During the winter 2016-1 OTS Field Ecology course I confirmed something that I didn’t want to believe: fish are being moved among streams and rivers within Costa Rica.
Peer into a stream in Costa Rica and you’re likely to see a fish community more impressive than anything you’ll see in a freshwater aquarium, but this diversity is under increasing threat from changes to both terrestrial and aquatic ecosystems. One way that streams and rivers are being changed that we know very little about is through species introductions. Commercially important fish species have been introduced into Costa Rica from all over the world, and inevitably a number of these species have escaped culture and established self-sustaining populations. Tilapia (Oreochromis sp.) were introduced from Africa for aquaculture, but some escaped and now wild populations are established in lowland streams and rivers throughout the country (Bussing 1998). Similarly, rainbow trout (Onchorhynchus mykiss), a cold water species native to North America, were introduced for aquaculture at high elevations, such as at the Cuericí Field Station. Rainbow trout are now established in the mountain streams near Cerro de la Muerte (Don Carlos Solano, pers. comm.). Another growing threat of unknown risk is transplanting native species within Costa Rica.
Costa Rica has a rich freshwater fish fauna that is divided into three different fish ecology provinces (Bussing 1998). Costa Rica’s geological history and unique climate zones have lead to the colonization and evolution of different species on the east and west versants (Figure 1) and unique fish fauna to the north and south. Few species are found in both the north and the south on the Pacific side and even fewer cross the east and the west versants. As a result, each region is home to a number of endemic fish species that are found nowhere else on earth.
Now there is evidence that fish species are being moved within Costa Rica. I recently posted a video to the Stroud Center Facebook account of a school of cichlids in the Rio Rincon on the Osa Peninsula in southwest Costa Rica (Figure 2). Bussing (1998) surveyed this river but did not report any large cichlids from it, but he did see Amphilophus (Astatheros) lyonsi in a nearby river. The fish in the video didn’t look exactly like A. lyonsi, but this species was my tentative identification based upon the assumption that it was either missed by Bussing or was a recent colonist from a nearby source population, and that the difference in spotting pattern was a result of phenotypic plasticity. But not everyone was buying it. Someone on Facebook said that the fish in the video was Parachromis loisellei, a species native to the north Atlantic versant of Costa Rica. Indeed, the fish in the video did look more like P. loisellei, but there was no way that species got into the Rio Rincon on its own. Was it really more likely that a species was transported across the country? Without an individual in-hand to confirm the identification it would be impossible to know for sure.
I went back to the Rio Rincon the next year and brought electrofishing gear, seines and cast nets, but never saw another mystery cichlid. I was stumped. Fortunately, in the meantime, I came across a fish survey from a tributary to the Rio Esquinas, which is near the Rio Rincon, and wouldn’t you know it, they found P. loisellei (Pichler and Schiemer 2008). I also showed the video to Dr. Ron Coleman from UC Sacramento while at the La Selva Biological Station in northeast Costa Rica during the winter 2016-1 OTS Field Ecology course. Dr. Coleman is an expert on the cichlids of Costa Rica, and he too believed that the fish in the video was P. loisellei or P. dovii, both species that we could see in the streams and rivers around La Selva. Based upon Pichler and Schiemer (2008) there was evidence that P. loisellei had been transported across the country, and now I believe that it was also the species in the Rio Rincon.
What have I taken away from this experience? Humility is just the beginning. Evidence that P. loisellei has been transplanted within Costa Rica raises a number of questions. First, why would someone transport this species? According to Dr. Coleman it was most likely for fishing. I did see people fishing near where the P. loisellei was spotted (Figure 3), but we probably will never know who introduced the species to the Rio Rincon and Rio Esquinas. At the same time, it is unrealistic to expect species transplants to slow or stop without working with people on the ground in the places where introductions are occurring.
As for the impact P. loisellei is having, 12 of 19 Costa Rican endemic fish species live in the southeast near Rio Rincon (Bussing 1998). Species introductions are experiments that are difficult to reverse. Once an introduced species becomes established it may be impossible to eradicate.
I also want to acknowledge how my bias influenced my perception. I didn’t want to believe that fish were being transplanted within Costa Rica, so I proposed what I thought to be the simplest answer. A robust study design that included voucher specimens was critical to allow others to look at the evidence and come to their own conclusion. Pictures and video may not be adequate vouchers for most studies, but in this case they were sufficient for others to evaluate and challenge my identification. Unfortunately, those findings do not bode well for Costa Rica’s native fish fauna.
Bussing, W. A. 1998. Peces de las aguas continentales de Costa Rica (Freshwater fishes of Costa Rica), 2nd ed. Editorial de la Universidad de Costa Rica, San Jose, C. R. 468p
Pichler, C., and F. Schiemer. 2008. Ecology of fishes of Quebrada Negra, Costa Rica, a first order neotropical lowland stream. Pages 495–508 in A. Weissenhofer, W. Huber, V. Mayer, S. Pamperl, A. Weber, and Aubrecht, editors. Natural and Cultural History of the Golfo Dulce Region, Costa Rica. Plöchl-Druck, Werndlstraße 2, 4240 Freistadt, Austria.