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).