Introduction
Maintaining sustainable fish stocks is complicated because
ten-to-hundreds of target species coexist and the daunting task of
selecting species for formal management has been equated to a form of
triage that fisheries managers are faced with
(Levin & Stunz 2005;
Patrick & Link 2015). Desirable stock
assessments that estimate population sizes, fish size-and-age
structures, and catch rates are not feasible for small and disconnected
multispecies fisheries, such as coral-reef fisheries. Instead, data-poor
approaches take representative snapshot(s) of size-and-age structures,
and assess the degree to which compensatory density dependence can
replenish stocks based upon several size-related metrics of fecundity
and recruitment (Nadon et al.2015; Prince et al. 2015;
Eikeset et al. 2016). These
approaches assume that density dependence is the primary driver of
population replenishment, which is well supported by studies
demonstrating how density dependence begets ecosystem stability, even in
complex food webs (McCann et al.1998; Berlow et al. 2009;
Houk et al. 2018b). However, the
expected replacement of large, old fish with mid-sized younger
counterparts has not had ubiquitous support, especially among coral-reef
fisheries. The causes of species variability and consequences of
treating species similarly forms the basis for our study.
Recently, (Houk et al. 2018a)
documented changes in abundances and size structures across 25 years of
exploitation in a coral-reef fishery and found contrasting responses
among herbivore/detritivore species with similar maximum body-sizes and
growth rates (Figure 1). Species were classified as ‘winners’ if they
persisted as major contributors to landings over decades despite having
size-and-age truncation. In contrast, species were classified as
‘losers’ if they slowly disappeared from landings while having
little-to-no shifts in their size structure. Similar responses have now
been reported from many Pacific islands based upon both
fisheries-dependent and fisheries-independent data
(Houk et al. 2012;
Houk & Musburger 2013;
Houk et al. 2015;
McLean et al. 2016;
Houk et al. 2017;
Cuetos-Bueno et al. 2018).
These disparate responses have remarkable consequences for fisheries
management and can improve our contextual understanding of ecological
niches. For instance, stock assessments based upon size structures mayincorrectly recommend that no management is required for ‘losing’
species while they disappear from landings and reefs, so long as
little-to-no size-and-age truncation is observed
(Lorenzen & Enberg 2002;
Hordyk et al. 2014;
Nadon et al. 2015;
Prince et al. 2015). More broadly,
the disparate density dependence responses may also predict niche
attributes, such as dominance and breadth, and lead to better
predictions of ecosystem services and stability. Our goal was to
investigate phylogenetic attributes as a proactive and inexpensive means
towards predicting compensatory density dependence responses.
Recent studies have seen an increase in the combined use of ecological
data and phylogenetic information to understand the structure and
assembly of communities and predict the vulnerability of certain taxa to
overexploitation (Jennings et al.1999b; Godoy et al. 2014). We
hypothesized that phylogenies may help predict the relative strength of
density dependence responses within a diverse coral-reef fishery and
identify ‘winning’ and ‘losing’ species proactively. Our hypothesis
builds upon Darwin’s observation that competition should be stronger
between closely related species with more similar traits
(Darwin 1859), potentially as a result of
higher realized niche partitioning, and lead to low species persistence
with exploitation. In contrast, we hypothesized that greater
phylogenetic isolation, defined by larger distances between sister
species, may equate to stronger density dependence and persistence, and
might infer greater niche dominance or breadth.
Our hypothesis was tested by combining an extensive network of
fisheries-dependent datasets from many Pacific islands with an objective
measure of phylogenetic isolation. Genetic material was collected from
primary target species that accounted for 90% of the landings within
four dominant families common to coral-reef fisheries in the Pacific:
parrotfishes, surgeonfishes, snappers, and emperorfishes. We then
examined novel relationships between phylogenetic isolation and
species-based responses to fishing pressure and revealed a proactive
means towards species-based management before stocks show signs of
decline.
Brief Methods