Fishing for answers in Palo Verde

 
Palo Verde 
Palo Verde National Park is a beautiful place full of thorny trees and cacti in a seasonally dry tropical forest, an open wetland that serves as a sanctuary for many migratory birds (and a few small crocodiles) and large limestone outcroppings which allow for some amazing views of the park.  At the OTS Biological Station in this park, we conducted independent research projects, and two of my friends enticed me to work with them on a project about fish.  We decided to study the effect of conventional agriculture on behavioral response to a predator and on carotenoid concentrations in the fins used for sexual displays in a small poeciliid guppy.  Carotenoid pigments can be used as an indicator for health, and agrochemicals are known to reduce carotenoid levels and impare the ability of fish to respond to predators.  We compared fish from canals in rice fields in the Bagazi Agricultural Project, which is just outside the park, with fish from the Palo Verde marsh.

I was a little skeptical about the project at first because I was afraid we would not be able to catch enough fish from the rice field canals.  The first day we went there we saw hundreds of fish, but after around an hour seining and swiping at the water with nets we only had one fish in a bag to show for it.  However, with more experience and the help of the course driver Carlos and our TA José we were able to catch a lot more fish.  I grew to really enjoy standing as still as I could over the canals, waiting to strike with my net like an egret, and tramping around through the mud in the marsh. 

We tested the behavior of the fish by putting them in plastic arenas filled with a small amount of water, with a small shelter on one side, a larger shady shelter on the other, and a model egret held above the center of the arena.  We predicted that agrochemical exposure would increase the boldness of the fish under threat of predation, and hoped that the bolder fish would emerge from the small shelter more quickly, be more likely to pass directly under the model predator than shy fish, and reach the large shelter more quickly.  We tested about 140 fish over three days, and when I closed my eyes at night all I could see was little guppies swimming around.  After the behavioral test we photographed the fish so we could analyze the length and sex of the fish and the carotenoid content of their ventral fin using the ImageJ program. 

We ran into some difficulties determining the species of fish that we were catching.  In our first few days of naiveté and inexperience we thought that all our fish were one species in the genus Gambusia.  However, a few days in our professor Erika pointed out that some of the fish had black bands on their tails and others did not; we must have two species.  To get some help identifying them, Erika sent a few pictures out to a colleague of hers who is a fish specialist.  The expert analysis was that we actually had three fish species, and non of them were Gambusia!  The species that we caught the most individuals of, and the only one we found significant numbers of in all our capture sites, was Poecilia gillii, so that is the only species we used in our study.

We determined that the fish from the outflow canals, larger fish, and male fish all took longer to reach the large shelter during the behavioral test.  Larger fish were also more likely to cross in the middle of the enclosure than small ones.  These results were interesting but since there wasn’t a significant difference in boldness between fish from the Bagazi rice fields and those in the marsh they do not support the prediction that agrochemicals are impacting the response of fish to a predator.  We found that carotenoid concentrations were higher in larger fish and in females.  They were also higher in the fish from the marsh than in those from the outflow canals, and this did suggest that agrochemicals were having a negative impact on the diet or health of fish.

Sequoia Grettenberg