SPINED ZOOPLANKION CAUSES FEEDING
AVERSION IN JUvENILE STEELHEAD

Vincent A. Balcer


Department of Biology and Chemistry
Lake Superior State University
Sault Ste. Marie, MI 49783, USA



Abstract. The exotic zooplankton, Bythotrephes cederstroemi was first detected in the
Great Lakes in 1984. Bythotrephes can rarely be found in the stomachs of juvenile fish
less than 5 cm. in length. The spine of the Bythotrephes seems to irritate most juvenile
fish, which tend to experience difficulty swallowing them. Feeding trails were performed
to observe the response of the juvenile steelhead feeding on Bythotrephes over time.
Eight naive juvenile steelhead were videotaped to record their change in response to
Bythotrephes for each trail. The steelhead captured and ingested Bythotrephes with some
difficulty, but with experience the steelhead were more likely to reject a Bythotrephes they
had captured. The goal of the study was to determine if the fish recognized the
Bythotrephes before attacking or only determined it was undesirable during capture.
Changes in behavior prior to capture where then monitored over time to observe any
changes associated with Bythotrephes experience. The pursuit distance and pursuit time
were then measured to determine if the steelhead had learned to identify the Bythotrephes.
Using a linear regression, it was determined that the pursuit distance and pursuit time did
decrease with experience to Bythotrephes (p-value=.0249). The observed change in
behavior prior to a capture of the Bythotrephes indicates they recognize the Bythotrephes
visually and modify their attack strategy.




SELECTED REFERENCES

Barnhisel, DR. 1991. Zooplankton spine induces aversion in small fish predators.
	Oecologia 88: 444-450.

Evans, B.I., and W.J. O'Brien. 1988. A reevalution ofthe search cycle of planktivorous
arctic grayling, Thymallus arcticus Can. J. Fish. Aquat. Sci. 45: 187-192.