Maternal age effects on Atlantic cod recruitment and implications for future population trajectories

Abstract

Exploited fish populations frequently exhibit truncated age-structure. To address a basic question in fisheries science and conservation biology—how does age truncation affect population dynamics and productivity?—we explored the effect of age-structure on recruitment dynamics of ten stocks of Atlantic cod (Gadus morhua). Based on six alternative stock–recruitment relationships, we compared models that included and excluded maternal age-structure effects on recruitment. In all ten stocks, a recruitment model that included a maternal age-dependent effect was preferred over the standard Ricker model and in seven of the ten stocks, the preferred statistical model included a positive effect of either maternal age or mass on recruitment. Simulations comparing standard and maternal age dependent recruitment two decades into the future suggest that the inclusion of maternal age in recruitment models has little effect on projected biomasses. However, this similarity in biomass trajectory masked an increased sensitivity of populations with maternal age-dependent recruitment to stock age-structure. In particular, simulations with maternal age-dependent recruitment responded strongly to changes in fishing mortality on the oldest age classes, while simulations using standard recruitment models did not. Populations with maternal age-dependent recruitment can exhibit increased biomass catch even if fishing mortality on older individuals was reduced. Overall, simulations suggested that the influence of maternal age on population dynamics are more nuanced than suggested by previous research and indicate that careful consideration of the effects of age-structure on populations may lead to substantially different fisheries management reference points—particularly with respect to age-specific fishing mortality—than classical models. While these results suggest a link between maternal age and population productivity, future research requires the incorporation of biologically reasonable and empirically defensible mechanisms to clarify the effect of age on population dynamics.

The use of spatial management tools in rights‐based groundfish fisheries

“…A potential benefit of protected areas for fisheries is recovery of a more complete natural age structure (Berkeley et al., ), although this was only identified as an explicit objective in Alaska (Table ). The disproportionate reproductive value of big old fat fecund female fish, or ‘BOFFFFs’, may play an important role in maintaining stock productivity and stability (Berkeley et al., ; De Leo & Micheli, ; Hixon et al., ; Hsieh et al., ), although the mechanisms behind the effect remain a topic of debate (Shelton, Munch, Keith, & Mangel, ; Shelton et al, ). Older fish often produce more eggs per unit of body weight and higher quality eggs, and spawn more often over the course of the year.…”

Ten principles from evolutionary ecology essential for effective marine conservation

“…Egg quality differences due to population age and size structure do not necessarily contribute meaningfully to long‐term population dynamics (Shelton et al. , ; Le Bris et al. ).…”

“…; Shelton et al. ), and whether spatial closures or fisheries management is the most effective tool of conservation biologists (Edgar et al. ; MacNeil et al.…”

Trends and management implications of human‐influenced life‐history changes in marine ectotherms

“…For example, based on evidence that age truncation will lead to increased recruitment variability (Secor 2007), precautionary management might seek to increase resilience by increasing the number of age classes contributing to reproductive output. Notably, when a positive effect of maternal age is taken into account, reducing harvest rates on older, larger individuals may actually increase catch compared to the status quo, as productivity increases and more young fish offset the foregone harvest on older fish (Shelton et al 2015). Adaptive management would mean that management strategies would be flexible and could respond rapidly as new information is received (e.g.…”