The River Tiger: Golden Dorado, by Connor Parsons

In the rivers of South America there lives a fish that is quickly becoming one of the most prized and sought after freshwater sport fish species in the entire world. This amazing fish is known as the Golden Dorado (Salminus brasiliensis) and is found in rivers that flow through Brazil, Paraguay, Uruguay, Bolivia and also northern Argentina (Lathrop 2015). This visually stunning gamefish gets its name from its radiant golden color that is lined with black holographic stripes. The Dorado is commonly called the “River Tiger” not only because of its looks but also because of its intense behavior. This fish is an extremely aggressive and strong piscivore that wanders up and down rivers searching for schools of baitfish. As indicated by its taxon, the Dorado is a relative to the salmon and shares a similar muscular body shape (The Freshwater Dorado, 2015). Attached to the body is a giant head that houses a set of jaws that are full of tiny razor-like teeth. The Golden Dorado faces many pressures in today’s world and conservationists are worried about its future due to the increased fishing pressure and loss of habitat where they are found. Something must be done or we just might lose this magnificent species. 

 

Golden Dorado, a popular sport fish in South American rivers.  Source

            This iconic charismatic fish has been getting quite the attention this past decade.  Anglers from all over the world come to South America to try and catch one of these beasts and in turn they have created important economic support for these local communities found along the Dorado’s rivers. Kirk Deeter (2015) explained in his article the reason why people come from all over to get a Dorado on the end of their line. “Imagine a fish that attacks a fly like a smallmouth bass, bolts like a bonefish, leverages like a wild steelhead, and then also jumps like a tarpon.“ That sure sounds like the ultimate freshwater sport fish to me. Luckily, these visitors are usually very passionate anglers and believe in catch-and-release fishing. Recent studies are being done to try and understand the impact that this kind of fishing is having on the species’ population (Gagne and Ovitz 2015). At the same time that these fish are being caught for sport, they are also being harvested for food by the people native to the areas of South America where these fish are found. The current management for the harvest of these fish is next to nonexistent so no one really knows the differences in population sizes in different rivers.  

Golden Dorado head.  source

            On top of the increased popularity as a sport fish and the overharvesting, this fish is also being influenced by habitat degradation (Block 2015). These fish are a migratory species much like their North American salmon relatives. Little is known about their migration patterns but we do know that humans are making it much harder or impossible to complete these journeys to spawn. One study focused on tracking these fish so scientists had a better idea of how they migrate. Seventy-three Dorados were radio-tracked using aerial surveys in the Upper Uruguay River. Surprisingly 40% of the fish that were tagged were never detected which means they probably were either harvested, post release mortality, or the fish migrated to tributaries outside of the tracking range. However, the study brought in the first data on the migrations of these fish and showed that part of the population was pretty active in migration (Hahn et al 2011). 
 

Salto Grande Dam completed in 1979 on the Uruguay River. Source

Hydroelectric dams put in place to create cheaper energy for the people of South America have blocked many of the Golden Dorado’s spawning routes. On top of blocking important waterways for them, the dams are reducing the flow of vital soil nutrients, which is affecting entire ecosystems that these fish inhabit (Fraser 2015). The great migrations that used to take place in the upper Uruguay River Basin are now blocked off by the Salto Grande Dam that was finished in 1979 (Fraser 2015). Due to the elimination of an important area for its reproductive cycle, the population has been steadily decreasing since. In one study, genomic DNA was taken from muscle cells from 67 individuals from nine locations spread out in Uruguay and Paraguay. From this sampling, eight microsatellite loci were found and could be used for suitable markers in population studies (Rueda et al 2011). It’s not only dams that are hurting the environment; intensive farming, deforestation, industrial pollution and the demands of a growing population for water and energy are drastically affecting the rivers here (Fraser 2015). 

