Size-specific fish avoidance of the spined crustacean Bythotrephes: field support for laboratory predictions

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Abstract
Stomach contents of juvenile fish and zooplankton samples taken simultaneously with fish were analyzed to determine whether young fish in Lake Superior utilize Bythotrephes (cederstroemi form), a spined crustacean present in the Great Lakes since the 1980s. Bythotrephes made up approximately 11% of the zooplankton samples on our sampling date; however, it comprised approximately 0.3% of the fish diet (N = 358 fish, 2.8–23.9 cm in total length). No fish < 7.0 cm in length (N = 208) contained Bythotrephes. According to the Manly–Chesson selectivity index, fish negatively selected or avoided the spined zooplankter. A Spearman rank correlation analysis indicated that although the number of Bythotrephes in the gut significantly increased with fish length, preference did not. These field results support predictions derived from laboratory experiments that (1) Bythotrephes is not a preferred prey item for young fish, (2) there is a threshold size of fish that can utilize Bythotrephes as a food source, and (3) as fish increase in size, they are more likely to consume Bythotrephes.