Discussion
The present study provided context to the common assumption that density dependence is the primary driver of fish population dynamics. We demonstrated that 51% of target coral-reef fishes did not show the expected compensatory density dependence response when exposed to fishing pressure, but instead have been slowly disappearing from landing and reefs (Houk et al. 2015; McLean et al. 2016; Houk et al. 2017; Cuetos-Bueno et al. 2018; Houk et al. 2018a; Houk et al. 2018b). Previous studies have found that the smaller of two closely related sister species will replace the larger counterpart in landings as fishing pressure grows (Jennings et al.1999a; Jennings et al. 1999b). However, we add broader context showing that (i) replacements can also occur between fish with similar maximum body sizes, and most importantly, (ii) there was a phylogenetic distance threshold beyond which compensatory density dependence became stronger than species replacements. While high sensitivity to fishing pressure has been documented for the largest reef fish with body-sizes approaching 1 m, reef-associated sharks, and other iconic megafauna (Martin et al. 2016; Hamilton et al. 2019), we emphasize that disparate sensitivities existed among common target species from four families that are essential for both food security and ecosystem resilience across the Pacific (Bellwood et al. 2012; Mumby et al. 2013; McLean et al. 2016).
Darwin’s foundational hypothesis proposed that more closely related species would be in stronger competition with each other and therefore reduce access to resources, or reduce niche dominance (Darwin 1859). If so, smaller phylogenetic isolation may infer greater present-day competition and realized-niche partitioning of available resources, as seen with protist species and Cuban lizards (Knouft et al. 2006; Violle et al. 2011), and hypothesized generally by Hutchinson (Hutchinson 1959). In contrast, we hypothesize that greater phylogenetic isolation may indicate increased niche breadth that leads to species persistence. In support, we contrast two parrotfishes, Hipposcarus longiceps and Chlorurus microrhinos , and two surgeonfishes, Naso unicornis andAcanthurus xanthopterus , that had similar sizes-at-capture and asymptotic lengths. Growing fishing pressure has resulted in size/age truncation for H. longiceps and N. unicornis that had greater phylogenetic isolation, while C. microrhinos and A. xanthopterus slowly disappeared from landings with little change to their size structure. Interestingly, both H. longiceps andN. unicornis have larger than expected growth rates based upon standard body-size growth curves for their respective families (Choat et al. 2002; Taylor & Choat 2014; Taylor et al. 2014), and both travel in large roving schools that afford greater access to resources and/or protection. In contrast, C. microrhinos has a small home range and notable morphological features that provide a competitive advantage to access a limited resource pool (Bellwood 1995; Lokrantz et al. 2008; Bonaldo & Bellwood 2009), while less is known about A. xanthopterus . Thus, disparate responses to fishing pressure may equate to disparate differences in life-histories, diets, functional traits, and ecosystem services (Bellwood et al. 2012). If so, phylogenetic characteristics have the potential to proactively determine a suite of