Dispersal patterns (SP 7)

Mechanisms controlling porpoise dispersal in the North Sea

Construction at DanTysk wind farm in the German North Sea 2013. Photo: Vattenfall

The aim of the project described here is to quantify the large-scale harbor porpoise movement patterns in the North Sea and to evaluate how shifts between dispersal and foraging are triggered by environmental cues.

The study is an important contribution to the DEPONS project, which aims at reparameterizing an existing individual-based population model to make it suitable for the porpoise population North Sea. The simulation tool will facilitate the evaluation of population-level consequences of various management actions.

This specific study aims:

  1. to obtain more data on large-scale porpoise movement patterns by equipping porpoises with satellite telemetry tags
  2. to investigate the mechanisms that induce dispersal
  3. to incorporate these mechanisms into the simulation framework

A key feature in individual-based models is that they facilitate the prediction of population responses to changed environmental conditions. This is particularly the case if they build on a low-level understanding of the mechanisms that cause animals to change behaviour under different conditions.

As an example, porpoises may be known to forage in a particular area during summer, but disperse in the fall. If dispersal is induced by decreasing prey densities associated with decreasing water temperatures, the model is more likely to retain its predictive power under changed environmental conditions if specifically including the mechanism that induces dispersal than if merely forcing the simulated animals to disperse at particular times of the year.

Further, when aiming to use a model to predict the population consequences of a particular management action, it is important to incorporate stochastic environmental and demographic variations into the predictions. Nabe-Nielsen suggested including stochastic fluctuations in disease-induced mortality into future simulations of porpoise population dynamics, but climate change scenarios may be equally important.