What’s for lunch? Novel method for gut content analysis elucidates food resource availability and selectivity

Authors: Bixby, Rebecca, Burdett, Ayesha, Tave, Douglas, Hutson, Alison

Year: 2016 (xlviii)

Abstract

Gut content analysis of invertebrates and fish has long been used to examine food resource selectivity and determine food web structure. Most gut content analyses identify vertebrate and invertebrate prey taxa. Some studies identify algal taxa but ver y few identify both animal and algal taxa from the same fish gut, giving a more complete picture of available food resources. We have developed methodologies to analyze both algal and invertebrate composition from environmental samples and individual fish gut s to utilize Ivlev’s Electivity Index (IEI) to assess feeding habits of a grazing fish, the Rio Grande silvery minnow (Hybognathus amarus). Samples were collected in 2011 and 2012 from the Los Lunas Silvery Minnow Refugium, a conservation fish culture facility where the fish forage for natural food resources with no supplemental food provided in the outdoor environment. Periphyton and macroinvertebrates were collected monthly (July-October) from four natural substrates (concrete run, sediment in pools, macrophytes, water column) at the facility. Standard methods were used to identify and enumerate periphyton and invertebrates from these environmental samples. In addition, larval fish were also collected monthly from the facility. In the lab, standard body length and total gut length were measured. Macroinvertebrates and zooplankton were identified to the lowest practical level (often family) and enumerated from each fish gut. Algae were identified to genus, including diatoms to specie s, and enumerated from the same gut samples. Total algal and invertebrate numbers then were calculated per densities per individual fish gut, giving the ability to combine algal and invertebrate data sets and compare fish diets of different ages. In the environmental samples, periphyton was dominated by two diatoms Nitzschia perminuta (17% across all samples) and Epithemia sorex (9%) and green desmids (Cosmarium spp.; 13%), while the macroinvertebrate community was dominated by ostracods (33%) and chironomids (26%). Gut contents reflected opportunistic resource use: ostracods, diperans, upright-growing diatoms, and unicellular green algae were important food resources (IEI>~0.5). In contrast, other algal and invertebrate taxa were only re corded in the environment but not in the guts (IEI<0). Results from this study demonstrate the potential of this more comprehensive method to evaluate food resources available for grazers and also provides a window into the foraging habits of grazing fish.