An experimental assessment of factors affecting the sink rates of spanish-rig longlines to minimize impacts on seabirds

Abstract

ABSTRACT1. The Spanish system of longline fishing is used in a range of demersal and semi-pelagic 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 increase the sink rate of longlines to reduce interactions with seabirds.2. The experiment examined the effect of setting speed, distance between line weights and the weight of the line weights on the sink rates of hook lines.3. The most powerful effect was line weighting, which affected sink rate independent of setting speed and distance between weights. Distance between weights was also important, but its effect depended on setting speed. Overall, however, the effect of setting speed was minor and the principal determinants of sink rates were the mass of the weights and distance between the weights.4. Based on published accounts of seabird mortality in Spanish system fisheries, 8 kg/30 m is the preferred weighting regime to minimize interactions with seabirds. Sink rates from this regime are the fastest that can be expected from Spanish system gear without overly compromising fishing operations. Used in combination with bird scaring streamer lines and the slower setting speeds the weighting regime has the potential to reduce mortality of albatrosses and deep diving seabird species to very low levels.

Seabirds and demersal longliners in the northwestern Mediterranean: factors driving their interactions and bycatch rates

“…This makes sense, as greater distances and lighter weights slow the longline sink rate (Robertson et al 2007) and therefore increase seabird access to the baited hooks. We found these characteristics in the bottom longliners used by small-scale vessels in the study area.…”

Experimental determinations of factors affecting the sink rates of baited hooks to minimize seabird mortality in pelagic longline fisheries

“…Thus, it is often necessary to precede seabird avoidance experiments by operational, or gear‐related, experiments to identify the most important factors to manipulate experimentally against seabirds. This two‐stage approach was useful with research on the sink rates of gear with the autoline (Robertson et al , 2006) and Spanish methods (Robertson et al , 2008a) of deep water longlining due to the complex gear designs (especially with the Spanish system) and uncertainty about some of the key determinants of sink rate. The approach is equally relevant to pelagic (surface) longline fisheries because of the number of features that could potentially affect sink rates and therefore the frequency of interactions with seabirds.…”

Seabird bycatch mitigation trials in artisanal demersal longliners of the Western Mediterranean

“…Setting speed also influenced sink rates but to a lesser extent [ 57 ]. The seabird access window in the Non-floated longline was significantly shorter than in the Floated 2 (139 m vs. 203 m) and similar to Floated 1, probably caused by its lower setting speed, thus indicating that lowering the setting speed of the vessels can decrease bycatch risk by reducing the seabird access window.…”

“…The seabird access window in the Non-floated longline was significantly shorter than in the Floated 2 (139 m vs. 203 m) and similar to Floated 1, probably caused by its lower setting speed, thus indicating that lowering the setting speed of the vessels can decrease bycatch risk by reducing the seabird access window. However, setting speed also determines the final arrangement of the fishing gear on the seabed; faster speeds allow a tension of the mainline which keeps the longline straight and expanded along the sea bottom [ 57 ]. Vessel speed is sometimes adjusted to obtain an appropriate arrangement of the gear due to the speed and direction of the currents or other operational reasons, and so regulating setting speed to manage bycatch risk would thus be impractical.…”