|
~y
<br />~~
<br />~r~sact ns of the American Fisheries Society ! l6a 17-L l9, 1987
<br />© Copyright by the American Fisheries Society 1987
<br />K
<br />Digestive Tract Contents of Adult Razorback Suckers in
<br />Lake Mohave, Arizona-Nevada
<br />PAUL C.~MARSH
<br />Center far Environmental Studies
<br />and Department of Zoology
<br />Arizona State University
<br />Tempe, Arizona 85287, USA
<br />Abstract, -Digestive tracts of 34 preserved adult ra-
<br />zorback suckers Xyrauchen texanus from Lake Mohave,
<br />Arizona-Nevada, were examined. Contents were dom-
<br />inated by planktonic crustaceans, diatoms, filamentous
<br />algae, and detritus. The species is equipped for and ap-
<br />parently pursues a planktivorous habit, but benthic ma-
<br />terials in the diet indicate bottom feeding also occurs.
<br />The razorback sucker Xyrauchen texanus has
<br />received considerable recent attention because of
<br />dwindling numbers and imperiled status through-
<br />out its native range in the Colorado River basin
<br />(McAda and Wydoski 1980; Minckley 1983).
<br />Many facets of its life history such as diet and
<br />mode of feeding are little known, and the species
<br />is now so rare that the sacrifice of individuals for
<br />examination of their digestive tracts is scarcely
<br />justified. However, 34 specimens preserved for
<br />other purposes became available, and their diges-
<br />tive tract contents are reported here.
<br />Methods
<br />Adult razorback suckers were collected by tram-
<br />mel net from shoreline areas of Lake Mohave,
<br />Arizona-Nevada (Minckley 1983) in May 1980
<br />(N = 8) and in January (N = 10), February (N =
<br />3), and April (N = 13) 1984. Fish were killed and
<br />preserved intact by formalin injection. They were
<br />measured for standard length (SL, nearest mm),
<br />and sex was determined by internal examination.
<br />Digestive tracts were excised and 1.0-mL aliquots
<br />of the contents from either the anterior third (1980
<br />fish) or from each of the anterior, middle, and
<br />posterior thirds (1984 fish) were removed. Each
<br />aliquot was diluted in 5.0 mL of water, and 1.0
<br />mL of that suspension was pipetted into a stan-
<br />dard Sedgwick-Rafter counting chamber. Mate-
<br />rial in triplicate subsamples was identified by light
<br />microscopy at 35 to 400 x magnification and fre-
<br />quency of occurrence (percentage of digestive tracts
<br />
<br />containing a particular item} was determined
<br />among all specimens which were not empty (N =
<br />32). Predominance (percentage of tracts in which
<br />an item made up the greatest proportion of total
<br />volume) was estimated by examination of entire
<br />tracts as an index to clarify relative importance of
<br />various items. Animal foods were enumerated in
<br />digestive tracts of specimens collected in 1984 only
<br />(N = 26, of which two were empty).
<br />Results
<br />Fish were representative in size of the Lake Mo-
<br />have population (Minckley 1983; Bozek et al.
<br />1984). Males averaged 431 mm SL (range, 398-
<br />467 mm) and females 492 mm (411-530 mm).
<br />Digestive tract samples were pooled for analysis
<br />because contents varied among fore-, mid-, and
<br />hindgut samples from individuals, but not among
<br />fish, nor were there evident differences between
<br />sexes or among dates. Two (6%) digestive tracts
<br />were empty and were excluded from calculations.
<br />Planktonic crustaceans, rotifers, diatoms, detri-
<br />tus, and filamentous algae occurred in at least 44%
<br />of digestive tracts (Table 1). Bosmina sp. occurred
<br />in all tracts and was the most abundant item. Oth-
<br />er cladoceran genera occurred, but only Daphnia
<br />sp. was common (72% of fish). Rotifers, benthic
<br />ostracods, copepods, and chironomid dipteran
<br />larvae were found in 53, 53, 34 and 3% of diges-
<br />tive tracts, respectively but, except for rotifers,
<br />numbers were low. Among plants, diatoms, pri-
<br />marily Fragillaria crotenensis, were found in near-
<br />ly 90% of tracts and filamentous green algae were
<br />in 44% of all fish. Detritus and amorphous organic
<br />matter occurred in 56% of tracts. Inorganic ma-
<br />terials occurred in 16% of fish.
<br />Items with highest frequencies of occurrence in
<br />digestive tracts were generally those which con-
<br />tributed greatest volume to total contents. How-
<br />ever, rotifers, ostracods, and copepods all were in
<br />more than one-third of tracts yet predominant in
<br />none. Both frequency and predominance must thus
<br />be examined to gain a meaningful interpretation
<br />of relative importance of various items ingested
<br />by razorback suckers.
<br />117
<br />
|