Preface

This statement is based on unpublished research intended for journal publication in 2004 and should be used for informational purposes only. We (the authors) request that this statement not be formally quoted or referenced because our analyses and interpretation of our data are not complete. Questions or comments may be directed to either of us.

Introduction

Internal injuries (spinal damage and muscle hemorrhage) in fishes caught by electrofishing (EF) have been recognized as a sampling risk since the mid-1900s. However, the issue has drawn widespread attention among fisheries scientists and manager since the late 1980s. Since then, many research projects have been performed to identify the factors affecting risk of injury to EF-caught fish and the means to reduce that risk. This research has largely shown that risk of injury increases with increasing fish size and frequency (pulses per second or hertz) of pulsed DC; risk also apparently increases with electrical intensity (voltage, current, power) and the use of AC (versus DC) but the relationships are less clear.

The application of all this research has suffered because of the "apples-and-oranges" effect, that is, limited comparability and generalization of results due to differing methods and measurements among projects. Furthermore, most of this research has been done on salmonid species, particularly rainbow trout Oncorhynchus mykiss the limiting the scope for application.

To address the "apples-and oranges" issue, we undertook a study during 1998-2000 to 1) document the scope of the EF -induced injury problem among various North American freshwater fishes; and, 2) evaluate the relationships of voltage gradient (in-water measure of voltage), electrical waveform (Dc and pulsed DC), fish size (length), and induced behavioral response (escape, forced swimming or immobilization) to fish injury. We approached our objectives by conducting a series of controlled (tank) experiments at various locations across the U.S., using the same experimental design and equipment on different fish species. Our approach eliminated sources of variability common in field studies (e.g., habitat and equipment differences), while allowing better resolution of the relative importance of treatment factors (e.g., voltage and electrical waveform) to risk or probability of injury.

However, our approach also raises a basic question: Are the results of tank experiments applicable to EF operations? We addressed this question by assuming that fish behavioral response to electrical stimulus applies equally to any setting – laboratory or field. Fish response depends on the in vivo power achieved in a fish. For a level of in vivo power, a fish will exhibit the same response even though its surroundings may vary. We believe that fish response is a valid and meaningful basis for comparing electrical effects between controlled and uncontrolled environments.

Paradigm

We propose that the risk of electroshock-induced injury in fish is primarily a function of fish response, fish size and vertebral count (see figure below).

Fish response can be easily categorized as escape (ESC, upright avoidance swimming), forced swimming (FSW, unbalanced swimming) or immobilization (IMM, no swimming motions) during EF. Fish response is determined by the nature (shape) of the electrical waveform and its intensity (amplitude). Risk of injury tends to be low for the ESC and FSW responses and increases significantly with the IMM response; tetany, severe muscle contraction, is often associated with the IMM response.

Given a waveform of sufficient amplitude (threshold), an electroshock will produce muscle contraction in fish, resulting in a response that may be favorable for capture. The force of contraction is directly proportional to fish mass or volume and the maximum force along the fish body occurs at the point of maximum girth (explaining why most injuries are located at mid-kength). Thus, risk of injury increases with force of contraction and force of contraction increases with fish size. We used fish length as an easily-measured surrogate for the force of contraction. Risk of injury significantly increases among fish > 20-30 cm.
The force of contraction is mitigated by the strength of the spinal column as a resistance to the force. Vertebral count is a surrogate measure of the strength of the spinal column. High counts are characteristic of the supple spines found in strong-swimming body swimmers (e.g., salmonids); these spinal columns offer low resistance to the force of muscle contraction. Low counts are characteristic of the robust spines found in weak-swimming tail swimmers (e.g., centrarchids); these spinal columns offer high resistance to the force of contraction. Risk of injury increases significantly in species with vertebral counts >40.

Conclusions

The results of our experiments indicate that small-size fishes and those with low vertebral counts are at low risk for electroshock-induced injury. Large fish, and those with high vertebral counts, are at high risk, but this risk can be reduced by avoiding electrical waveforms and intensities that immobilize (thus, tetanize) fish. Even small fish, or those with lower vertebral counts, may be injured if the electrical waveform and intensity are sufficient to produce severe contractions. We recommend that EF strategies include efforts to capture fish by inducing forced-swimming behaviors, not immobilization and the tetany that accompanies it.