By Peter B. Moyle & Andrew L. Rypel

If you ever watched National Geographic television and are interested fishes and rivers, you likely have some familiarity with Dr. Zeb Hogan. He hosted a series of shows on giant freshwater fishes, called Monster Fish. He and a colleague also recently published a fascinating book (Hogan and Lovgren 2023) on global adventures searching for giant freshwater fishes. This book is likely to interest California Water Blog readers for several reasons.

• Zeb obtained a PhD in Ecology from UC Davis working with Bernie May, Peter Moyle, and other faculty. His dissertation included a study of the biology and conservation of giant catfish in the Mekong River, documenting it was close to extinction.
• His new book discusses sturgeon conservation at length and provides additional background useful for saving the white and green sturgeon in California.
• The book is an entertaining travelogue featuring trips to rivers around the globe to answer the question: What is the biggest freshwater fish? It also features a strong conservation message, showing why “megafishes” are so important for aquatic conservation.
• It calls attention to the Mekong River in particular, an amazingly diverse and threatened ecosystem. The Mekong supports 500 endemic fish species and its fisheries feed millions of people. Giant catfish, carp (barbs) and sting rays are part of the river’s fish native fauna. Mekong fish and fisheries are especially threatened by hydropower dams (Hogan et al. 2004). Some scientists are convinced that such dams can be built and operated in ways that don’t affect fish populations, a proposition we are skeptical of, given our experience with California dams and fish.

Zeb confined his megafish search to species that spend their entire lives in freshwater. If he had included fishes that occur in freshwater but spend much of their lives in saltwater, the search would be short. He did not consider these fishes because going out to sea or into an estuary gives fish access to marine resources that allow them to grow rapidly to large size. This is the primary reason salmon and steelhead go out to sea. If anadromous fish were included, sturgeon would win the big fish contest, fins down!  Number 1 would be Beluga sturgeon from Russia, which have been recorded as long as 8 m in length and 1.4 tons. Number 2 would be white sturgeon from the west coast North America, which have been recorded as long as 6.7 m long and 800 kg (1764 lbs).  

This book recognizes and discusses how sturgeon species across the globe face many shared problems. This is important for California, because there are likely to be common solutions for our problems from other sturgeon populations. Many sturgeons have been reasonably well-studied from their value as caviar producers, as commercial and ‘game’ or ‘sport’ fishes, and as ancient survivors of the mass extinction at the end of the Cretaceous period. Of course, all this has not kept sturgeon from facing extinction recently. There are 27 known sturgeon species/ESUs globally; all of them are rated as in danger of extinction by the International Union for the Conservation of Nature (IUCN). And while the threat to white and green sturgeon has historically been rated as low, this assessment is changing rapidly. In California, factors such as the recent red tide die-off of white sturgeon in San Francisco Bay is generating broad reevaluation of conservation practices (Schreirer et al. 2023).                     

Here are some megafish lessons for California sturgeon:

• The Beluga sturgeon, the largest and oldest of sturgeon species, is approaching extinction in the wild due to overharvest because its caviar is the most valuable of all sturgeon caviar. Despite regulations that ban fisheries and international trade in Beluga caviar, poaching continues. The sturgeon is now subject of an intense aquaculture industry which raises Belugas for their caviar; this growing industry assures that Beluga sturgeon will continue to exist, if not in the wild. Yet the mystique surrounding wild Beluga caviar enhances its value and demand, which basically ensures poaching of wild fish will continue. It is now, listed by IUCN as critically endangered and it will likely be extinct in the wild soon. In California, white sturgeon are also increasingly cultured for caviar (and meat), and again, poaching similarly continues. The wild white sturgeon population in California is in decline, but also supports a sport fishery which is increasingly efficient in detecting and harvesting sturgeon. For example, it is fairly simple and relatively inexpensive to buy an efficient fish finder. Using just this tool, one can cruise through known sturgeon areas of the estuary looking for large whites. And because sturgeon don’t move much, they are easily captured using hook and line and simple baits.

• More importantly, poor water quality is now killing adult white sturgeon, and therefore limiting natural production. For decades the white sturgeon has been held up as a positive example of fisheries management. Now, having white sturgeon may survive in the future solely as a cultured fish now seems likely. The red tide that spread across the San Francisco Estuary late last summer killed an unknown number of white sturgeon, and some green sturgeon. Large red tides, harmful algal blooms and corresponding fish kills are often brought on by intense heat waves (Till et al. 2019; Griffith and Gobbler 2020; Tye et al. 2022). Thus, deterioration of water quality in the estuary is linked almost directly with climate change impacts in California. These events are probably just beginning, suggesting that California’s white sturgeon population is becoming less resilient overall. So conservation strategies for the fishery must change.

