UK dolphin and porpoise conservation strategy: technical report

The technical report for the proposed UK Dolphin and Porpoise Conservation Strategy, describing the process used to assess the vulnerability of the populations of the nine named species of cetaceans to current pressures in UK waters.

Section 4: Approach to Assessing Vulnerability to Pressures.


19. The vulnerability of a species is a measure of the risk of impact to the UK population of each of the named species from a pressure to which it is sensitive.

The assessment process

20. Each species covered by this strategy has been assessed for vulnerability to pressures in UK waters, on a UK scale. These vulnerability assessments support the identification of priorities and consequent development of actions with a view to conserving UK cetacean populations.

21. The vulnerability of a species to a pressure within UK waters is a combination of its sensitivity and exposure to the pressure.

22. The components of vulnerability are defined as:

1) Sensitivity: the likelihood of change when a pressure is applied to an individual animal in terms of its ability to:

1. tolerate or resist change (resistance) and;

2. recover (resilience).

2) Exposure: the action of a pressure with regards to the area over which the pressure extends, the intensity and duration of the pressure and the proportion of the population experiencing the pressure as indicated by the spatial distribution of individuals within that population.

23. It is only those changes that have consequences for fecundity (reproductive ability) or survival of the individual that are considered in this assessment of sensitivity.

Assessing vulnerability

24. To assess vulnerability to a population, the matrix below uses the combination of sensitivity at the individual level and exposure at the population level to score how vulnerable the species is to each pressure:

Sensitivity of Individuals
Exposure of population L L L L

Assessing sensitivity

25. To assess sensitivity, the potential for mortality or impact on fecundity or survival of an individual when exposed to a pressure was assessed. For example, if an individual of a species were to encounter a pressure, is it likely that the result would be mortality or a direct impact on reproductive capabilities?

26. Sensitivity for a species is graded as below, considering the available evidence on individual responses to pressures:

  • High sensitivity to a pressure: the pressure impacts the individual of a species directly and it would be unable to resist change and could not recover (e.g. direct mortality as a result of bycatch) OR the pressure acts indirectly (e.g. through prey consumption) but the consequence directly impacts survival/fecundity and it would be unable to resist change and could not recover (e.g., bioaccumulation of contaminants).
  • Medium sensitivity to a pressure: the pressure may impact the fecundity/fitness of an individual of a species indirectly e.g. the pressure elicits behavioural/physiological change that may have consequences on fecundity/fitness Individuals may tolerate and/or recover (e.g., noise disturbance).
  • Low sensitivity to a pressure: the pressure does not impact survival or fecundity, directly or indirectly, of the individual of a species. Individuals are resistant to the pressure.

Assessing exposure

27. To assess exposure, the distribution of individuals of each species was compared to the distribution of each pressure at the UK scale. Any spatial overlap was graded High, Medium or Low. These gradings were then adjusted to take account of the persistence and intensity of the pressure at a UK scale. For example, a very wide-ranging pressure with a high overlap with species distribution, but infrequent and of short duration, would be expressed as Medium or Low exposure.

28. This latter stage is largely based on expert opinion of how often the pressure tends to occur and its magnitude/intensity. Where exposure to a pressure is locally or regionally different from the overall UK picture, this is noted in Table 4 and the UK level scorings would not necessarily apply.

29. Exposure is entirely based on an assessment of the pressure and did not consider how the behaviour or ecology of a species may mediate exposure. Such information was not used because there is insufficient evidence to consistently apply such considerations across all species.



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