EFFECTS OF CHEMICALS ON A SCHOOLING FISH, KUHLIA SANDVICENSIS

¹ Dept. of Zoology, and Entomology and Dept. of Chemistry, University of Hawaii, Honolulu 14, T. H.

1. The objective of this study was to find, by judicious selection and testing, whether or not certain chemical compounds in exceedingly dilute concentrations would arouse sensations in fish which would alter the sensory bonds between them, thus effecting dispersal of schooling fish.

2. The basic considerations in selecting chemicals capable of dispersing schools of fish were (1) that they be grossly irritating to fish rather than merely toxic or narcotic, and (2) that they be rapid in action, reaching, in our experiments, a maximum effectiveness within a period of two or three minutes.

3. A preliminary classification of the 87 compounds tested included insecticides and insect repellents, substances irritating or toxic to mammals, piscicides and fish repellents, and special irritants. Suitable chemical dispersers were added to the relatively insoluble compounds. Tests were made at dilutions of 20.0, 10.0, 2.0, 1.0, 0.2, 0.1, 0.05 p.p.m.

4. The intensity, nature and rapidity of response to introduced chemicals were recorded by observation and by motion pictures. Although a clear perceptual pattern of the mode and site of action of certain chemicals known to arouse localized sensations in terrestrial animals was not definitely discernible, certain reactions suggested that special areas were intensely stimulated. General skin irritants (phenol, allyl isothiocyanate) strongly stimulated the entire body; lachrymators (phenacyl chloride, phenacyl bromide) seriously irritated the eyes and impaired vision; some general nerve poisons (isobornyl thiocyanoacetate, ethyl mercaptan) induced tetanic paralysis; whereas another (thiocyanic acid 5, 5, 5-trichloro amyl ester) resulted in flaccid paralysis; and respiratory impairment was brought about by a reducing agent (sodium bisulfite) and by probable respiratory poisons (sodium cyanide, thiocyanic acid 5, 5, 5-trichloro amyl ester).

5. Examples of the highly selective nature of the sensations aroused by chemicals on different organisms were clearly demonstrated. High molecular weight thiocyanates which have little or no effect on the common chemical sense of mammals are highly effective irritants for fish. Others (mercaptans, certain amines, some nitriles, skatole), exceedingly irritating to man, appear to have little or no effect on fish. Substances classed as intense skin irritants or as lachrymators (allyl isothiocyanate, phenacyl chloride, phenacyl bromide) for man appear to be equally effective for fish. Underlying reasons for such disparities and similarities in reaction were suggested.

6. Certain characteristics of molecules appear to be significant in predicting success for irritants, and thus contribute toward a framework for a theory establishing relations between chemical constitution and physical properties on the one hand and aggressive action on the other. Increased aggressiveness with sulfur in the molecule was noted, and the bond type, ionic or covalent, apparently exerts a governing effect with irritants as shown by contrast between the very effective covalent thiocyanates and the ineffective ionic thiocyanate salts.

7. A striking proportionate parallelism seems to exist for stimulus-response reactions of all the chemical senses between humans and the fish used for these tests.

8. Results thus far indicate that substances perceived as odors and tastes for humans do not arouse sensations capable of dispersing fish in schools, but that general skin irritants, lachrymators and nerve poisons may be capable of doing so. Types of compounds which now seem most effective for dispersing fish in schools are multi-halogenated organic compounds, organic thiocyanates, organic isothiocyanates and halogenated ketones.