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Electrofishing and fish health

spinal-injury-feature

A dorsal view X-ray of a rainbow trout revealing spinal fracture caused by electrofishing (source: N. G. Sharber).

As fish lovers and conservationists, we have a vested interest in maintaining fish health. As fisheries scientists, we have the responsibility of ensuring that our sampling techniques are as ethically transparent and non-invasive as possible. Many of us have moved from passive gear types, such as gill nets, to what are considered ‘less harmful’ methods, such as electrofishing. 

For many practicioners, however, electrofishing remains very much a dark art. This is understandable given the “very dynamic and complex mix of physics, physiology, and behavior” at play, but we must not fish without thinking. We must not fool ourselves into a false sense of security regarding (a) the scientific validity of our sampling or (b) the potential effects on fish health. Instead we must ask: are we hurting fish; if so, how so; and, if so, is the level of harm justifiable when compared to other gear types and the positive conservation outcomes?

In his 2003 report, Darrel Snyder, Colorado State University, began answering some of these questions, by first detailing the mechanics of electrofishing using field and power transfer theory, together with fish models. He then investigated the evidence relating to fish injuries in two main categories, non-spinal internal injuries and spinal injuries, and reviewed the response thresholds for various North American species, mostly salmonids. He found the latter group to be particularly susceptible to alternating currents. Direct current (DC) and low frequency pulsed DC (<30 Hz, the lower the better), in contrast, caused fewer spinal injuries and hemorrhages (p.98).

trout-injury

Hemorrhages and associated tissue damage in a rainbow trout (source: N. G. Sharber).

Snyder also discussed the relationship between the spatial characteristics of electric fields and fish injuries. He suggested that reducing the area of most intense power (near the anode/s) may not reduce fish injuries, if the threshold for injury is, in fact, occurs at much lower powers (i.e., towards the outside of the field where fish exhibit a twitch response). He concludes:

Except in very severe cases, electrofishing injuries in fish heal and seldom result in immediate or delayed mortality. Instead, most electrofishing mortalities appear to result from asphyxiation due to extended tetany or poor handling. However, electrofishing injuries may significantly reduce subsequent growth, at least until they fully heal. When sufficiently severe, spinal injuries may affect physical appearance or swimming ability. Still, even for highly injury-susceptible species, such as the salmonindae, significant effects at the population level are unlikely except in the case of very small or very extensively and intensively sampled populations, as is sometimes the case for threatened and endangered species.

Electrofishing is a valuable tool for fishery management and research, but when resultant injuries to fish are a problem and cannot be adequately reduced, we must abandon or severely limit its use and seek less harmful alternatives. This is our ethical responsibility to the fish, the populace we serve, and ourselves.

Like the subject matter itself, Snyder’s text is complex. But, for experienced practicioners wanting to be more honest with themselves, this report is a great place to start. Can you address the knowledge gaps identified by Snyder as an add-on to your existing sampling?

Download Snyder’s report here.

The top photo shows a dorsal view X-ray of a rainbow trout revealing spinal fracture caused by electrofishing (source: N. G. Sharber).

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