Best practice seabird by-catch mitigation for Spanish-rig longline vessels with emphasis on line weighting regimes

The Spanish system is used in a range of demersal and semi-pelagic longline fisheries throughout the southern hemisphere and has been the source of a large number of seabird fatalities. An experiment was conducted on a chartered Spanish-rig vessel to improve the sink rate of longlines to reduce interactions with seabirds. The benchmark sink rate was that of integrated weight (IW) longlines, as used by autoline vessels, that are effective in reducing the mortality of white-chinned petrels, one of the world’s most difficult seabird species to deter from baited hooks. Results to shallow depth (0-2 m) were given priority because Spanish system gear, while fast to deeper depths (10-20 m), is slow to clear surface waters. The experiment determined the effect of setting speed (6 knots, 8 knots and 10 knots), distance between weights on longlines (30 m, 40 m and 50 m) and mass of the weights (4 kg, 6 kg and 8 kg) on sink time profiles of longlines. The variable was the sink time to six target depths from 1-20 m as determined by time-depth recorders attached to lines. Separately, a trial was conducted to determine differences in sink rates between traditional Spanish system line weights (netting bags of rocks) and elliptically-shaped steel weights. In the vessel charter experiment there was a statistically significant interaction between setting speed and distance between weights to ? 10 m depth. Overall, distance between weights and mass of the weights were the principal determinants of sink times to target depths. Longlines with weights 30 m apart and either 6 kg or 8 kg traditional weighs, or 40 m and 8 kg weights, most closely approximated the sink profile of IW-autoline in the shallow depth ranges irrespective of setting speed. In the weight-comparison trial 4 kg steel weights and 8 kg traditional weights sank at comparable rates and are considered to be interchangeable. Best practice seabird bycatch mitigation for line setting operations would involve a) setting longlines with weights 30 m apart (minimizes lofting in propeller turbulence), b) use of 5-6 kg elliptical steel weights instead of traditional weights, c) limiting setting speed to the 6-8 knots range, d) lining bait compartments of setting baskets and stern setting surfaces of vessels with marine grade stainless steel (to reduce incidences of hook-ups and line tension astern), e) releasing line weights before line tension occurs (eliminates a source line tension astern), f) use of dual, hydraulically-driven bird scaring streamer lines (to reduce crew work load and improve compliance) to CCAMLR standards and with 50-60 m aerial extent, g) attaching streamer lines to vessels ?5 m either side of the position the hook line leaves the vessel and use of a ‘lazy’ line to increase effectiveness in cross winds and to enable streamer lines to be positioned according to the weather conditions each time longlines are set. Strict compliance to the recommendations above has the potential to eliminate albatross mortality and reduce mortality of deep diving seabird species to very low levels.