festival fell on hard times. For a while, the festival imported frozen crawfish from China but but this apparently did not work well. The revival of the festival in 2023 was possible because live red swamp crayfish were available in abundance from Louisiana, where it is raised in rice paddies and other habitats.
Red swamp crayfish. This crayfish (Procambarus clarki) is typically red and does inhabit swamps, where it burrows into the mud. This behavior can cause distress to farmers if the ‘swamp’ happens to be a rice paddy or its levee. On the other hand, farmers who harvest the crayfish consider them to be a bonus crop (Brady 2013). This crayfish is also very versatile, abundant in streams with warm to cool water, variable water quality, and mud to rocky bottoms. It is aggressive and has apparently displaced other crayfish, and an occasional swimmer, in many places where introduced. Introduced into California in the 1930s, it is now the most commonly encountered crayfish in the central and southern parts of the state and subject of a trap fishery in the Delta and Central Valley, mostly for bait.
Buciarelli et al. (2018) demonstrated how red swamp crayfish can change stream ecosystems that historically lacked crayfish. They preyed on or displaced dragonfly larvae in low gradient streams. The dragonfly larvae were more efficient predators on mosquito larvae than crayfish, so fewer dragonflies resulted in more mosquitoes. Red swamp crayfish have also been shown to be aggressive to other crayfish species and appear to have displaced signal crayfish from some streams in Oregon, as well as in Spain (Pearl et al. 2013). This may account for the observation that signal crayfish in California are often confined to cold headwater (trout) streams) when red swamp crayfish are present in the warm lower reaches of the streams.
Virile (Northern) crayfish (Faxonius virilis) are native to much of northeastern and midwestern USA and are one of the most widely introduced crayfish worldwide. They are best known as Orconectes virilis but were recently reclassified as Faxonius virilis by Crandall and de Grave (2017). They have been widely introduced around the western USA, including California, apparently because of their popularity as bait. The first records from the Central Valley were of crayfish in ponds near Chico State College, where they were kept for teaching purposes starting in the early 1940s (Riegel 1959). Today they seem to be common in southern California and abundant in the Central Valley. Virile crayfish, however, can live in a wide variety of habitats including flowing streams, preferring warm water. Like the red swamp crayfish, they create burrows into which they can find refuge as their habitat dries up. The fact their broad habitat requirements are similar to those of red swamp crayfish suggests the two species co-occur and perhaps compete for food and space at times.
Crayfish had an easy time invading California. Here are some reasons.
- People like to eat them or use them as bait for game fish.
- They are hardy and easy to transport with minimal water.
- A population can be established by a single ‘berried’ female carrying 100-300 fertilized eggs or newly hatched young. The young can mature in 1-2 years and live up to 5 years, longer in captivity.
- They can live in a wide variety of streams, reservoirs, and other aquatic habitats, with signal crayfish doing well in cold waters (e.g. trout streams) and red swamp crayfish and virile crayfish widespread in warmer waters.
- They quickly spread once introduced into a new area, making them nearly impossible to eradicate once established.
- We have a poor understanding of how crayfish affect aquatic ecosystems and native aquatic species in California.
Because crayfish, especially non-native crayfish, are so widespread and abundant in California, they tend to be taken for granted. They are present in habitats from warmwater ditches to coldwater mountain lakes and appear to be thoroughly integrated into our aquatic ecosystems, even waters like Lake Tahoe. They do especially well in habitats thoroughly altered by people, such as reservoirs, regulated streams, and rice fields. But there is much we don’t know about them. Some potential research questions include:
- What is the distribution of crayfish species in California today, native and non-native? Such information could allow us to see if they are useful indicators of habitat quality and change. Sampling e-DNA might be a useful approach to this question.
- Do the species replace one another in different habitats? Is the red swamp crayfish today the dominant crayfish in most habitats?
- Are dominant crayfish suppressing invertebrates and plants in streams, lakes, and sloughs throughout California, changing the nature of the ecosystems? This seems to be true in Lake Tahoe.
- Would removal of crayfish return a given aquatic ecosystem to its original state, favoring native species?
- Are other crayfish species likely to invade California? For example, the rusty crayfish (Faxonius rusticus) is an ecosystem damaging, aggressive crayfish that is spreading across North America.
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