Application of Stable Isotope Techniques to Trophic Studies of Age-0 Smallmouth Bass

Abstract

Naturally occurring stable isotope ratios of carbon (d13C) and nitrogen (d15N) can be used to differentiate pelagic and benthic prey items and to characterize the trophic position of aquatic organisms. The isotopic signatures of age-0 smallmouth bass Micropterus dolomieufrom six broods in Lake Opeongo, Ontario, were tracked between June 18 and July 24, 1995. Posthatch embryos (4-5 mm in total length) had elevated d15N values (9‰) that were attributed to the parental origin of their nitrogen pool. The d 15 N decreased rapidly, approaching 2‰ for 15-mm smallmouth bass; this complete dilution of the parentally derived d15N pool corresponded with metamorphosis from larvae into juveniles. The dramatic decline in d 15 N provided an opportunity to model the relative importance of somatic growth and tissue turnover in isotopic shifts; tissue accumulation (from exogenous feeding) accounted for 86% of the observed decline in d 15 N. Ni- trogen isotopes indicated a dietary shift and an increase in trophic position between 17 and 46 mm. By the final sampling date (July 24), body size of age-0 fish ranged from 38 to 46 mm; a positive relationship between d15N and body size suggested that intrapopulation trophic differences may be responsible for the observed variation in body size. The d 13 C values of premetamorphosis (,15-mm) smallmouth bass (223.2‰ to 226.1‰) were generally higher than adult d13C values (225.0‰ to 228.4‰). The d 13 C of juvenile smallmouth bass increased with body size (from 224‰ to 221‰), indicating a dietary shift from a mix of benthic and pelagic prey towards reliance on benthic food items.