Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Non-protected bycatch
Fishery discards are an important component of the diet of chicks, but Francis et al. (2008) were not
able to assess whether the associated positive effect on population growth (e.g., from increased
breeding success) is greater or less than the negative effect of fishing-related mortality.
5.4.3.4.2. Quantitative models for black petrel
Francis and Bell (2010) analysed data from the main population of black (Parkinson’s) petrel
(Procellaria parkinsoni), which breeds on Great Barrier Island. Abundance data from transect surveys
were used to infer that the population was probably increasing at a rate between 1.2% and 3.1% per
year. Mark-recapture data were useful in estimating demographic parameters, like survival and
breeding success, but contained little information on population growth rates. Fishery bycatch data
from observers were too sparse and imprecise to be useful in assessing the contribution of fishingrelated
mortality. Francis and Bell (2010) suggested that, because the population was probably
increasing, there was no evidence that fisheries posed a risk to the population at that time. They
cautioned that this did not imply that there was clear evidence that fisheries do not pose a risk.
Subsequent analysis (Bell et al. 2012) included an additional line transect survey in 2009/10 in which
the breeding population was estimated to be ~22% lower than in 2004/05 (the latest available to
Francis and Bell, 2010). Updating the model of Francis and Bell (2010) made little difference to
estimates of demographic parameters such as adult survival, age at first breeding, and juvenile
survival (which had 95% confidence limits of 0.67 and 0.91). The uncertainty in juvenile survival
gave rise to uncertainty in the estimated population trend, with a mean rate of population growth over
the modelling period ranging from ‐2.5% per year (if juvenile survival = 0.67) to +1.6% per year (if
juvenile survival = 0.91, close to the average annual survival rate for older birds) (Figure 5.18). Bell et
al. (2012) concluded that the mean rate of change of the population over the study period had not
exceeded 2% per year, though the direction of change was uncertain.
Figure 5.18 (reproduced from Bell et al. 2012): Likelihood profile for annual probability of juvenile
survival showing: A, the loss of fit (the horizontal dotted line shows a 95% confidence interval for this
parameter); and B, population trajectories corresponding to different values of juvenile survival, together
with population estimates from transect counts (crosses with vertical lines indicating 95% confidence
intervals. Note that the 1988 population estimate was not used in the model.
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