Line weights of constant mass (and sink rates) for Spanish-rig Patagonian toothfish longline vessels

CCAMLR Conservation Measure 25-02 requires Spanish system longline vessels attach 8.5 kg weights at 40 m intervals on longlines to minimise interactions with seabirds. The weights are collections of rocks enclosed in netting bags. The netting bags abrade on the seabed, rocks are lost and weights become progressively lighter, requiring ongoing weighing and repair. This problem can be solved by use of torpedo-shaped steel weights. Steel weights are smaller, lighter for equivalent mass, more hydrodynamic than their rocks counterparts and require no maintenance. An experiment was conducted on a chartered Spanish-rig longline vessel to determine the statistical relationship in sink rates of longlines equipped with bags of rocks (4 kg, 6 kg and 8 kg) and lines 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 to the sink rates associated with the line weighting requirements of the conservation measure. Both the traditional Spanish method and the newly-developed Chilean method (a modified version of the former method to avoid fish loss by toothed whales) were examined in the experiment. Traditional method longlines with 8 kg/40 m rock weights averaged 0.24 m/s to 2 m depth, which would be equal to or exceeded by lines with 5 kg steel weights. Sink rates of Chilean method longlines greatly exceeded those of the traditional method, ranging from 0.68 m/s (4 kg rocks) to 1.41 m/s (8 kg steel) in the shallow depth ranges. We recommend that for operational simplicity and to facilitate compliance to the conservation measure irrespective of fishing method, operators be given the option of using either 8.5 kg rock weights or 5 kg torpedo-shaped steel weights.