Male Fitness, Body Size and Timing of Reproduction in Smallmouth Bass, Micropterus Dolomieui

We conducted a mark-recapture study and a survey of nesting male small-mouth bass, Micropterus dolomieui, to determine the relationship between age at first reproduction and fitness among males of several cohorts. Males that spawned at 3 yr of age obtained on average many more eggs than males that delayed reproduction, suggesting that age at reproduction is a conditional strategy. Body size appears to be the trait used to determine the value of the life history decision; small age-3 males were more likely to delay reproduction than large males. The tactic adopted by males may also depend on events that occur well before the time of the life history decision, as males of a cohort that were large at age 3 were also large at age 1. Furthermore, growth over that time period was depensatory. Thus, environmental influences during early ontogeny could potentially direct males into alternative life history pathways. Males that delayed reproduction ultimately reproduced at a larger body size than males that spawned at age 3. Due to size-dependent reproduction within a spawning season, and indeterminate growth, males that delayed reproduction also reproduced relatively early within a spawning season. The progeny of males that spawn at age 3 should consequently experience a relatively short period of growth prior to winter; males that delay reproduction should produce progeny that are large at age 1, whereas the progeny of males that spawn at age 3 should be relatively small. These patterns suggest that the association between timing of reproduction and body size within a spawning season will produce a negative paternal effect for age at reproduction such that the life history alternates in successive generations; males that spawn at age 3 produce progeny that delay reproduction and vice versa. Environmental changes at various stages of the life cycle could alter the proportion of males that adopt each tactic. Our findings suggest that details of early ontogeny and inheritance, in combination with estimates of fitness, may be necessary to understand the evolution of life history pathways in some systems.