Abstract
Life history theory predicts selection for higher reproductive investment in response to increased mortality among mature individuals. We tested this prediction over the period from 1978 to 2013 for three populations of Atlantic cod ( Gadus morhua) off Newfoundland. These populations were heavily fished for a long period. We considered changes in standardized gonad weight as a proxy for changes in gonadal investment. We accounted for the allometry between gonad and body weight, individual body condition, water temperature, and potential spatial and density-dependent effects. Males display significant temporal trends in gonadal investment in all populations; in agreement with theoretical predictions, these trends show increased gonadal investments during the earlier part of the time series when mortality was high, with the trends leveling off or reversing after the later imposition of fishing moratoria. In contrast, females display patterns that are less consistent and expected; significant trends are detected only when accounting for density-dependent effects, with females in two populations unexpectedly showing a long-term decline in gonadal investment. Our results support the hypothesis that fisheries-induced evolution has occurred in gonadal investment in males, but not in females, and suggest that gonadal investment is more important for male reproductive success than expected in this lekking species.
Highlights
Reproductive investment is jointly determined by several traits, exhibits a strong link with fitness, and is tightly associated with the quantity and quality of offspring
Without any standardization other than by maturity stage, it appears that the metric traditionally used to measure gonadal investment, the GSI, shows high variability among individuals sampled in any given year, as well as between years, without any obvious temporal trends (Fig. 3)
Our results show that gonadal investment in cod off Newfoundland is highly variable, but shows significant long-term patterns
Summary
Reproductive investment is jointly determined by several traits, exhibits a strong link with fitness, and is tightly associated with the quantity and quality of offspring. Changes in mortality reshape the fitness benefits of current and future reproductive investments, and the optimal proportion of resources to be invested into reproduction at a given time. The timing of sexual maturation and the schedule of energy allocation to reproduction are expected to evolve toward precocity and increased reproductive investment for each reproduction opportunity when exploitation reduces life expectancy (Hirshfield and Tinkle 1975; Law and Grey 1989; Rijnsdorp 1993; Festa-Bianchet 2003; Heino et al 2013). Phenotypic changes resulting from the density-dependent availability of resources are regularly put forward as potential explanations for an observed increase in size-specific fecundity (Koslow et al 1995; Stares et al 2007)
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