for more information:
Brothers, N., Duckworth, A. R., Safina, C., & Gilman, E. L. (2010). Seabird Bycatch in Pelagic Longline Fisheries Is Grossly Underestimated when Using Only Haul Data PLoS ONE, 5 (8) : 10.1371/journal.pone.0012491
Huge numbers of critters are unintentionally trapped in deep sea fishery longlines. Turtles, dolphins, birds, fish, and many other species are routinely caught in lines up to one hundred kilometers long possessing thousands of hooks.
Sometimes a species is caught in such large numbers that longlines are contributing to their risk of extinction, e.g. in the case of loggerhead sea turtles and Amsterdam albatrosses. Twenty-six of the 61 bird species threatened by long lines are at risk of extinction.
Sinking weights attached to the lines, night deployment, and other efforts have been put into place to avoid catching birds. Nevertheless, hundreds of thousands of birds are caught in longlines every year.
Accurately determining the number of birds caught up in deep sea longlines is very important to determine their global threat. The current method of counting bycatch, i.e. after the longline has been reeled in rather than during longline deployment, is insufficient, and has been repeatedly challenged for two decades.
Eric Gilman (Hawaii Pacific University, United States) and coworkers have estimated how many birds are caught by deep sea longlines, via directly counting birds caught during longline deployment. They estimate that birds are caught at twice the numbers as previously thought.
Counting seabird bycatch.
This study reports the scientists' seabird bycatch efforts from 1988 to 2003 (15 years). Their study has been focused near Tasmania (Australia), the state of Western Australia, the north-central Pacific Ocean (midway between Hawaii and Japan), and the Coral Sea (Australia).
They collected data from 11 longlines (305 sets), possessing a total of over 780,000 baited hooks, deployed over deep water and far from land. Eighty-four percent of these observations were made during the day, typically for at least 30 seconds after deployment (the lines are generally underwater by this point), but binoculars were used for longer monitoring when lightweight longlines were deployed.
The scientists also recorded the time and distance from the ship when a bird interacted with the longline (floats affixed to the line of known distance apart were reliable indicators of distance). This enabled them to estimate when any caught birds would be hauled in to the ship, if they remained attached to the line, and thereby estimate the accuracy of counting bycatch only after the line has been hauled in.
Every hook and the complete haul was recorded whenever possible, but at times it was only possible to record part of the haul (the majority or the entirety of the line was always recorded). Ninety-three percent of the lines were observed being set, and 67% were observed being hauled in.
The scientists noted seabirds which capture the bait without being caught, fail to capture the bait but do not get caught, are caught, are possibly caught, or are of unknown fate. Seabirds which look for underwater bait were not included in the analyses.
Further noted was the mechanism of seabird capture: entanged yet not hooked, hooked in the throat or bill, or hooked in some other location. Evidence of bite marks (e.g. from sharks) was also noted (a partial indicator of possible predator success at removing seabird carcasses).
Importantly, most of this data was made of longlines intended to minimize seabird bycatch. This study should also be considered a test of seabird bycatch mitigation effort efficacy.
Seabird bycatch is vastly understated.
The scientists observed more than 25 seabird species caught in deep sea longlines (over 43,000 individuals). Approximately one-third of the seabirds (quite variable depending on the species) which attempted to take the bait were successful.
Nearly 80% of hooked seabirds were caught through the throat or bill. Five of them (i.e. very few) showed bite marks, indicating that if predators attempt to remove the birds, they generally remove the entire carcass.
Three percent of the seabirds, 176 individuals, were observed being caught (again, the percentage was highly variable among species). Importantly, only 85 of the carcasses were ultimately retrieved.
This means that if seabird bycatch were to be measured only after longline retrieval, it would be underestimated by approximately 50%. Underestimation was within the 45%-55% range for three of the four measured ocean regions, but was 25% for the Coral Sea.
Of the 553 birds recorded as possibly caught, 42 carcassess were retrieved, suggesting that bycatch underestimation is even greater than the 50% estimate. Of the 5,367 seabirds recorded as either successful or unsuccessful (i.e. not caught), only 5 carcasses were retrieved, indicating a low observation error rate.
Implications.
Deep sea longlines clearly catch far more seabirds than is commonly thought (twice as many). Possible reasons for the discrepancy include seabird dislodgement from the line and seabird removal by predators.
Deep sea longlines are a much greater threat to seabirds than is commonly realized. Governmental and nongovernmental agencies should take note of this discovery when drafting new laws regulating the "acceptable" bycatch of deep sea longline fisheries.
NOTE: The scientists' research was funded by the Blue Ocean Institute via the Gordan and Betty Moore Foundation.
for more information:
Brothers, N., Duckworth, A. R., Safina, C., & Gilman, E. L. (2010). Seabird Bycatch in Pelagic Longline Fisheries Is Grossly
Underestimated when Using Only Haul Data PLoS ONE, 5 (8) : 10.1371/journal.pone.0012491