ALBUQUERQUE, N.M., -- We’re standing knee-deep in Rio Grande mud, squinting in the bright sunlight. I close my eyes and drink in the lazy sounds and smells of the bosque. A Great Blue Heron wings by.
The cry “I’ve got silver!” sets us all in motion. The biologists have brought the nets ashore. Wriggling in the bag seine are a dozen small fish, among which are seven endangered Rio Grande silvery minnow (Hybognathus amarus). They are rapidly transferred from the fishing net to a water-filled bucket. When the scale is set up, they are quickly measured, weighed and released. All but one exceeds 70 mm in length. They are not young-of-the-year, but this is not surprising: this year’s drought meant that the spring runoff was not large enough to cue spawning.
Albuquerque District Fishery Biologist Michael “Mick” Porter is satisfied with the catch. It’s early fall 2013, and we are out on the river with biologists from USACE and SWCA collecting data for a study on silvery minnow populations in the Middle Rio Grande. Porter’s team is funded by the USACE Middle Rio Grande Collaborative Program to evaluate whether previous population censuses accurately counted fish. The standard method is to use a beach seine, a flat, rectangular net that is dragged upstream through a section of water. Using this method, the majority of silvery minnow sampled were small, young-of-the-year. Large, old adults were rarely caught. This led to the conclusion that silvery minnow are a short-lived species, with life-spans in the 1-3 year range.
However, collections at the University of New Mexico Museum of Southwestern Biology made earlier this century contained larger, older individuals. Had damming the river and diverting water for irrigation somehow reduced the average lifespan of individual fish? If large fish were not turning up in nets, did that mean they were not there, or did it mean they were not being caught using this method?
Porter suspected that the standard beach seining method used to census fish could be biasing the results. Larger, faster-moving fish could swim out of the way of the net while small, young, slower-swimming fish could not. So Porter designed a two-net system, consisting of the standard beach seine and a bag seine. Whereas the beach seine is a flat piece of net, the bag seine has a three dimensional shape that enables it to bulge in a downstream direction, better trapping fish. The two nets are deployed facing each other in the river at a measured distance. The beach seine is walked downstream to the stationary bag seine, and when the two meet, the catch is brought to shore.
Once on shore, the catch in each net is tallied by species and size. So far this season, the number of fish caught in each net is approximately equal, suggesting that both nets do equally well at catching fish. But, intriguingly, the minnow have almost all been caught in the bag seine. Porter points out that this year was a “lean year for minnows” in the sense that low spring water levels meant that few minnow reproduced. The minnows caught to date are almost entirely large fish. Because minnows grow continuously, age is correlated with size. In historic collections, the majority of fish in the size range being caught this year are estimated at 3 years old and older. Most intriguingly, the minnows caught in the bag seine are larger, presumably older individuals.
“It would be interesting to repeat this study in a ‘good’ year,” Porter says, “to see whether different age classes occur in different nets. I would expect either the young to be split evenly between the nets or to be found more frequently in the mobile beach seine. As they get older, I would expect more and more of them in the bag seine.”
If it turns out that larger minnow are able to avoid capture when beach seines are used for sampling, and smaller minnow are not, this would have broad implications for our understanding of minnow populations on the Middle Rio Grande. Current understanding is based mainly on population censuses using beach seines, which have suggested minnows are small, short-lived and vulnerable to population failure when drought stymies reproduction. The new data suggest that minnows may be larger, longer-lived fish with populations that may be somewhat larger than previously estimated. Importantly, a longer life span would offer greater resilience to drought and other causes of reproductive failure than is currently thought. The minnow data also suggest that undercounting of older age classes may be occurring for other fish species in the Rio Grande and elsewhere. Porter’s study is therefore being watched with great interest by fisheries biologists across the U.S.
Porter is optimistic that his preliminary observations will be upheld as the number of fish sampling events increases. He is hoping to continue the study through the end of Fiscal Year 2014 and, possibly repeat it in an out year when a large snowpack offers the chance of a good minnow reproduction year. But for now, the nets are empty. It’s time to sample at the next location and the team moves upriver under the scorching late summer sun.