Assessment of the risk to seabird populations from New Zealand commercial fisheries

We examined the risk of incidental mortality from commercial fishing for 64 seabird species in New Zealand trawl and longline fisheries. For each species, the risk was assessed by comparing the total number of birds potentially killed while fishing against the Potential Biological Removal (PBR) index. This index represents the amount of human-induced mortality a species can sustain without compromising its persistence. The PBR was calculated from the best available information on the species’ demography.

Because estimates of seabirds’ demographic parameters and of fisheries related mortality are imprecise, the uncertainty around the demographic and mortality estimates was explicitly considered. This allowed uncertainty in the resulting risk to be calculated, and also allowed the identification of parameters where improved precision would reduce overly large uncertainties. However, not all sources of uncertainty could be included, and the results are best used as a guide in the setting of research and management priorities. In general, both seabird demographic information and the distribution of seabirds within New Zealand waters were poorly known.

Amongst the 64 studied species, the black petrel (Procellaria parkinsoni) clearly stood out as the species the most at risk from commercial fishing activities within the New Zealand Exclusive Economic Zone. With an average number of potential annual fishing-related fatalities estimated to be almost 10 times higher than the PBR, our study suggests that this species should become the primary subject of more detailed research and management.

Seven other species had a number of annual potential fatalities significantly exceeding the PBR, as the 95% confidence interval of their risk ratio was strictly above one: the grey-headed albatross, the Chatham albatross, the Westland petrel, the light-mantled albatross, the Salvin’s albatross, the fleshfooted shearwater, and the Stewart Island shag. For a further 12 species, the confidence interval of the risk ratio included one.

Small inshore fisheries, especially trawl fisheries targeting flatfish, and small bottom and surface fisheries, appeared to be associated with the greatest level of risk to species. This was due to a combination of low observer coverage, high effort, and overlap with the distributions of many seabird species in these fisheries. In fisheries where there were few observations, the number of potential fatalities was estimated in a precautionary way, with the estimates being biased toward the high end of the range of values that were consistent with the observer data. In these poorly observed fisheries, the risk is primarily associated with the lack of information. Of the species that had a risk ratio greater than one, the risk for four of them (grey-headed albatross, Westland petrel, Chatham albatross, and light-mantled albatross) was associated with a lack of observer coverage in inshore fisheries that overlap with the distribution of these birds. Increasing the number of observations in inshore trawl and small vessel longline fisheries, especially in FMAs 1, 2, 3, and 7, would increase the precision of the estimated fatalities. The risk was estimated independently for each fishery, and there was no assumption that the vulnerability of seabirds to capture was related between different fisheries. This has the consequence that birds (such as light-mantled sooty albatross) may be caught infrequently in well observed fisheries, but still have high risk associated with poorly observed fisheries.

Many limitations were identified in the risk assessment. These may result in biased estimates (either too high or too low) of the risk of fishing to some seabirds. Moreover, some fisheries were not included in our analysis, and other sources of human-induced mortality were ignored. The conclusions of our results should therefore be interpreted with caution, as some species might be at risk, even if their risk ratio was estimated to be lower than one. Conversely, the fisheries-related fatalities may be overestimated in poorly observed fisheries. The risk assessment method assumed a high number of captures in the absence of observations to the contrary, so the estimated potential fatalities in poorly-observed fisheries may be higher than the actual fatalities.

Note that sections C and D mentioned in this report are part of the adjoining document providing supplementary materials (Richard et al. 2011).