2.2 | Ichthyoplankton Sampling
We collected ichthyoplankton samples every 10 days during daylight hours
from late April to September in 2014 to 2018. Sites were located 1 km
upstream and downstream and at the confluence of each tributary for a
total of nine sample locations (Figure 1). Each site consisted of three
tows, one at each different habitat types including a backwater,
thalweg, and channel border. We used ichthyoplankton tows (0.5 m
diameter, 2.5 m length, and 500 µm mesh net) to collect larval fishes by
conducting tows parallel to the flow at speeds to maintain no upstream
or downstream movement for a total of four minutes. We preserved larval
samples in 95% ethanol and recorded water flow through each net using a
General Oceanics 2030R flowmeter mounted at the mouth of the
ichthyoplankton net which we used to calculate water volume
(m3) filtered through the net. Across all sites and
years, we collected a total of 1,776 ichthyoplankton samples.
In the laboratory, we sorted and cleared ichthyoplankton samples from
debris with a minimum of two different individuals until no further
larvae were found. We identified larvae to the lowest taxonomic level
possible with visual identification using techniques described by Auer
(1982) and Chapman (2006). We identified larval fishes (ventral
fin-folds present) to family apart from certain taxa such as bigheaded
carp and freshwater drum that could be identified down to genus or
species. We selected bigheaded carp, freshwater drum, gizzard shad, and
Percidae for analysis due to variable reproductive strategies and
frequency of occurrence in our samples. We were only able to visually
identify larvae down to family Clupeidae, but are referenced as ‘shad’
hereafter, as gizzard shad are the dominant taxa found in the study
reach. Bigheaded carp and drum are both pelagic broadcast spawners while
shad and percids tend to spawn in lower water velocity environments of
channel borders and backwaters (Kolar et al., 2007; Holland 1986). Shad
and percids tend to spawn earlier during cooler water temperatures,
bigheaded carps tend to spawn later in the year during warmer water
temperatures, and freshwater drum spawning tends to be protracted across
a broad range of water temperatures (Becker, 1983; Bozek et al., 2011;
Kocovsky et al., 2012; Swedberg & Walburg, 1970).