Is the black petrel at risk from commercial fisheries?
Black petrels are relatively few in number and are sometimes caught on fishing lines and in nets. The effect of fishing-related deaths on the population is unknown.
The black petrel, also known as Parkinson’s petrel or Procellaria parkinsoni, is classed as Nationally Vulnerable by the Department of Conservation (DOC) due to its restricted range.
This small seabird breeds only in New Zealand, in burrows at over 400 m above sea level on Little Barrier and Great Barrier Islands in the Hauraki Gulf. Breeding starts in October, eggs are laid in December, and most hatch in early February. The adult birds feed in waters around northern New Zealand, returning to feed their chicks in the burrow until April, when the chicks fledge (become independent). During the winter months, black petrels feed in waters off the North Island west coast and in distant waters around South America .
Annual studies at the main breeding colony provide information about the black petrel population and the distribution of the birds at sea. The population of the main breeding area is thought to number fewer than 3500 breeding birds. The birds actively feed around fishing vessels, and fisheries observers have reported captures of black petrels on longlines and in trawl nets, especially in waters around northern New Zealand .
We do not know the actual number of black petrels that die each year because of fishing. However, there is concern that deaths from fishing may be an added source of mortality that could cause the population to decline.
Fisheries managers need to consider the effects of fishing on all aspects of the environment, so knowledge of the black petrel population trends, and the potential impact on the population of deaths from fisheries, is an important consideration for the management of commercial fisheries.
In this work for the Ministry of Fisheries, Chris Francis of NIWA developed the SeaBird modelling software and applied this to the black petrel data collected largely by seabird researcher Elizabeth Bell of Wildlife Management International to assess the population status of this seabird. Was the petrel population increasing, decreasing, or stable? How old are the birds when they first breed? How many successfully raise a chick?
The main aim of this work was to evaluate the effects of commercial fishing on the main population of black petrels on Great Barrier Island . Since 1995–96, researchers have banded black petrels as part of an ongoing mark-recapture population study designed to provide information about the status of the population.
Using a computer program called SeaBird developed in previous work, Chris Francis used several datasets to estimate the survival rates and breeding success rates for the black petrel population. The main set of data used mark-recapture data collected over 14 years when researchers visited black petrel burrows and recorded the location of each bird found. They also noted whether the bird had been banded in previous trips, was a chick or a breeding bird, and whether it had successfully raised a chick. Black petrels seen during the study visits were banded if they hadn’t been observed and banded in earlier visits. The second dataset provided estimates of population numbers (abundance) based on different studies at varying times over a 40-year period.
What do these data tell us?
- Over 3,300 birds have been banded, and 420 were seen at the study area in 2008–09
- Only 54 of almost 1700 chicks that were banded since the 1995–96 summer have been seen breeding.
- The youngest breeding bird was 4 years old, the oldest about 10
- Of the birds recorded as pre-breeders, 68% later bred in the study area
How do the SeaBird models work?
Our SeaBird software analysed multiple versions of 4 different models that used adult bird mark-recapture data, juvenile bird mark-recapture data, all mark-recapture data, or all mark-recapture data with abundance data to estimate:
- the initial population size
- the proportion of birds that survive each year (survival)
- the number of banded birds seen in subsequent seasons (resighted birds)
- the proportion that change status, for example from being a nonbreeder in one year to a breeder in the next year.
Some questions had to be considered or determined for the models.
- Could band loss be biasing the analyses?
- Were banded birds that were breeding more likely to be resighted than those taking a year off from breeding?
- Were birds that were observed breeding in one season more or less likely to survive to the next season than those that were not breeding?
The main conclusion was that there is no evidence that fisheries currently pose a risk to this black petrel population, but we can’t say there is clear evidence that fisheries don’t pose a risk. Although the abundance estimates and the mark-recapture data suggest that the population has grown by about 1.2–3.1% a year, the evidence for this is not strong.
What else did the models tell us?
- The mean age at breeding is 6.7 years
- New adults spend an average of 1.2 years at the colony as pre-breeders
- Of birds that appeared in the study area as pre-breeders and survived to breed, 68% bred in the study area
- 89% of breeding birds survive each year
- 80% of breeding birds breed each year and of those 77% successfully produce a chick.
What more do we need to know?
Information about juvenile survival and likely population growth or decline could be collected to better monitor the population. More data on at-sea distribution would be useful to show where black petrels and fisheries might share the ocean.
Further information: the SeaBird software was also used to assess the fisheries risks to southern Buller’s albatross and white-capped albatross.