Ontogeny of Similarity and Disparity in Length-at-Age of Coral Reef Fishes in the Family Acanthuridae between Marine Protected and Fished Areas

Abstract

Drastic reductions in the harvestable biomass of exploited fish populations on coral reefs and other marine coastal ecosystems have been hypothesized to be a consequence of astronomical and unrelenting fishing pressures, particularly in the coastal small-scale and sustenance fisheries of the world. It has recently been proposed that the causal relationship between fishing pressure and fish biomass is rooted in the negative effects of size-selective fishing mortality, which consequently instigates directional shifts in phenotype (i.e., fishing induced evolution). To mitigate the effects of unsustainable fishing pressures, parcels of traditional fishing grounds have been designated as Marine Protected Areas (MPAs). To date, the assessment of the efficacy of MPAs as fisheries management tools has been primarily limited to visual census surveys, largely because the “no-take” designation of MPAs precludes repeated sampling. As such, this investigation capitalized on a rare opportunity to perform an approved static sampling excursion in 2015 on four acanthurid species (Acanthuridae, Teleostei) between five pairs of MPAs and adjacent fished areas (AFAs) within a network of coral reefs along the coast of the Zambales Province, Philippines to introduce the novel use of a “back-calculation” technique into the field of MPA assessment. For every specimen, standard length, a proxy for fitness, was measured and otoliths extracted, after which ageing was performed in collaboration with the Age and Growth Laboratory of the Florida Fish and Wildlife Commission Fish and Wildlife Research Institute. Back-calculated lengths-at-age were examined for spatial (MPA vs. AFA) and temporal (interannual) similarities and disparities throughout the observed life history. Analyses revealed five major findings. First, newly settled A. nigrofuscus were phenotypically similar at ages one and two. Second, A. nigrofuscus transformed into significantly different length-at-age morphs between ages three and six, in which MPA populations were predominantly larger than AFA populations. Third, this phenotypic divergence disappeared towards the latter life history stages. Fourth, the observed life histories of the other acanthurid species were dominated by spatial phenotypic homogeneity. Fifth, all species displayed prolonged temporal growth. These observed patterns may result from (1) a release from fishing pressures within MPAs, (2) spatiotemporal variations in food and habitat resources between MPAs and AFAs and (3) density-dependent forces that could promote phenotypic change through phenotypic plasticity and, potentially, subsequent genotypic adaptation. Altogether, four major conclusions are drawn from this investigation. First, the results presented here confirm the success of this methodology, promoting the MFBC model as a bio-mathematical tool capable of accurately reconstructing the size-related life histories of coral reef fishes and thus conferring a novel solution to the logistic and legal restrictions placed on repeated sampling within protected areas. Second, analyses determined that the sampled MPAs are gradually restoring historical body sizes in exploited regions, both within and outside of their borders. Third, patterns of phenotypic convergence and divergence were variable, including within the same genus (Ctenochaetus), suggesting that these phenotypic responses are species-specific. As a result, assessments should not make assumptions that the phenotypic responses of a single or select few species are representative of all other species contained with the protected area. Fourth, the results of this investigation perfectly example the caution that needs to be taken when interpreting the findings of static spatial analyses that were performed in attempt to assess dynamic tools. A lack of phenotypic divergence between MPAs and AFAs has historically been interpreted as a failure of the MPA to instigate phenotypic enlargements, which is disproven in this investigation (i.e. C. binotatus, C. striatus and Z. scopas). As a result, investigators should refrain from using the results of independent static spatial analyses as a proxy of the biophysical performance of MPAs. Although these findings offer critical insight and a comprehensive provision of MPA functionality within the Northwest Philippine region of the Coral Triangle, the ubiquity of the observed phenotypic responses across other taxa and regions is unknown. Whilst further spatiotemporal analyses are required across other regions and taxa of differential exploitation and life histories, this investigation adds to the growing body of evidence that MPAs can provide positive conservation and fishery benefits. Continued studies identifying the positive outcomes of MPAs will be critical in garnering support for the establishment of MPAs in the coming decades.