Data collected by Australian observers, combined with logbook data supplied by
all Japanese vessels fishing in the Australian Fishing Zone were used to estimate
the total seabird catch and catch rates (and associated variances) within the
Australian Fishing Zone by season and area. The species composition of subsamples
of captured seabirds was used in conjunction with the estimates of total
by-catch to provide estimates of by-catch by species.
The total by-catch of all seabird species by Japanese longline within Australian
waters was estimated to be 2,981 (cv 17%) for the 1992 fishing year, 3,590 (cv
15%) in 1993 and 2,817 (cv 19%) in 1994. However, the observed seabird bycatch
may underestimate the total number of seabirds killed by 27%. The increase
in total seabird by-catch between 1992 and 1993 is partly due to the use of
monofilament nylon longline gear by two vessels in 1993, and the decrease from
1993 to 1994 was mainly due to a corresponding reduction in fishing effort.
For the three years examined, 78% of the total seabird by-catch was albatrosses,
with black-browed albatross (Diomedea melanophrys) and shy albatross (D.
cauta) caught in the greatest numbers. Catches of yellow-nosed albatross (D.
chlororhynchos), wandering albatross (D. exulans}, and grey-headed albatross (D.
chrysostoma) were also substantial. The proportions of individual species caught
showed considerable inter-annual variability.
Most seabirds caught and killed by longline fishing are captured during line
setting. Data collected by the observers were also used to determine the influence
of various environmental factors and mitigation measures on seabird catch rates.
Generalised linear models were used to determine the statistical significance of
the effect of each factor on the seabird catch rate. Results show that the
environmental factor that has the most influence is whether line setting was
carried out at night or during the day. For the data examined, the chance of
catching seabirds during day sets was five times greater than for night sets. For
night sets, the chance of catching seabirds during the full half-phase of the moon
was five times greater than during the new half-phase.
Other environmental factors with significant effects were the area and season
fished. Wind, cloud and sea conditions were not found to have a significant
influence. Considerable variation in the seabird by-catch rate among vessels was
found, which is probably due to differences in their implementation of mitigation
measures, as well as the clumped distribution of seabirds by area and time. Interannual
variation in the by-catch rate was found to be statistically significant but
the differences among years was small in comparison to other factors.
An examination of the influence of mitigation measures for sets made during the
day in summer in the Tasmanian area showed that the level of bait thawing and
unidentified factors related to individual vessels were most significant in
determining the seabird by-catch rate, followed by the use of a bait throwing
device. It was not possible to examine the influence of the use of bird scaring tori
poles and lines as these were used during all sets examined in detail. For this data
set, the amount of cloud cover had an influence, while moon phase, sea conditions
and wind strength did not.
| Identifer | oai:union.ndltd.org:ADTP/219101 |
| Date | January 1998 |
| Creators | Klaer, Neil E., n/a |
| Publisher | University of Canberra. Applied Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Neil E. Klaer |
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