Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Protected species: Sea lions
The BFG model is sensitive to several key parameters. Some relate mostly to uncertainty about the
productivity of the NZ sea lion population (including maximum population growth rate, abundance
relative to carrying capacity, maximum rate of pup production, and density dependence), whereas
others relate to how the fishery works and is managed (including strike rates and the survival of NZ
sea lions that interact with SLEDs but are not retained in the net). Conclusions drawn from the BFG
model results are sensitive to prior assumptions about how fast this NZ sea lion population is able to
grow. The maximum population growth rate (lambda, λ) for this population of NZ sea lions is not
known. Fitting the model to the observed data with an uninformative prior led to an estimated
maximum rate of less than 1% per year, potentially as a consequence of attempting to estimate λ for a
declining population. This is a very low maximum growth rate for a pinniped (some suggest a default
value of 12% per year, Wade 1998), so a prior of 8% was applied to the base model. In a sensitivity
run, the model was fitted using a prior of 5% per year, and the results were more consistent with the
observed data than when 8% was used.
The estimated abundance of NZ sea lions relative to the carrying capacity of mature individuals at the
Auckland Islands (K) is another source of uncertainty. When the model is run in the absence of
fishing, the median numbers of mature animals after 100 years was only 94.4% of K as estimated
from the model. Although the population is not presently near K, over this timescale, the population
would normally be expected to approach K. This is thought to be an artefact of the parameterisation of
survival rates in the model, which renders the model conservative when assessing performance
against K (Breen et al. 2010).
The density dependent response for this population of NZ sea lions is largely unknown, although there
is presently no evidence of a density dependent response in life-history traits such as pup mass, pup
survival or female fecundity (Chilvers 2012b). Ecological principles suggest that, as numbers in a
population decline, individuals compete less with one another for resources. Less competition may
result in NZ sea lions growing faster as well as having lower mortality rates and higher rates of pup
production and survival. The effect of this type of response is that populations tend to recover from
events that reduce their numbers, and populations with strong density dependence recover more
strongly than those with weak density dependence. In the BFG model, the shape of the density
dependent response was “hard wired” in the model and assumed to occur entirely in the mortality rate
of pups. The strength of this response is unknown, and there was no information to support a strong
preference for any of the assumed values used in sensitivity runs. This means the base model results
may be either conservative or optimistic.
The maximum rate of pup production for this population is not known but can be estimated in the
population model. Other modelling conducted for DOC (albeit using different assumptions, Breen et
al. 2010) suggests that the maximum rate of pup production is