Organic matter derived from kelp supports a large proportion of biomass in temperate rocky reef fish communities: Implications for ecosystem‐based management

Abstract

1. The relative availability of alternative organic matter sources directly influences trophic interactions within ecological communities. As differences in trophic ecology can alter the productivity of communities, understanding spatial variability in trophic structure, and the drivers of variability, is vital for implementing effective ecosystem-based management.2. Bulk stable isotope analysis (δ 13 C and δ 15 N) and mass balance calculations were used to examine patterns in the contribution of organic matter derived from macroalgae to food webs supporting temperate reef fish communities in two contrasting coastal waterways on the South Island of New Zealand: Fiordland and the Marlborough Sounds. Ten fish species common to both regions were compared, with up to 40% less organic matter from macroalgae supporting omnivorous species in the Marlborough Sounds. The largest differences in trophic position were found in those species exploited by fisheries.3. Furthermore, stratified surveys of abundance and species biomass combined with trophic position data were used to calculate regional differences in the contribution of macroalgae to whole fish communities in terms of density of biomass. In Fiordland, over 77% of the biomass of exploited reef fishes was supported by macroalgae, compared with 31% in the Marlborough Sounds. 4. Surveys of macroalgal density and species composition in the two regions indicated that regional differences in trophodynamics may be explained by a lack of macroalgal inputs to the food web in the Marlborough Sounds.5. The findings demonstrate large regional differences in the incorporation of benthic and pelagic sources of organic matter to food webs supporting reef fish communities, highlighting the need for ecosystem-based approaches to management to recognize spatial variability in primary production supporting coastal food webs.

Changes in trophic structure of an exploited fish community at the centennial scale are linked to fisheries and climate forces

“…Furthermore, from a modeling approach, overfishing has been shown to change species composition of fish communities and alter marine food webs 15 , 57 . Shifts in trophic architecture such as the ones observed in the present study were previously linked to increased energy cost of species occupying trophic levels and relying more on pelagic compared to coastal production 14 , 58 . When coupled with decreased pelagic production and increased metabolic rates of fishes due to ocean warming, these results likely indicate a reduction in marine fisheries production in the future.…”

Shifting trophic architecture of marine fisheries in New Zealand: Implications for guiding effective ecosystem‐based management

“…Fisheries‐induced shifts in community structure can have dramatic impacts on ecosystem function, affecting species trophic levels (Popp et al, 2007), food web dynamics and reproduction (Jack & Wing, 2010; Persson & Hansson, 1999), organic matter fluxes, energetics and distribution of biomass (Udy, Wing, Jowett, et al, 2019; Udy, Wing, O’Connell‐Milne, et al, 2019), and ultimately catch rates (Hinz et al, 2017). In the present study, average trophic levels of individual species were retrieved from a large online database (http://Fishbase.org).…”

“…In this context, preservation of intact trophic structure within fish communities is necessary to maintain fluxes of organic matter and nutrients through food webs providing resilience of marine ecosystems, and should be considered a vital component of essential fish habitat (Blanchard, Law, Castle, & Jennings, 2011; Thrush & Dayton, 2010; Trueman, Johnston, O’Hea, & MacKenzie, 2014). Understanding food web ecology of commercially important species and its spatial and temporal variation remains an extant challenge (Udy, Wing, O’Connell‐Milne, et al, 2019; Ward et al, 2016). Nevertheless, the principles of ecosystem‐based management, with focus on preservation of intact food web structure, have been tested at regional scales both internationally and in New Zealand waters (Lassalle et al, 2011; Sponaugle, 2010; Wing & Jack, 2013), with successful management outcomes (Jack & Wing, 2013; Nicoll, 2018; Wing & Jack, 2014).…”

Regional differences in kelp forest interaction chains are influenced by both diffuse and localized stressors

“…, Udy et al. , b). The exploitation of top predators in rocky reef habitats can reduce top‐down control on the structure of marine communities (Frank et al.…”