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POP2019-02: Fish shoal dynamics in north-eastern New Zealand (PDF, 5,711K)

Summary

A notable feature of north-eastern North Island, New Zealand waters are the large numbers of seabirds feeding in ‘workups’ – multispecies aggregations containing zooplankton and fish. Many seabird species are potentially dependant on prey (zooplankton and fish) advertised by and made available by the shoaling fish in workups. However, the processes that drive workup formation and dynamics are poorly understood in this region. Purse-seine fisheries in this region target fish species which form workups and may therefore be indirectly affecting seabirds which utilise workups for food. The degree to which this occurs is unknown and therefore it is important to better understand the relationship between seabird population trends and changes in abundance and distribution of workup forming fish shoals.

This study aimed to characterise the biological composition of workups by determining the associations among the presence of zooplankton, shoaling fish, and feeding seabirds. Nine fieldwork days were undertaken in the wider northern Hauraki Gulf between November 2019 and February 2020. Locations where seabirds were seen feeding were targeted for zooplankton sampling, fish captures, data collection on seabirds and fish species, underwater videography and environmental measurements. Three types of fish shoal events were defined and sampled: Mixed fish shoal, Kahawai school and Tuna school. Three types of non-fish shoal events where seabirds were feeding were defined and sampled: Current line, Krill patches, and Unknown. Zooplankton samples were subsampled as required and counted into seven groups: Copepoda, Malacostraca, Nauplii (krill), Thaliacea, Appendicularia, Fish eggs and Other. Each event type was able to defined by specific zooplankton, fish and seabird types/species and certain seabird feeding behaviours. Krill (Nyctiphanes australis) was found to be an important component of fish shoal events and preyed upon by both fish and seabirds. Krill was also found at high abundances at Krill patch events where fish shoaling did not occur but seabirds were feeding on the krill.

This season’s research was curtailed by COVID-19 restrictions resulting in a large 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 indirect effects of fisheries on seabirds in the wider Hauraki Gulf.

The appended report provides an update on analyses of fish shoal data from the aerial sightings database (aer_sight).