Methodology:

A licensed commercial longline fishery for Patagonian toothfish (Dissostichus eleginoides) operates in Subarea 48.3 (South Georgia). Since 2016, temperature loggers have been affixed to all longline deployments, subject to availability of serviceable units, to measure the temperature of the waters around South Georgia. Typically a single logger is affixed per longline, however in some instances where vessels were interested in recording the movement of the line, multiple loggers were affixed to a single line. Some idea of where on the longline the logger was positioned is recorded in the 'logger position' column. Longlines are comprised of multiple magazines; sections with ~1000 fishhooks. In 2019, temperature, maximum depth of the set on which the longline was deployed, taken from the logbooks, and pressure were recorded, but since 2021, logger depth has also been used instead of max. depth, and pressure no longer measured. Data from deployments of the temperature loggers have been matched to positional data stored in the South Georgia Data Portal (closed to the public). Data from 2019-2024 are collated here.

Data collection:

Instrumentation.

Data were recorded using the Star Oddi Starmon TD - https://www.star-oddi.com/products/data-loggers/time-depth-recorder-tdr-starmon

Data quality:

To ensure only in-water temperatures are presented, all data were subset to those measurements taken between the end of the set and the start of the haul. As there is some mismatch between the in-water times in recorded in logbooks and the time on the temperature loggers, further anomalous measurements were removed using a two-stage process:

1. Limits of -1.8°C (the freezing temperature of seawater) and 15°C were used as initial broad cutoffs.

2. Further anomalies were removed using the mean temperature ± 2 times the standard deviation.

Where depth data were available, bottom temperature measurements were selected by removing those shallower than the 75th depth quantile for each line.

Some missing values (NAs) are present:

2019 - Depth was not recorded, however, pressure was recorded on deployments by one vessel.

2024 - Some logger deployments recorded erroneous depths, so these have been removed resulting in NA.

Additionally, data from 2020 are limited due to difficulties with the COVID-19 pandemic which was occurring at the time. Data from 2016-18 and 2022 are still undergoing collation. Where possible these data will be integrated in future updates to this temperature time series.

As these data were collected opportunistically, the sampling interval varies across deployments.