Abstract
Micronekton are a key component of the pelagic food web of the Chatham Rise east of New Zealand. The Chatham Rise is an important fishing area for hoki (Macruronus novaezelandiae), New Zealand’s largest finfish fishery, and a predator on mesopelagic fish. Four fisheries oceanographic voyages provided multi-frequency acoustic data (18, 38, 70 120 and 200 kHz) and midwater trawls, which were used to define a classification tree to separate micronektonic organisms. We carried out validation and sensitivity analyses that showed that we were able to classify pearlside (Maurolicus australis) and euphausiids. Other mesopelagic targets (mainly myctophids) were classified together based on their acoustic frequency response. Using scripting in the open-source software ESP3, we applied our classification tree to an independent time series of acoustic data from trawl surveys on the Chatham Rise between 2009 and 2018, that was not used for model development or validation. Our methodology allowed us to study temporal and spatial patterns of M. australis, euphausiids, and total backscatter in the water column. Total backscatter associated with micronekton has varied over the last ten years, with no clear trend. The abundance of euphausiids showed a significant decreasing trend over the last ten years. Abundance of M. australis also decreased since 2012, though this was not significant. This work contributes to on-going efforts to monitor and detect changes in the pelagic ecosystems.
Highlights
The Chatham Rise is a submarine ridge that extends approximately 1500 km east of the South Island of New Zealand and has a total area of around 160,000 km2 (Nodder et al, 2012)
Because we only used Mark identification (mark ID) trawls where the catch was dominated by weight or number by one species or group for model training, the training dataset for mesopelagic fish at 38 kHz included only 15 trawls: 6 from TAN0802; 5 from TAN1116; 1 from TAN1511; and 3 from TAN1516
Mark ID trawls targeted mesopelagic fish marks based on observations made on the echosounders at 18 or 38 kHz, at which euphausiids are weak sound scatters, their presence in the catch was incidental
Summary
The Chatham Rise is a submarine ridge that extends approximately 1500 km east of the South Island of New Zealand and has a total area of around 160,000 km (Nodder et al, 2012). This region is highly productive and it is characterized by a stable oceanographic feature where warm subtropical and cold subantarctic waters meet to form the Subtropical Convergence Zone (Heath, 1985; Murphy et al, 2001; Sutton, 2001).
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