 

Jirau Dam built on Madiera River in 2012.  Photo by Noah Friedman-Rudovsky

            Fortunately, it has become apparent that something must be done or we risk losing the Golden Dorado. Professor Andy Danylchuk, a fisheries biologist from the University of Massachusetts Amherst, began to work in spring 2015 with Argentina’s Ministry of Environment and other regional partners like Tigres del Rio and Patagonia Inc. wanting to get involved in the conservation of these species (Lathrop 2015). Their goal with their data is to develop new conservation strategies and new ways to sustainably manage these fish. Their study will measure certain impacts of catch and release such as fight time, air exposure, the method used to catch the Dorado. Then when the fish are caught, physiological biomarkers like blood lactate, blood glucose, and blood pH will be recorded and used to determine the effects of sport fishing (Lathrop 2015). Some findings so far in the study have indicated that a careful set of scientifically backed best practices should reduce the stresses on the Dorado.  With this knowledge, conservationists can express the right way to catch, handle, and release these fish so that they are affected as little as possible. With the increased awareness of conservation at the community levels, locals can learn the techniques that will diminish the amount of overharvesting (Gagne and Ovitz 2015). Most natives do not realize how they are actually negatively impacting their ecosystems when they take these fish out of the river at any rate that they desire. Newer dams like the two on Brazil’s Madeira River are using only a part of the river to reduce the size and avoid blocking off the whole waterway (Fraser 2015). Fish passages such as fish ladders have been constructed to assist in the successful migration of the Dorado but little is known on how effective these structures are. They can be quite selective and can promote the migration of non-target species, which can severely alter the environment and even hurt the Dorado even more (Braz et al 2008). Recently there have been propositions to include a channel through future dams that will allow fish to swim through or around them but even if they are built, there is no guarantee that the fish will find them. 

            The Golden Dorado has been through a lot these past few decades due to the increase in sport fishing, the overharvest of these fish, and the habitat alterations due to increases in the human population and its need for space as well as energy. The Dorado has come to symbolize conservation for these South American river systems and has really raised the awareness of our impacts as humans.

References

 Agostinho, AA., Pelicice, FM., & Gomes, LC.. (2008). Dams and the fish fauna of the Neotropical region: impacts and management related to diversity and fisheries. Brazilian Journal of Biology, 68(4, Suppl.), 1119-1132.

Block, E. 2015. "New Program Supports Research and Protections for Golden Dorado." MidCurrent. http://midcurrent.com/2015/04/22/new-program-supports-research-and-protections-for-golden-dorado/. (accessed April 19, 2016.)

Deeter, Kirk. "The Golden Dorado: Fly Fishing's Pound-for-Pound Toughest Fish in the World." Field & Stream. http://www.fieldandstream.com/blogs/flytalk/the-golden-dorado-fly-fishings-pound-for-pound-toughest-fish-in-the-world. (accessed April 19, 2016.)

Fraser, B. 2015. "Amazon Dams Keep the Lights On But Could Hurt Fish, Forests." National Geographic. http://news.nationalgeographic.com/2015/04/150419-amazon-dams-hydroelectric-deforestation-rivers-brazil-peru/. (accessed April 19, 2016.)

Hahn, Lisiane, et al. (2011), Use of radiotelemetry to track threatened dorados Salminus brasiliensis in the upper Uruguay River, Brazil. Endangered Species Research 15.2: 103-114.

Lathrop, Janet. "Protecting South America's Iconic Golden Dorado Fish." Office of News & Media Relations. http://www.umass.edu/newsoffice/article/protecting-south-america’s-iconic-golden. (accessed April 19, 2016.)

Ovitz, K., & T. Gagne. "Fish Mission: Golden Dorado - Moldy Chum." Moldy Chum.. http://www.moldychum.com/fish-mission-golden-dorado/. (accessed April 18, 2016.)

Rueda, E. C., Amavet, P., Brancolini, F., Sommer, J. and Ortí, G. (2011), Isolation and characterization of eight polymorphic microsatellite markers for the migratory characiform fish, Salminus brasiliensis. Journal of Fish Biology 79: 1370–1375.

"The Freshwater Dorado." Golden Dorado (River Tiger). http://www.amazon-angler.com/dorado/4577908513. (accessed April 18, 2016.)