Counting Fish in a Fluid Environment
September 11, 2013
ALBUQUERQUE, N.M., -- Where do fish occur in a river? To the casual observer, all parts of a stream seem alike: wet! But fish experience the river as complex habitat, and discern many subtle variations in water velocity, depth, temperature, turbidity, food content, dissolved oxygen, and other attributes. For a given species, some places have a more favorable mix of attributes than elsewhere, and individuals of a species will tend to congregate in these "favorable areas." Over the course of the year, as shifting air temperature, precipitation, and water levels change the location of favorable areas, so too individuals of a species will adjust their movements to track changes in the location of favorable habitat. This evolutionary dance between a species and its environment, honed over eons, is a wondrous thing.
Unless you are trying to count fish.
On a hot, sultry, mid-August day, I'm standing thigh-deep in the slow, muddy Rio Grande watching U.S. Army Corps of Engineers' Fishery Biologist Dr. Michael "Mick" Porter and Aquatic Ecologist Justin Reale; Eric Gonzalez, Michael Hatch, Matt McMillan, biologists with SWCA Environmental Consultants; and TetraTech biologist Michael Marcus seine for fish. A seine is a fishing net that hangs vertically in the water with floats at the upper edge and sinkers at the lower edge. The men are monitoring population levels of the endangered Rio Grande silvery minnow (Hybognathus amarus) in stretches of the river close to USACE ecosystem restoration sites. They want to not only understand how many fish are present, but what size classes are represented. These data are critical if the Middle Rio Grande Endangered Species Collaborative Program is going to successfully implement minnow recovery in the Middle Rio Grande. But to census fish, they have to find them and this turns out to be easier said than done.
The silvery minnow measures about two inches at the end of its first summer, growing over several years until it reaches the size of a man's index finger. According to previous research, most wild silvery minnows only live two to three years; larger and therefore older individuals are rarely caught during seining. Silvery minnow spawn during the height of the spring runoff on the flooded overbank area where slackwater nursery habitat sustains fry, (the young minnows) until they are able to swim in the river current. Water resources development and drought have reduced the size of the spring runoff pulse and frequency of overbank flooding, dramatically decreasing the number of minnows that are successfully recruited into the population each year. In 1994 the Rio Grande silvery minnow was listed as endangered by the U.S. Fish and Wildlife Service because the fish had been lost from 90 percent of its former range.
The nets come to shore and the counting begins. The fish from each net are identified and measured; if a silvery minnow is present, it is measured first and also weighed before being released. To minimize stress on the fish, the counting is done quickly; if many fish are in the net, they are transferred to a bucket while waiting to be measured and released.
Mike Hatch and Eric Gonzalez do most of the counting; Mike Marcus writes it all down. "CypLut 21!" Hatch shouts, shorthand for 21 mm long Red Shiner (Cyprinella lutrensis). He quickly releases the fish while Gonzalez, crouched next to him, shouts "PlaGra 44!" and puts back a 44 mm long flathead chub (Platygobio gracilis). In less than a minute, the nets are cleared and rolled for transport. The team moves on.
Pausing midstream to choose the next seining site, the conversation focuses on the kind of mesohabitat patches to choose from in that stretch of river. Small habitat areas measuring several dozen meters on a side are termed "mesohabitats" and an area of a particular mesohabitat type is called a "patch." Each seining effort focuses on a particular mesohabitat patch, with the length of the haul adjusted to the size of the patch. The abundance and distribution of mesohabitat patches shifts as a function of stream flow over the course of the year. A particular patch may be a different mesohabitat type at each visit.
But looking at how the flow adjusts itself to each sand bar, snag, and change in bottom sediment, you see that even within mesohabitat patches there is a great deal of variation. If one seines the whole patch, what kind of habitat did one seine? The scientists classify each seining site in terms of a specific mesohabitat type, but also record a description of it, including characteristics thought to be important for minnow use: water velocity, depth, oxygen level, turbidity, temperature, and time of day sampled.
The catch in each seining attempt is highly variable and Porter ponders why this variation exists. Today they'll seine 27 mesohabitat patches along the river upstream of Alameda Bridge in Albuquerque, N.M. To me, these reaches are largely similar: muddy, thigh-deep, briskly flowing water over shifting muddy sand. Yet in one seine haul they catch seven silvery minnow, in another two, and in the remaining 25, none. In this drought year, almost all are larger, older individuals; however, one is small enough to be this year's young. Following the spring spawn, young-of-the-year usually dominate the catch. What do this year's catch numbers indicate for the chance of species survival and recovery? Does the near absence of young-of-the-year in this year's nets really indicate few young were produced, or did the minnows spawn during the summer? Conversely, does a majority of larger fish in the nets indicate a senescing and aging population, or a resilient one?
Despite the low overall catch numbers today, Porter remains optimistic that they'll sort out these questions in time to recover the species. He reminds everyone that today's silvery minnow catch is a project record. "Don't forget to brag that we got nine minnows today!" he says as we part.
USACE ecosystem restoration work was funded under the USACE Civil Works Program. Monitoring of fish populations was conducted with funding from the USACE Collaborative Program.