Abstract

In aquariums, fish may be exposed to low levels of electricity originating from electrical problems such as improper grounding, breakdown of watertight seals allowing water to contact wiring, and electrical equipment imposing an electromagnetic field on nearby tanks and their inhabitants. Exposure to excessive ambient electrical fields has been hypothesized as one contributor to generalized ill thrift, and to head and lateral line disease. This study was designed to quantify physiologic changes associated with exposure to voltage levels similar to those present in tanks with poor or marginal electrical conditions. Changes in specific analyses were interpreted relative to analyze levels in unexercised controls and in conspecifics following 35 min of forced exercise.

Teleosts are known to react with a host of biochemical changes to various stressors such as physical, environmental, and seasonal changes.¹ Their primary response is an acute rise in ACTH, catecholamines, and corticosteroids followed by secondary changes in hematological and biochemical parameters such as hematocrit, glucose, chloride, sodium, and cholesterol.² Within minutes of an acute stressful experience, plasma cortisol, the main stress hormone, may increase 10- to 100-fold from its baseline of ~0.5 µg/dl. Levels return to baseline within one to three days, however, cortisol may remain elevated with chronic stress.^3,4

The stress response to high levels of electricity utilized in electro fishing has been extensively studied. Electro fishing utilizes voltages ranging from 100-1100 V and currents up to 60 Amps. After electro fishing shock, cortisol increases two to five fold within minutes, remains elevated for several hours, and returns to baseline in eight to forty-eight hours.⁵

Summer flounder were exposed for 35 min to a 1.9 V, .3 Amp electrical field, much less than that used in electroshock fishing but greater than background bioelectric fields generated by animals and their movements. Blood samples were collected prior to the initiation of the stressor, three times during the stress, and then three times during the 48 h following the application of the stressor.

Plasma cortisol, glucose, and sodium concentrations began to rise within five minutes in both exercised fish and fish exposed to electricity, but were unchanged in unstressed controls. Resting levels of these analyses were restored within twenty-four hours. Cortisol in exercised fish nearly tripled, whereas electricity-exposed fish showed a 13.5-fold elevation. Both groups showed a comparable rise in glucose, ~two-fold, as a consequence of increased cortisol. Sodium increased 6.4% in exercised fish but only 2.4% in the fish exposed to electricity, reflecting changes in gill water and ion permeability in combination with increased ventilation in the exercised fish.

These results suggest that brief exposure to even low levels of electricity is associated with demonstrable physiological changes comparable in some respects to those observed in forced exercise. Because the voltage levels used in this study are below those that are palpably detected, these electrical conditions could go unrecognized unless specifically tested for using a voltmeter. Long-term exposure would be expected to result in sustained elevations in cortisol with adrenal exhaustion, and potential sequelae such as immunosuppression, disrupted endocrine function, impaired reproduction, and poor growth. However, additional study would be needed to document such long-term effects.

Acknowledgements

This study was reviewed and approved by Mystic Aquarium's Institutional Animal Care and Use Committee. We thank Greg Charbeneau and the Fish and Invertebrate Husbandry staff for the care of the study animals obtained from Millstone Nuclear Power Station of Waterford, CT.

Thanks to Bob Carr from Engineering and Maintenance for building the electrical apparatus. Finally, we greatly appreciate all of Gayle Sirpenski's assistance in shipping, editing, photo-documentation, and other general support.

References

1.  Wendelaar Bonga, S.E. 1997. The stress response in fish. Physiological Review 77(3):591-625.

2.  Groff, J.M. and J.G. Zinkl. 1999. Hematology and Clinical Chemistry of Cyprinid Fish. Veterinary Clinics of North America Exotic Animal Practice. WB Saunders Company, Philadelphia, Pp 741-776.

3.  Kakizawa, S., T. Kaneko, S. Hasegawa, and T. Hirano. 1995. Effects of feeding, fasting, background adaptation, acute stress, and exhaustive exercise on the plasma somatolactin concentration in rainbow trout. General and Comparative Endocrinology 98(2)'137-46.

4.  Pickering, A. 1981. Stress and Fish. Academic Press, New York, Pp 367.

5.  Maule, A.G. and M.G. Mesa. 1994. Efficacy of electro fishing to assess plasma cortisol concentration in juvenile chinook salmon passing hydroelectric dams on the Columbia River. North American Journal of Fisheries Management, 14(2):334-339.