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The Influence of Light and Other Factors on the Seaward Migration of the Silver Eel (Anguilla anguilla L.)

Rosemary H. Lowe
Journal of Animal Ecology
Vol. 21, No. 2 (Nov., 1952), pp. 275-309
DOI: 10.2307/1963
Stable URL: http://www.jstor.org/stable/1963
Page Count: 35
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Influence of Light and Other Factors on the Seaward Migration of the Silver Eel (Anguilla anguilla L.)
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Abstract

1. Observations on the conditions, particularly floods and the phase of the moon, which affect the seaward migration of silver eels were collected for three years from six field sites in the English Lake District. 2. These observations confirmed that silver eel runs occur only between late summer and early winter. The higher upstream the eels the earlier in the season they are trapped. 3. The migration was confined to very few nights, on which large numbers of silver eels travelled seawards; eels ran on the same nights at widely separated sites. 4. Most eels migrated when floods coincided with moonless periods. Few eels ran when there was a full moon; the run could be checked experimentally by artificial light. The start of the eel season was delayed until there was a flood, after which some eels ran on dark nights when there was no flood. The greatest catches were made on rising floods. Weather conditions were not as important as floods in determining whether eel runs occurred. 5. Eels tended to run in the early hours of the night except on a rising flood. Evidence whether eels congregate near a lake outflow before migrating downstream was inconclusive. 6. Attempts to induce runs of eels and to re-trap marked eels were not very successful, but both contributed evidence that silver eels have to be stimulated into a `migration mood', a state of extreme activity, before they will migrate. This mood appeared to be related to conditions of flood, possibly fall in temperature and darkness of the night. Eels kept in tanks in the laboratory were particularly active the same nights that eels were migrating in the field. 7. Experiments in the field and laboratory have shown that the downstream migration of silver eels may be (a) checked, and (b) deflected by lights. At Cunsey Beck a series of underwater lights stopped the run of eels for half-hour periods. For the deflexion of eels into traps at the side of a river underwater lights must be thrown well upstream; lights shining vertically downward into the water are less effective. Upstream-shining lights of about 5 and 0.7 c.p. were effective in Cunsey Beck; evidence whether brighter lights would deflect more eels is conflicting. The extreme activity of the silver eel associated with migration appears to lower the threshold of response to light, artificial lights having most effect on the nights when the eels were most active. There is no reason to believe that the response to light differs according to the sex of the eel. 8. In an artificial river in the laboratory the downstream swim was delayed by light and eels chose a dark channel in preference to a lit one; there was evidence from laboratory experiments that once the shock effect of the light wore off the eels responded photokinetically instead of negatively phototactically. 9. In the artificial river the eels swam upstream against a flow of 1.1 ft./sec., but tired rapidly against a current of 1.7 ft./sec. 10. It was concluded that flood, or some associated factor, possibly fall in water temperature, acts as a trigger stimulus starting the downstream migration. Eels then run continuously, except when inhibited by light during the day and on nights with bright moonlight or starlight, until the effect of the activating stimulus wears off. Most eels will probably need restimulating several times before they reach the sea.

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