INDIVIDUAL VARIATION IN THE TIMING OF ONTOGENETIC NICHE SHIFTS IN LARGEMOUTH BASS

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The transition to piscivory is a crucial ontogenetic niche shift for many primarily piscivorous fishes. An early transition to piscivory may increase growth, decrease mortality, and therefore enhance lifetime fitness. Although much is known about the extent and causes of variation in the timing of the shift to piscivory among species and among cohorts within a species, little is known about the extent and causes of variation in the timing of the switch to piscivory among individuals within a single cohort. Here, I link otolith age and growth analysis to direct diet and stable isotope analyses to examine variation in the timing of the transition to piscivory and its causes among individual members of the 1994 largemouth bass cohort in Paul Lake, Michigan. Stable isotope and direct diet analyses indicate that only a few members of the 1994 cohort were able to shift to and sustain piscivory in their first summer of life (early piscivores), while most cohort members would have to wait until their second summer of life to become piscivorous (late piscivores). Significant differences in growth rate between early and late piscivores emerged shortly after 18 June, the first date of possible piscivory by early piscivores, after which early piscivores began to grow at rates nearly twice that of late piscivores. Otolith and stable isotope analyses combined indicate that an early hatching date was necessary, but not sufficient, to explain variation in the timing of the transition to piscivory. All early piscivores were hatched early in the summer, but many early-hatched members of the 1994 cohort did not shift to piscivory in their first summer of life. A combination of at least 10 days of variation in hatching dates and higher-than-average growth rates was required for early piscivores to switch to and sustain piscivory in their first summer of life. Individuals that were able to make the early transition to piscivory most likely benefited from both increased survival and fecundity over much of their life, the combination of which would confer a substantial fitness advantage upon individuals able to make the early transition to piscivory.