The discard of processing waste by fishing trawlers attracts seabirds, putting them at risk from being caught in nets or being struck by fishing gear. In order to minimise the impact of fishing on seabird populations, techniques are required for managing processing waste so that fewer birds are attracted to the stern of trawlers.

A simple method, that may reduce the number of birds attracted to trawlers, is to discharge the waste in batches: holding the waste and dumping it periodically, rather than discharging it continuously. In a previous experiment, waste was held for a range of intervals (between 30 minutes and 8 hours), and the influence of discharge interval on seabird abundance was determined. It was found that there were fewer birds behind the vessel when the interval between batches was either 4 or 8 hours long, rather than when it was either 30 minutes or 2 hours. In February and March 2010, a similar experiment was carried out on a trawler that was mainly targeting hoki. The key difference was that this experiment compared the number of birds behind the vessel during continuous discharge with the number that were present with either a 30-minute or 2-hour interval between waste discharges.

During the experiment, fishing was carried out normally, with the waste discharge being controlled as part of the experiment. For each day of the experiment a particular discharge regime was followed (either continuous, 30-minute, or 2-hour). A fisheries observer recorded the abundance of seabirds (large seabirds, Cape petrels, and other small seabirds) within 40 m from the stern of the vessel, making series of up to 12 observations spaced 5 minutes apart. During batched treatments, the observations began 5 to 10 minutes before a discharge event.

In this report, a preliminary analysis of data from the experiment is presented. Although the level of statistical significance was not tested, the seabird abundance at the back of the vessel was similar between treatments when waste was discharged, but the abundance was reduced by half in absence of discharge. As a consequence, across all the observation periods the seabird abundance was lowest when waste was discharged every two hours, slightly higher when the discharge interval was 30 minutes, and highest when the waste was continuously discharged. Observations of the 2-hour batch discharge treatment did not cover the whole period between discharge events, and so a larger difference between the 2 hour and other treatments may be expected. During the 2-hour treatment, waste discharge took an average of around 9 minutes, whereas during the 30-minute treatment waste discharge took an average of 3 minutes. The rapid response of seabird numbers and behaviour to the individual discharge events could not be determined, as observations were made 5 minutes apart. A fixed video camera was used to continuously record a view looking back from the stern of the vessel. This footage has not yet been analysed.

Both large and small seabirds responded similarly to the discharge treatments, and there was no apparent relationship between seabird abundance and other variables, such as vessel speed, swell height, or wind strength. The experiment was successfully conducted, with the protocol being closely followed. The preliminary descriptive analysis suggests that batching fishing waste may represent an efficient strategy in reducing the number of seabirds that are attracted to trawlers. Statistical modelling of the data would help determine the significance of this key result.