APPLICATION OF STABLE-ISOTOPE TECHNIQUES TO TROPHIC STUDIES OF AGE-0 SMALLMOUTH BASS

Naturally occurring stable isotope ratios of carbon (delta(13)C) and n itrogen (delta(15)N) can be used to differentiate pelagic and benthic prey items and to characterize the trophic position of aquatic organis ms. The isotopic signatures of age-0 smallmouth bass Micropterus dolom ieu from six broods in Lake Opeongo, Ontario, were tracked between Jun e 18 and July 24, 1995. Posthatch embryos (4-5 mm in total length) had elevated delta(15)N values (9 parts per thousand) that were attribute d to the parental origin of their nitrogen pool. The delta(15)N decrea sed rapidly, approaching 2 parts per thousand for 15-mm smallmouth bas s; this complete dilution of the parentally derived delta(15)N pool co rresponded with metamorphosis from larvae into juveniles. The dramatic decline in delta(15)N provided an opportunity to model the relative i mportance of somatic growth and tissue turnover in isotopic shifts; ti ssue accumulation (from exogenous feeding) accounted for 86% of the ob served decline in delta(15)N. Nitrogen isotopes indicated a dietary sh ift and an increase in trophic position between 17 and 46 mm. By the f inal sampling date (July 24), body size of age-0 fish ranged from 38 t o 46 mm; a positive relationship between delta(15)N and body size sugg ested that intrapopulation trophic differences may be responsible for the observed variation in body size. The delta(13)C values of premetam orphosis (<15-mm) smallmouth bass (-23.2 parts per thousand to -26.1 p arts per thousand) were generally higher than adult delta(13)C values (-25.0 parts per thousand to -28.4 parts per thousand). The delta(13)C of juvenile smallmouth bass increased with body size (from -24 parts per thousand to -21 parts per thousand), indicating a dietary shift fr om a mix of benthic and pelagic prey towards reliance on benthic food items.