Validation of sink rates of longlines measured by two different methods

We measured sink rates of longlines using two different methods: one was string wrapped around bottles and the other electronic time-depth recorders (TDRs). Bottles provided faster sink rates TDRs. Results obtained from the bottle method were more variable, and hence less reliable, among individual deployments than TDRs requiring repeated deployments on a longline. Sink rates measured to 2 m with bottles averaged 0.13 ± 0.02 m.s-1 compared to 0.21 ± 0.07 m.s-1 recorded by TDRs. This difference was statistically significant (t5 = -2.720, P = 0.042). Measuring sink rates to > 10 m proved difficult and unreliable with bottles. Bottles had a higher failure rate (51 – 68 %) than TDRs (> 90 %), for example, with string becoming entangled or bottles vanishing from sight. We recommend while bottles may be an appropriate means for measuring sink rates to shallow depths (> 5 m) that they should not be used for estimating sink rates to greater depths.