Validation of sink rates of longlines measured using two different methods

Sink rates of integrated weight longlines (lines with 50 g.m-1 lead integrated into two strands of the ground line - IW-50 lines) were measured during commercial fishing operations using two methods: electronic time depth recorders (TDRs) and pieces of string of known length wrapped around empty plastic bottles (bottle method). Sink rates measured to 2 m with bottles averaged 0.23 plus or minus 0.07 m.s-1 compared to 0.17 plus or minus 0.03 m.s-1 recorded by TDRs. This difference was not statistically significant (t12 = -0.181, P = 0.859). When the target depth was 15 m, sink rates measured using the bottle method (0.20 plus or minus 0.02 m.s-1) were significantly slower than those measured by TDRs (0.24 plus or minus 0.03 m.s-1, t10 = -3.851, P = 0.003). Measuring sink rates to 15 m proved difficult with bottles because they were too far behind the vessel, i.e. out of the observer's sight, when they reached the target depth. Bottles had a high failure rate (60%) due to string becoming entangled during line setting, or bottles vanishing from sight behind waves or in congregations of seabirds before the target depth was reached. No TDRs were lost during the trial. Bottle tests were most useful, depending on sea state and weather conditions, for measuring sink rates to shallow depths when instant readings were required. TDRs can be used to measure sink rates to depths of more than 2 m down to the seabed. A major advantage of the TDRs is the archival nature of the data collected.