Fish foraging patterns, vulnerability to fishing, and implications for the management of ecosystem function across scales

Abstract

Abstract. The function of species has been recognized as critical for the maintenance of ecosystems within desired states. However, there are still considerable gaps in our knowledge of interspecific differences in the functional roles of organisms, particularly with regard to the spatial scales over which functional impact is exerted. This has implications for the delivery of function and the maintenance of ecosystem processes. In this study we assessed the allometric relationship between foraging movements and fish body length at three sites, for 20 species of herbivorous reef fishes within four different functional groups: browsers, farmers, grazer/ detritivores, and scraper/excavators. The relationship between vulnerability of species to fishing and their scale of foraging was also examined. We present empirical evidence of the strong, positive, log-linear relationship between the scale of foraging movement and fish body length. This relationship was consistent among sites and between the two different movement metrics used. Phylogeny did not affect these results. Functional groups foraged over contrasting ranges of spatial scales; for example, scraper/excavators performed their role over a wide range of scales, whereas browsers were represented by few species and operated over a narrow range of scales. Overfishing is likely not only to remove species operating at large scales, but also to remove the browser group as a whole. Large fishes typically have a significant role in removing algae on reefs, and browsers are key to controlling macroalgae and reversing shifts to macroalgal-dominated states. This vulnerability to exploitation has serious consequences for the ability of fish assemblages to deliver their functional role in the face of anthropogenic impacts. However, identification of the scales at which herbivorous fish assemblages are susceptible to fishing provides managers with critical knowledge to design management strategies to support coral-dominated reefs by maintaining function at the spatial scales at which vulnerable species operate.

A quantitative framework for assessing ecological resilience

“…The underlying assumption is that the discontinuous organization of ecological systems, in terms of nonlinear distributions and availability of shelter, food, and other resources in the environment and the interactions between species, is ultimately mirrored in the size or mass structure of animal communities (Scheffer andvan Nes 2006, Nash et al 2014). The body mass discontinuity approach has been used, for example, with forest and woodland birds in fragmented agricultural landscapes (Fischer et al 2007) and coral reef fish subjected to fishing pressure (Nash et al 2013). Both studies found that communities subjected to human impacts are less resilient because of the selective extinction of particular body mass and functional groups.…”

Herbivory and the resilience of Caribbean coral reefs: knowledge gaps and implications for management

“…However, herbivorous fishes also vary greatly in how they use space (e.g. Nash et al 2013), and we know little about the foraging scales of most herbivorous fishes in the Caribbean.…”

Resource partitioning along multiple niche axes drives functional diversity in parrotfishes on Caribbean coral reefs

“…Guilds with higher levels of redundancy among species may be more resilient to overharvesting and better able to fulfil their ecological function. However, herbivores vary widely in a number of traits in addition to diet that will modulate their impacts on reef ecosystems and influence patterns of redundancy, including size (Lokrantz et al 2008), movement patterns (Nash et al 2013), preferred habitats (Robertson and Gaines 1986), and the specific substrates they target while foraging (Brandl and Bellwood 2014). Coexistence theory predicts that species that are similar in one niche dimension, such as diet, will be different in others, such as habitat selection (MacArthur and Levins 1967).…”