Estimated capture of seabirds in New Zealand trawl and longline fisheries, 2002–03 to 2006–07

Seabirds are caught during commercial fishing, most frequently either by being hooked during longlining, caught in trawl nets, or struck by trawl warps. In order to understand the impact of fishing on seabird species, estimates of the total mortality from fishing activity must be obtained. In New Zealand commercial fisheries, government observers are present on some vessels, and they record any captures of protected species that occur. These observer data provide a consistent basis for estimating total captures. In this report, generalized linear models were used to estimate total captures of seabirds by trawl and longline methods from observer data. Captures were estimated for the fishing years 2002—03 to 2006—07, by fishing within the New Zealand Exclusive Economic Zone (EEZ). For trawl, bottom longline, and surface longline fisheries, statistical models were built of captures of five species groups: white-capped albatross (Thalassarche steadi), sooty shearwater (Puffinus griseus), white-chinned petrel (Procellaria aequinoctialis), other albatross species, and other birds. The models were fitted using Bayesian methods, with the captures represented as samples from a negative binomial distribution.

The total number of seabirds that were estimated to have been caught within New Zealand waters during the 2006—07 fishing year was 3554 (95% c.i.: 2629 to 5270). There were 1328 (95% c.i.: 967 to 2002) estimated seabird captures in offshore trawl fisheries, 1199 (95% c.i. 746 to 2155) captures in surface longline fisheries, and 921 (95% c.i.: 430 to 2330) captures in the bottom longline fisheries for which estimates were made. Due to low observer coverage, trawl fishing targeting inshore fish species, bottom longline fishing from small vessels (< 34 m), was not included in the modelling. Of the five species groups used for the modelling, the other birds and other albatross groups had the highest number of estimated captures during 200—07 (median estimates of 1204 and 876, respectively). In this year there were also median estimated captures of 721 sooty shearwaters, 274 white-capped albatrosses, and 268 white-chinned petrels.

Across all offshore trawl fisheries, there was a significant decrease in the total number of birds caught between 2002—03 and 2006—07, with the total number of captures falling by 49% between the 2002—03 and 2006—07 fishing years. In surface longline fisheries, there was a small non-significant increase in the number of albatross captures between 2002—03 and 2006—07, but the number of petrels caught per year has decreased markedly falling by 69% between 2002—03 and 2006—07. Although the uncertainties were large, in the modelled bottom longline fisheries there was a 33% decrease in the median estimated captures of seabirds over the five year period. All these decreases were associated with decreases in the fishing effort.