Fishing and habitat condition differentially affect size spectra slopes of coral reef fishes

Abstract

Marine food webs are structured through a combination of top‐down and bottom‐up processes. In coral reef ecosystems, fish size is related to life‐history characteristics and size‐based indicators can represent the distribution and flow of energy through the food web. Thus, size spectra can be a useful tool for investigating the impacts of both fishing and habitat condition on the health and productivity of coral reef fisheries. In addition, coral reef fisheries are often data‐limited and size spectra analysis can be a relatively cost‐effective and simple method for assessing fish populations. Abundance size spectra are widely used and quantify the relationship between organism size and relative abundance. Previous studies that have investigated the impacts of fishing and habitat condition together on the size distribution of coral reef fishes, however, have aggregated all fishes regardless of taxonomic identity. This leads to a poor understanding of how fishes with different feeding strategies, body size‐abundance relationships, or catchability might be influenced by top‐down and bottom‐up drivers. To address this gap, we quantified size spectra slopes of carnivorous and herbivorous coral reef fishes across three regions of Indonesia representing a gradient in fishing pressure and habitat conditions. We show that fishing pressure was the dominant driver of size spectra slopes such that they became steeper as fishing pressure increased, which was due to the removal of large‐bodied fishes. When considering fish functional groups separately, however, carnivore size spectra slopes were more heavily impacted by fishing than herbivores. Also, structural complexity, which can mediate predator‐prey interactions and provisioning of resources, was a relatively important driver of herbivore size spectra slopes such that slopes were shallower in more complex habitats. Our results show that size spectra slopes can be used as indicators of fishing pressure on coral reef fishes, but aggregating fish regardless of trophic identity or functional role overlooks differential impacts of fishing pressure and habitat condition on carnivore and herbivore size distributions.

Biak and Wakatobi reefs are the two hottest hotspots of coral reef fish diversity and abundance in the Indonesian Archipelago

“…Using the fairly district-to-district comparison placed Raja Ampat on the third rank of coral reef fish diversity and abundance in the Indonesian Archipelago. Raja Ampat has six locally managed marine protected areas (Carvalho et al, 2021). The finding that the Pacific Ocean region host higher coral reef fish diversity and abundance was also supported by observable evidence.…”

“…The targeted fish are carnivore demersal fish, such as Lutjanidae, Lethrinidae, and Serranidae (Pattiasina et al, 2021). Carnivore fish are more sensitive to fishing pressure than herbivores (Carvalho et al, 2021). Heavy carnivore fishing pressure in Biak left herbivores predominating coral reefs.…”

Non-senescent species are not immortal: stress and decline in two planaria species

“…Values of b can vary substantially with estimates ranging from −6.75 to −0.40 been reported (Arranz et al . 2021; Carvalho et al . 2021).…”

“…2021). To estimate b , for each species separately, we used the abundance density function (Carvalho et al . 2021): where n is the number of individuals, x is the area of each individual (mm 2 ), x max and x min are the maximum and minimum area of individuals that were digitized.…”

Gear restrictions create conservation and fisheries trade‐offs for management

“…Fishing is, however, one of the primary drivers of marine ecosystem structure and function and has had profound negative effects, both direct and indirect, on coral reefs (Pandolfi et al., 2003). For example, fishing can directly impact reefs by reducing fish abundance, biomass and size structure and indirectly through compromising ecosystem functions (Carvalho, Setiawan, et al., 2021; DeMartini & Smith, 2015). The ability of coral reefs to continue providing ecosystem services to coastal communities relies on the development of management strategies that can support fisheries production and preserve biomass of fish that perform vital ecosystem functions.…”