Line weights of constant mass (and sink rates) for Spanish-system Patagonian toothfish longline vessels
CCAMLR Conservation Measure 25-02 requires Spanish-system longline vessels to attach 8.5 kg weights at 40 m intervals on longlines to minimise interactions with seabirds. The weights typically used are collections of rocks enclosed in netting bags. During fishing operations the netting bags abrade on the seabed causing rocks to be lost and the weights to become progressively lighter, requiring ongoing repair. This problem can be solved by use of hydro-dynamically shaped (e.g. torpedo-shaped) steel weights, which are smaller for equivalent weight, and require no maintenance. An experiment was conducted on a Spanish-system longline vessel to determine the relationship between the sink rates of longlines equipped with bags of rocks (4, 6 and 8 kg) and those with steel weights of equivalent masses. The purpose of the experiment was to provide vessel operators with the option of substituting steel weights for rock weights while remaining in compliance with the sink rates associated with the line-weighting provisions of Conservation Measure 25-02. Both the Spanish system and the newly developed Chilean longline (a modified version of the former method to avoid fish loss by toothed whales) were tested in the experiment. Spanish-system longlines with 8 kg rock weights every 40 m averaged 0.22 m s-1 to 2 m depth, which would be equal to, or exceeded by, lines with 5 kg steel weights. Sink rates of Chilean longlines greatly exceeded those of the Spanish system, ranging from 0.68 m s-1 (4 kg rocks) to 1.31 m s-1 (8 kg steel) in the shallow depth ranges. Hydro-dynamically shaped steel weights weighing 5 kg would be an appropriate substitute for 8.5 kg rock weights irrespective of fishing method.