Trophic structure through the history of exploitation in New Zealand fish communities
Creators: name: Durante, L.M.
Publisher: name: NIWA-hosted NSCs: Deep South and Sustainable Seas National Science Challenges
Year: 2024
Subjects: classificationCode: 410404
subject: Environmental management
classificationCode: 310305
subject: Marine and estuarine ecology (incl. marine ichthyology)
classificationCode: 410203
subject: Ecosystem function

description: Marine ecosystems have been under a considerable amount of stress due to human activities, especially after industrialization. The abundance and ecological interactions of exploited fishes are influenced by fishing activities, climate change, and natural environmental variability, but there is a lack of knowledge on how these community parameters have changed over time. In the present study, temporal shifts in the trophic structure and composition of an important commercial fish community along the east coast of the South Island of New Zealand was investigated. The Marine Trophic Index (MTI) and species composition of the reported commercial fisheries landings for the whole country were used to identify periods of fisheries expansion and shifts in the trophic architecture of the landings. A stable isotope approach was used to estimate resource use and trophic level of fishes, comparing these parameters spatially and temporally. It was demonstrated that it is possible to access carbon and nitrogen stable isotope values from preserved fishes, for both bulk tissues and specific amino acids, which enabled this work to access ecological information from specimens stored in museum samples. The results demonstrate that fisheries industrialization occurred between 1970 and 2000 in New Zealand, increasing MTI values and shifting the relative landings composition from inshore to offshore and deep-water species over time. Regional comparisons demonstrated that modern fish communities from Otago relied more heavily on macroalgae production when compared to those from Kaikōura, linked to higher fishery yields in the former. When analyzing the same parameters from museum specimens collected before the full expansion of industrialized fisheries, modern species inhabiting the outer shelf and slope also showed a reduced reliance on macroalgae production, together with an increased trophic level. When considering continuous changes in the trophic parameters, environmental factors and prey abundance had a significant relationship with the variation of trophic parameters for most fish assemblages. Trophic parameters of the inner shelf assemblage also showed a relationship with MTI throughout time, indicating the earlier effects of coastal fisheries. Although Kaikōura and modern communities showed a wider niche breadth compared to Otago and historical samples, respectively, tarakihi was the only species to show both regional and temporal trends. Tarakihi stocks in the region have been overexploited and are sensitive to habitat degradation, which could be linked to the shift in trophic structure observed here. Furthermore, temporal changes in community composition towards a relatively higher abundance of lower trophic level species were observed from the analysis of long-term trawl survey data. Species with decreasing abundance that presented shifts in trophic parameters with time, such as red cod, had their trophic level and resource use occupied by species with increasing abundance, like spiny dogfish. The present study demonstrates how the trophic structure and composition of important commercial communities have changed over time and how they are related to natural and anthropogenic effects, with the potential to alter regional fishery yields. This study highlights how uncovering ecological baselines before human impacts is an important step towards the implementation of an ecosystem-based management approach to fisheries resources.