The aim of this and preceding research projects (POP2017-06, POP2019-02) is to better understand the relationship between the diet of sea-surface foraging seabirds, and prey items associated with fish workups. This knowledge can then be used to help identify any mechanisms that could be changing the distribution, occurrence and/or abundance of workups and how that might in turn affect seabird populations. This report presents a summary of the analysis of zooplankton samples collected in the 2020-2021 sampling season (Gaskin et al. 2021) and their relationships with different types of seabird-feeding events. It forms a continuation of the fish school dynamics and zooplankton research conducted between 2017 – 2020 (2017-2018: Gaskin et al. 2019a; 2018-2019: Gaskin et al. 2019b; 2019-2020: Kozmian-Ledward et al. 2020). Six fieldwork days were undertaken in the wider northern Hauraki Gulf between 6 October 2020 and 26 March 2021. The vessel followed a similar route each day, and locations where seabirds were seen feeding were targeted for zooplankton sampling, fish captures, observations on seabirds and fish species, underwater videography, and environmental measurements. Four types of pelagic fish schooling events with associated feeding seabirds were sampled: Mixed Fish School, Kahawai School, Koheru School, and Fish School – unknown type. Two events where no schooling fish were seen but seabirds were feeding were sampled: Current Line and Unknown. Zooplankton samples were subsampled as required and counted into seven groups: Copepoda, Malacostraca, Krill, Thaliacea, Fish, Fish eggs and Other. Each event type was generally able to be defined by specific zooplankton, fish and seabird types/species and certain seabird feeding behaviours. Krill was found to be an important component of many of the fish school events that were sampled, with the krill preyed upon by both fish and seabirds.
This season’s research was curtailed by Covid-19 restrictions resulting in a reduction in data collected and subsequent analysis. This needs to be considered when looking at data trends given in this report. There is a need to continue to develop the multi-disciplinary approach used here to fully investigate the associations between oceanographic factors, zooplankton, pelagic schooling fish and prey availability for seabirds in the wider Hauraki Gulf.
Kozmian-Ledward, L., Jeffs, A. and Gaskin, C. 2022. Seabird feeding associations with fish shoals. BCBC2020-08 final report summarising analysis of zooplankton samples 2020-21. Prepared by Northern NZ Seabird Trust for the Department of Conservation. 36 pp.