Dear MARMAM colleagues,

Detecting population decline before it's too late: What dolphin teeth revealed about a hidden crisis

We're excited to share our new study in Conservation Letters demonstrating the first evidence of declining viability in common dolphins (Delphinus delphis) in the Bay of Biscay. Using age-at-death data from 759 stranded specimens (1997-2019), we documented a dramatic decline in female longevity from 24 to 17 years - representing 2-3 fewer breeding events per female and a 2.4% reduction in population growth rate. This occurred despite stable abundance estimates, highlighting the Bay of Biscay as a demographic sink.

The challenge: Making the most of stranding data

Many of us collect teeth from stranded marine mammals and invest significant resources in aging specimens. Yet analyzing these cross-sectional age-at-death data has traditionally required restrictive assumptions (particularly stationarity) or complex models that are difficult to fit and don't easily accommodate covariates or allow temporal trend detection. This often means pooling data across years and potentially missing critical early warning signals of population decline.

Our solution: A flexible, accessible framework

We developed a pragmatic modeling approach using stratified random sampling and a flexible linear regression framework that can:

The complete methodology is published and fully reproducible, with R code available on GitHub. Our Monte Carlo validation study (Ecological Modelling 2024) demonstrates the approach works even with sparse data and model misspecification.

Full text (open access): Rouby et al. 2025. Conservation Letters: https://doi.org/10.1111/conl.13142

Methods paper:
Rouby et al. 2024. Ecological Modelling: https://doi.org/10.1016/j.ecolmodel.2024.110647

Code repository: https://github.com/erouby/dolphin_viability_cons_letter

We hope this framework proves useful for others working with stranding data to assess marine mammal population viability. We welcome questions and feedback!

Best regards,

Etienne Rouby