Do harbour porpoise mortality records reflect living population structure? A matrix population model diagnostic

Michaela Kirstine Hjelm Hansen, Magnus Wahlberg, Owen Russell Jones

Effective conservation of marine mammals depends on reliable demographic information, yet acquiring such data for highly mobile cetaceans is challenging. Harbour porpoises (Phocoena phocoena) are widely used as sentinel species, but much of what is known about their demography comes from opportunistic sources, such as stranding and bycatch records. While invaluable, these data may be subject to selective demographic filtering due to drift dynamics, detection probability, reporting effort, and age-specific vulnerability.
We developed a matrix population model (MPM)-based diagnostic framework to quantify how observed stage compositions in mortality datasets deviate from asymptotic demographic expectations. Using a stage-structured MPM parameterised from published vital rates, we derived the stable stage distribution (SSD) and compared it with age-class distributions from 10,863 classified harbour porpoise strandings (of 16,181 total records) in Danish and North Sea waters (1990–2017), bycatch and mixed-mortality records from four independent source datasets, and hunting captures from Greenland (1988–1989, 1995) and Denmark (1941–1944). Deviations from SSD were assessed using goodness-of-fit tests, distributional distance metrics (Keyfitz’s Δ and Hellinger distance), and tests for juvenile over-representation.
Strandings showed strong and consistent departures from SSD expectations, with juveniles markedly over-represented across spatial and temporal scales. These patterns were robust to sensitivity analyses that addressed sample-size thresholds, missing age classes, and uncertainty in SSD estimates. Distance metrics indicated moderate to strong divergence from the asymptotic stage structure, with pronounced spatial heterogeneity and significant positive temporal trends in 3 of 6 regions. Bycatch, mixed-mortality, and hunting captures showed comparable SSD divergence, indicating that juvenile-heavy mortality composition is not an artefact of stranding sampling alone.
Our results show that harbour porpoise stranding, bycatch, and even hunting data should not be assumed to be demographically representative of living populations. Researchers and managers relying on stranding or bycatch data for demographic inference should treat stage composition as a selective, biased signal rather than a population-representative sample.