• The Fraser River (British Colombia, Canada) supports the largest white sturgeon population in Canada, and a valuable sport fishery. Yet in the early 1990s, there was an unexplained die-off of large sturgeon that prompted reevaluation of the fishery. The conservation response was a major, swift and stakeholder-driven. One early action was a switch to catch-and-release fishing. Once this was established, a volunteer tagging program for anglers was organized. “This was a widely successful program with tens of thousands of sturgeon tagged; it yielded valuable data-rich information about their abundance, movements, and growth. A decade into the program, the decline of the sturgeon had been reversed… (Hogan and Lovgren 2023, p. 83)”. Angler attitudes towards sturgeon have broadly changed. Not too long ago, sturgeon species in many regions were classified as ‘rough fish’ or ‘other fish’ and afforded little to no protection via protective regulations (Rypel et al. 2021). The Fraser River example demonstrates a high potential for recovering the Sacramento River population. And why not engage anglers to to tag and release sturgeon they catch to improve information on the population and fishery, especially if these changes lead to more fish?

• The Kootenai River is a tributary to the Columbia River, and supports a unique, land-locked population of white sturgeon. Although the Kootenai River once supported a white sturgeon fishery, in recent decades the population has become endangered because of pollution from mines, overharvest, and most importantly, construction of Libby Dam in 1974. The dam changed how sediment was transported in the river and resulted in the only spawning site for the sturgeon to be covered in sand, a substrate that is very poor for early life survival. The sturgeon population persisted for decades without natural reproduction only because of their longevity. To save the fish, the Kootenai Tribe built a sophisticated hatchery on the river and now releases juvenile sturgeon into the habitat to augment the population of the few remaining adults. “A hatchery can buy you time to restore the river, assuming there is the knowledge, money, and political will to do so. But these restoration efforts often fall short, in which case species like the white sturgeon will depend on hatcheries in perpetuity (Hogan and Lovgren 2023, p.85.”). For more discussion of hatcheries see Rypel and Moyle (2023).

• China has, or had, populations of Chinese sturgeon, Yangtze sturgeon, and Chinese paddlefish, a close sturgeon relative. The Chinese paddlefish was recently declared extinct (Zhang et al. 2020) and the two sturgeon species are extinct or near extinct in the wild, except for those released into the Yangtze River from hatcheries (Zhuang et al. 1997; Zhuang et al. 2016). The root cause of decline for these fishes is the fundamental transformation of the river by a chain of dams (including the Three Gorges Dam, by some measures the largest dam in the world) that eliminated spawning habitat and most other suitable habitat. Extinctions do happen, and this is a possible future for white and green sturgeon in the Sacramento River and estuary if essential conditions for all life history stages are not maintained.

• The lake sturgeon of eastern North America is also a contender for biggest freshwater fish because it spends its entire life-cycle in lakes and rivers, reaching up to nine feet long (275 pounds) and living at least 150 years. It was once one of the most abundant fish in Lake Erie and other large natural lakes, but it was decimated by unregulated fisheries during the 19^th century. Cumulative impacts ultimately left just a tiny fraction of the original population and resulted in the species being listed as ‘endangered’, by IUCN and number of US states. In Wisconsin, the fishery was first banned (1915) but then allowed to resume as a sport fishery under close supervision in 1934 while the life-history and population ecology were better studied, especially in Lake Winnebago and its main tributaries. Over time, biologists collected data on abundance, sex ratios of spawners, and growth changes of fish every spring during the spawning migration. Using these meticulously collected data, it was originally estimated, and ultimately confirmed, that ~5% of the adult population could be sustainably harvested every year; this quota is taken mainly by spearfishing during a short ice season in February. Access to the fishery is tightly restricted via a lottery system. Biologists and anglers also work in tandem with fishers to collect data on all harvested fish. This management system is popular among anglers because the lucky quota winners could potentially catch fish approaching the historic maximum size. For them, it is a once-in-a-lifetime experience. Abundance has, in turn, increased dramatically over time given the excellent management. During spring now, hundreds of large sturgeon move upstream to spawn, an event that attracts many viewers, including local volunteers who go so far as protecting the large fish from poachers at night (a.k.a. ‘sturgeon guards’). This shows the high potential for “what science-driven management, community support, and a long-term commitment to the preservation of a large freshwater fish can accomplish (Hogan and Lovgren 2023, p.214)”.

Conclusions

A major point of the megafish book is that large fish are among the most vulnerable to decline and eventual extinction from human-made causes. Sadly, many of the species Zeb searched for may not be around in the near future unless action is taken to protect them, especially through habitat protection and management. Science and proper monitoring is extremely critical, and we must keep learning about the biology of these interesting animals to understand how to protect them better. For example, we only just realized that there are actually two migration behaviors in California green sturgeon (Colborne et al. 2022; Colborne et al. 2023). Community engagement is also essential – these are fishes that the public is often willing to protect, and will work hard to do so (Schmitt Kline et al. 2009). If their extinction occurs on our watches, it is a stain on all of us. The best hope for “monster fish” is that they are used as flagship species to encourage habitat conservation on a large scale. In California that would be a great role for white and green sturgeon!

