Development of a method for the quantitative assessment of fish grazing pressure in temperate reefs

Ryoji Fukui¹, Sylvain Agostini¹

¹University of Tsukuba, Japan

Kelps and other macroalgae are critical habitat-forming species in temperate reefs, creating underwater forests that support high biodiversity, contribute to carbon fixation, and provide vital ecological services. However, these ecosystems are increasingly threatened by global warming and other climate-driven changes, leading to their widespread decline. One of the key drivers for kelp loss is overgrazing by tropical fish, which rapidly expand their ranges polewards as sea temperatures rise. Although many studies have investigated the interactions between macroalgae and herbivorous fish, the methodologies applied vary significantly, making it challenging to obtain quantitative data across different sites. This research aims to develop a standardized and practical method for quantifying fish grazing pressure in temperate reefs. We present a simple and cost-effective basic approach inspired by longline fishing, which is easy to make, deploy, and retrieve. For broader accessibility, the method primarily uses widely available dried macrophytes, such as Laminaria, as bait. As a case study, the proposed method was applied in Shimoda (Japan) to investigate the grazing pressure by range-extending tropical fish (including Calotomus japonicus) and its contribution to the decline of the local kelp population, providing insights into the impacts of herbivory on macroalgal ecosystems in the region. While certain challenges remain, this study provides a framework for collecting generalized data on fish grazing pressure that can be compared across locations and time periods. Such standardized datasets are crucial for advancing our understanding of ecosystem functions and informing effective ecological conservation and restoration strategies. The proposed method offers a valuable tool for protecting macroalgae-dominated ecosystems that are at risk of disappearing.