Retention of<i>Bythotrephes</i>Spines in the Stomachs of Eastern Lake Erie Rainbow Smelt

Abstract

The exotic cladoceran Bythotrephes longimanus is an important food resource for eastern Lake Erie rainbow smelt Osmerus mordax. Bythotrephes possesses a long, chitinous caudal spine that is generally used to quantify its presence in fish diet studies despite concerns that it may be retained within fish stomachs longer than other prey items. We investigated spine retention in rainbow smelt collected in the eastern basin of Lake Erie in June (day and night), July (night), and October (night) of 1998 and 1999. To assess differential retention of Bythotrephes spines in rainbow smelt stomachs, we compared counts of Bythotrephes caudal spines to direct counts of Bythotrephes bodies or mandible pairs. Retention of caudal spines was evident in yearling and older (age-1+) rainbow smelt caught during the day in June as well as in age-1+ fish caught at night in July and October and in young-of-year fish captured at night in October. Smelt stomach fullness decreased with time after sunset, indicating lower nighttime feeding rates. Spine retention was positively correlated with sample collection time and negatively correlated with stomach fullness, verifying that most prey items, including Bythotrephes bodies, are evacuated at a faster rate than caudal spines. Although spines are likely evacuated within 24 h, October age-1+ rainbow smelt may have contained spines from earlier feedings when caught at sunset. The use of caudal spines to quantify the relative importance of Bythotrephes in fish diets can significantly overestimate predation rates on this species and is an unreliable metric.