So, what is the biggest freshwater fish? You will have read the book to find out. Suffice it to say that your reading trip of discovery will be most enjoyable, despite what you may have concluded from our focus on sturgeon problems. Zeb Hogan and co-author Stefan Lovgren have done a great job of introducing the reader to some of the world’s most interesting fishes, aquatic habitats, and fish people.

Peter B. Moyle is a Distinguished Professor Emeritus at the University of California, Davis and is Associate Director of the Center for Watershed Sciences. Andrew L. Rypel is a professor of Wildlife, Fish & Conservation Biology and Co-Director of the Center for Watershed Sciences at the University of California, Davis.

Further Reading

Colborne, S.F., L.W. Sheppard, D.R. O’Donnell, D.C. Reuman, J.A. Walter, G.P. Singer, J.T. Kelly, M.J. Thomas, and A.L. Rypel. 2022. Intraspecific variation in migration timing of green sturgeon in the Sacramento River system. Ecosphere 13: e4139.

Colborne, S.F., L.W. Sheppard, D.R. O’Donnell, D.C. Reuman, J.A. Walter, G.P. Singer, J.T. Kelly, M.J. Thomas, and A.L. Rypel. 2023. Green sturgeon in California: hidden lives revealed from long-term tracking. https://californiawaterblog.com/2023/02/12/green-sturgeon-in-california-hidden-lives-revealed-from-long-term-tracking/

Griffith, A. W., and C. J. Gobler. 2020. Harmful algal blooms: A climate change co-stressor in marine and freshwater ecosystems. Harmful Algae 91:101590.

Hogan, Z. and S. Lovgren. 2023. Chasing Giants: In Search of the World’s Largest Freshwater Fish. University of Nevada Press, Reno NV USA.

Hogan, Z. S., P. B. Moyle, B. May, M. J. Vander Zander, and I. G. Baird. 2004. The imperiled giants of the Mekong. American Scientist 92: 228-237.

Rypel, A. L., P. Saffarinia, C. C. Vaughn, L. Nesper, K. O’Reilly, C. A. Parisek, M. L. Miller, P. B. Moyle, N. A. Fangue, and M. Bell‐Tilcock. 2021. Goodbye to “rough fish”: paradigm shift in the conservation of native fishes. Fisheries 46(12):605-616.

Rypel, A.L., and P.B. Moyle. 2023. Hatcheries alone cannot save fish and fisheries. https://californiawaterblog.com/2023/04/30/hatcheries-alone-cannot-save-species-and-fisheries/

Schmitt Kline, K., R.M. Bruch, F.P. Binkowski, and B. Rashid. 2009. People of the sturgeon: Wisconsin’s love affair with an ancient fish. Wisconsin Historical Society Press, Chicago IL USA.

Schreier, A., P.B. Moyle, N.J. Demetras, S. Baird, D. Cocherell, N.A. Fangue, K. Sellheim, J. Walter, M. Johnston, S. Colborne, L.S. Lewis, and A.L. Rypel. 2022. White sturgeon: is an ancient survivor facing extinction in California? https://californiawaterblog.com/2022/11/06/white-sturgeon-is-an-ancient-survivor-facing-extinction-in-california/

Till, A., A. L. Rypel, A. Bray, and S. B. Fey. 2019. Fish die-offs are concurrent with thermal extremes in north temperate lakes. Nature Climate Change 9(8):637-641.

Tye, S. P., A. M. Siepielski, A. Bray, A. L. Rypel, N. B. Phelps, and S. B. Fey. 2022. Climate warming amplifies the frequency of fish mass mortality events across north temperate lakes. Limnology and Oceanography Letters 7(6):510-519.

Zhang, H., I. Jarić, D. L. Roberts, Y. He, H. Du, J. Wu, C. Wang, and Q. Wei. 2020. Extinction of one of the world’s largest freshwater fishes: Lessons for conserving the endangered Yangtze fauna. Science of the Total Environment 710:136242.

Zhuang, P., F. e. Ke, Q. Wei, X. He, and Y. Cen. 1997. Biology and life history of Dabry’s sturgeon, Acipenser dabryanus, in the Yangtze River. Environmental Biology of Fishes 48:257-264.

Zhuang, P., F. Zhao, T. Zhang, Y. Chen, J. Liu, L. Zhang, and B. Kynard. 2016. New evidence may support the persistence and adaptability of the near-extinct Chinese sturgeon. Biological Conservation 193:66-69.