Modeling the potential impacts of maternal age on recruitment variability: an exploration of two hypothesized portfolio effects

Background/Question/Methods

Many rockfish species are long-lived and are thought to be susceptible to being overfished. In recent years, hypotheses about the importance of older females to population persistence have led to arguments that marine reserves are needed to ensure the sustainability of rockfish populations. However, the implications of these hypotheses for rockfish population dynamics are still unclear. We modeled two mechanisms by which reducing the proportion of older fish in a population is hypothesized to influence sustainability and explored whether these mechanisms influenced mean population dynamics and recruitment variability. We explored whether populations with these mechanisms could be managed more sustainably with a marine reserve policy in addition to a constant fishing mortality rate (F) than with a constant F alone. Both hypotheses can be seen as portfolio effects whereby risk of recruitment failure is spread over a “portfolio” of maternal ages. First, we modeled an offspring size effect which assumed that older mothers produced larger offspring than younger mothers where the survival rate of offspring was high regardless of maternal age when environmental conditions were favorable, but offspring of older mothers had the highest survival rates when environmental conditions were poor. Secondly, we modeled a spawning site effect whereby mothers of different ages spawned in different times or locations. Thus, offspring of younger mothers experienced different environmental conditions than those of older mothers each year. 

Results/Conclusions

Recruitment variability was 55-65% lower than for control models in the absence of fishing and increased with increases in Fs for both effects.  The reproductive rate specified in a typical age–structured model is the same as that output from the model. However, an offspring size effect caused lower output reproductive rates. Therefore, a standardization was conducted such that the offspring size effect and control models had the same realized reproductive rates. A flaw in previous analyses of offspring size effects is that reproductive rate is not standardized and thus comparisons of models with and without an offspring size effect are comparisons two non-equivalent species with different reproductive rates.

A comparison of long-term catch, the probability of falling below a biomass threshold, and recruitment variability over a range of exploitation rates (catch divided by total biomass) for models with an age–related portfolio effect showed no benefit to a marine reserve implemented in addition to a constant F (as compared to a constant F alone) for populations with sedentary adults and sedentary or mobile larvae.