No abstract

In the laboratory, we assessed the effects of electroshocking embryos and early larvae of razorback sucker Xyrauchen texanus with square-wave pulsed DC in homogeneous fields. Embryos at early epiboly, early tail bud. or fin fold developmental stages and pre-swim-up larvae were exposed for 10 s to simple pulse currents of 30 Hz (12% duty cycle). 60 Hz (24% duty cycle), or 80 Hz (40% duty cycle), or to a fixed complex pulse current of three 240-Hz, 2.6-ms pulses delivered at 15 Hz (12% duty cycle). Peak-voltage gradient for each current was 1.2 V/cm (power density = 936 jiW/cm 3 ). Tests were also conducted with the 60-Hz current at peak-voltage gradients of 5.0 V/cm (16,250 jiW/cm 3 ) and 10.0 V/cm (65.000 jiW/cm 3 ). Survival of embryos from treatment through hatching improved significantly (P ^ 0.05) at successive developmental stages; embryos at early epiboly were most sensitive to electric shock. Mean survival of embryos

No abstract

Ichthyoplankton drift samples from the lower James River, South Dakota, during the 1978 spring and summer, were composed of 16 taxa representing six families. The predominant taxa were freshwater drum Aplodinotus grunniens, gizzard shad Dorosoma cepedianum, Catostomidae, Cyprinidae, Ictaluridae, and Centrarchidae in decreasing order of abundance. Peak drift densities (460/1,000 m •) occurred on July 6 when the predominant ichthyoplankters were larval freshwater drums (323/1,000 m •) and gizzard shad (58/1,000 m•). The average weekly number of fish larvae moving through the study area was approximately 2.52 million. Collectively, the lower James River ichthyoplankton exhibited no significant diel periodicity, but four taxa did so. Gizzard shad, ictalurids, and catostomids were nocturnal drifters; their densities began increasing at dusk, peaked at 0100 hours, and subsequently declined at 0500 hours. Larval freshwater drums were diurnal drifters; greatest densities occurred between 0900 and 1700 hours. The diurnal drifting of freshwater drums was primarily evident on four dates when large numbers of protolarvae and mesolarvae were collected. Cyprinids and centrarchids did not exhibit diel periodicity.Tondreau (1979) examined several potential sources of recruitment of larval fishes to the mainstem Missouri River and identified tributaries as major contributors. One objective of this study was to determine the contribution of fish larvae from the lower James River, South Dakota, to the Missouri River during a major portion of the spawning season. Another objective was to identify the species included in this recruitment, and characterize various aspects of their downstream movement in the James River, particularly diel patterns. The heterogeneity of ichthyoplankton distribution within an ecosystem complicates quantitative and qualitative sampling. Numerical abundance depends upon spatiotemporal variability in spawning and diel movements of larvae. Diel periodicity is a recurring daily activity pattern; light intensity and the rare of light intensity change .are the main phase-setting mechanisms (Corbet 1966). Ichthyoplankters frequently display a diel periodicity in their activity and generally are classified as diurnal, nocturnal, or crepuscular (Hoar 1953; Northcote 1962; Lindsey and Northcote 1963; Geen et al. 1966; Clifford 1972). Developmental phases within a species exhibit different spatial and temporal distribution patterns. Accordingly, an understanding of diel variability in the rate and composition of ichthyoplankton drift (including active downstream movement by larvae) is essential to studies designed to obtain unbiased, "representative" estimates of fish egg and larva abundance. Methods The James River, a major tributary of the Missouri River, is a shallow prairie stream originating in central North Dakota and flowing generally southward through southeastern North Dakota and eastern South Dakota. It drains an area approximately 57,000 km 2, and has the smallest stream gradient of any river in South Dak...

We conducted laboratory experiments to address concerns about potential harmful effects of electrofishing on juvenile humpback chubs Gila cypha. Four types of square‐wave pulsed DC in homogeneous fields were tested: 30 Hz (12% duty cycle), 60 Hz (24% duty cycle), 80 Hz (40% duty cycle), and a complex pulse train of three 240‐Hz, 2.6‐ms pulses delivered at 15 Hz (12% duty cycle). We first determined peak‐voltage gradients for each current sufficient to induce the electroshock responses of taxis, narcosis, or tetany in captive‐reared early juvenile humpback chubs (49–96 mm total length) and bonytails G. elegans (46–79 mm). Bonytails were intended as surrogates for humpback chubs in most subsequent tests. However, mean voltage‐gradient response thresholds were 8–43% lower for humpback chubs than for bonytails. We then exposed 30 humpback chubs for 10 s to the complex pulse current at mean tetanizing field intensity and 60 bonytails for 10 s to one of the four currents at mean field intensities required for each of the three responses. All humpback chubs and half of the bonytails were euthanized, frozen, and later examined for internal injuries; remaining bonytails were reared for 98 d to assess effects on growth and survival. No mortalities, external injuries, or vertebral injuries were observed in either species. Moderately severe spinal hemorrhages were found in 20% of shocked humpback chubs and 13% of shocked bonytails. Shocked humpback chubs had a significantly higher (P ≤ 0.05) incidence of injuries than unshocked control fish. The number of injured bonytails was significantly higher (P ≤ 0.05) than controls in the 80‐Hz taxis treatment and the 30‐, 60‐, and 80‐Hz tetany treatments. Differences in the number of shocked bonytails with injuries among currents at each response threshold and among response thresholds for each current were not significant. No significant differences in injury rates were detected between humpback chubs and bonytails exposed to the complex pulse current at tetanizing field intensities. Growth of bonytails was not affected by shocking. Results suggest that electrofishing could cause spinal hemorrhages in some early juvenile humpback chubs but does not affect short‐term growth or survival. Studies are needed to evaluate the significance of electrofishing injuries in humpback chubs at the population level.