Cause – effect chain modelling of sand mining using Sandwich terns

Monitoring recommendations

For the prediction of effects of specific dredging projects on tern populations, monitoring the following parameters in advance is recommended.

  • The number of fishes that can be caught by terns per day. If terns are able to catch a large number of fishes easily, the amount of available time for foraging is not limiting. On the basis of observations of prey capture rates, it would be possible to estimate a probability density function for the prey capture rate at optimal conditions. If this information is available, using conservative assumptions on this variable is not necessary anymore. Using measurements, instead of these conservative assumptions, leads to a more realistic and probably less pessimistic estimate of the impact on terns (see van Kruchten & van der Hammen (2011), Chapter 4, results Run2a and 5a).
  • The background turbidity at the foraging location (SPM- and chlorophyll-a concentrations). The background turbidity has a large influence on the expected impact from dredging activities.
  • In addition, it is relevant to know whether large spatial differences exist in turbidity levels within the foraging area (see van Kruchten & van der Hammen (2011), section 4.2).

If the probability of occurrence of significant effects, predicted in advance, is not negligible, monitoring capture rates and turbidity during the dredging process is recommended. By substituting the uncertain input variables in the model with the measured values, the prediction of the ecological impact can be adjusted. If the probability on significant effects increases to an undesirable value, adaptation of the dredging process will be desirable. Preferably the additional SPM-concentration, caused by the dredging process, is hindcasted during the dredging process. This makes it possible to distinguish the effects of changes in the background turbidity (reference situation) from the effects that are caused by the dredging activity.

Regarding the adaptation of the dredging process, attention should be paid to the effectiveness of the possible measures. As tern populations are expected to be more affected by long term, far field effects of the dredging activities than by short term effects close to the dredging site, a certain time lag will exist between the moment of taking measures and the effect of these measures in the foraging area. It is recommended to do research on this time lag, as information on this time lag is highly relevant for the development of an effective, adaptive monitoring strategy. In case of the tern population: measures that are taken on the basis of today’s monitoring results should have a positive effect in the foraging area before the breeding period ends.

Information on the development of the tern population is also valuable. If information on the population size and composition is available, it can be used to validate the population dynamical model. In addition, in the assessment of the predicted impact, it may be relevant to know whether the population shows an autonomous increase or decrease.