Predicting sustainable population growth by linking age-specific mortality and growth rate (M’/G’) to biomass in a fluctuating environment

Background/Question/Methods

The ratio of age-specific mortality and somatic growth (M’/G’) is believed to be most variable at early life stages. Population biomass tends to decrease when mortality is high relative to growth (M’/G’>1) and increase when mortality is low relative to growth (M’/G’<1). The “transitional age” when M’/G’=1 has become an important indicator of recruitment success in fisheries science. Physical processes (e.g., hydrology) may contribute to biomass loss, but it is unclear how hydrology influences the timing of this important life-history transition. We evaluated age-specific mortality and growth for six marsh fishes along a hydrological gradient. Fish were collected for three years at six hydrologically variable sites for analysis of age-specific growth rates. Individuals were aged using saggital otoliths and the von Bertalanffy, Gompertz, and Logistic growth models were fit to length-at-age data. Akaike information criterion was used to select the best growth model. We estimated age-specific mortality by following 10-day cohorts in a 20-year time series of population density collected at 21 sites in the Everglades. The transitional age was then back-calculated using these relationships, log-transformed, and regressed against annual density and biomass. We then compared the transitional age to hydrological variables generated from monitored water gauges at each site.

Results/Conclusions

The von Bertalanffy model best described growth for each fish species in 75.9% of the samples collected. Analysis of Covariance revealed species-specific growth rates were not consistent among sites and years. The transitional age also differed between Gambusia holbrooki (14.96 ⁺/-0.13 days), Lucania goodei (15.78 ⁺/-0.17 days), Heterandria formosa (8.83 ⁺/-0.12 days), Jordanella floridae (33.23 ⁺/-1.76 days), Fundulus chrysotus (31.47 ⁺/-1.19 days), and Poecilia latipinna (26.25 ⁺/-1.15 days). Annual changes in both biomass and density were inversely correlated with the transitional age for most observations in the time series. This relationship was weakest for J. floridae (Mean R²=0.16⁺/- 0.04) and F. chrysotus (Mean R²=0.21⁺/- 0.04) and strongest for L. goodei (Mean R²=0.43⁺/- 0.05) and H. formosa (Mean R²=0.45⁺/- 0.04). The mean transition age increased as disturbance intensity increased for G. holbrooki, L. goodei, H. formosa; however, no pattern was seen for J. floridae, F. chrysotus, and P. latipinna. This may be caused by life-history differences associated with dispersal among these species. Highly mobile species were less likely to have transitional ages that increase due to disturbance intensity. Overall, evidence suggests that the transitional age (M’/G’=1) is a useful life-history parameter for predicting sustained biomass and density.