Guam Sea Grant

This paper aims at investigating and highlighting the hydrological benefit of vegetation by way of root water uptake (RWU) in preventing or at least delaying rainfall-induced slope failures in the island of Guam. In particular, the results of field instrumentation and monitoring by way of moisture and suction sensors, laboratory and field testing of geotechnical and hydrological soil properties and rainfall-induced numerical simulations of transient seepage coupled with slope stability analysis are presented. Two case studies, one with a 45° slope and another with a 60° slope, were studied for three different case scenarios. Only one set of vegetation and root reinforcement data was idealised, and the results confirm that matric suction was induced within shallow depths due to RWU helping in providing a buffer, which prevents slope failure during lower-precipitation events. However, this stabilisation effect ceases after saturation during long-duration precipitation. On the other hand, complementary analysis considering the mechanical root reinforcement effect showed that it is a dominating factor in improving the factor of safety even after the slope becomes saturated. In general, native hill slopes with a 45° angle were stable, but the one with a 60° angle failed when a real case rainfall event with climate–vegetation data was applied in this study.

A ridge-to-reef framework was developed for 26 watersheds around Guam. Dissolved inorganic nitrogen (DIN) data were collected for one year at the base of streams while coral and fish surveys were conducted on adjacent reefs. Two independent analyses revealed a similar 0.10 mg/l DIN threshold beyond which negative impacts to water quality and coral reefs existed. The influence of DIN was next partitioned with respect to a second primary stressor, fishing pressure. While coral diversity was negatively influenced by DIN, the cover of some stress-tolerant corals increased, such as Porites rus, making coral cover alone a poor indicator of watershed pollution. Less intuitive, DIN predicted increased food-fish biomass that was accounted for by generalist herbivores/detritivores, representing homogenized assemblages, while fishing pressure reduced biomass. Our DIN thresholds resonated with a similar study in American Samoa suggesting broader guidance for water quality legislation may be emerging.

Abstract Patterns of cohort decline are key demographic traits that provide a unique temporal perspective vital to understanding population dynamics. The discovery of multidecadal lifespans in tropical surgeonfishes in the 1990s created a paradigm shift to the notion that they are highly vulnerable species with low population recovery rates; however, research into the mortality patterns of surgeonfishes (Acanthuridae) has been sparse until recently. Recent studies on this family have demonstrated an unusual (possibly unique), two‐phase pattern of adult catch‐at‐age, whereby there is a population‐level shift from higher‐than‐expected rates of decline early in life to lower‐than‐expected decline rates for the remainder of the lifespan. To examine the geographic and phylogenetic ubiquity of this pattern, we compiled age‐based information from demographic samples of 70 populations of 25 tropical species spanning the Indo‐Central Pacific and Central‐Western Atlantic. Overall, we found that 79% of populations exhibited strong biphasic patterns, including 88% of populations across the Indo‐Central Pacific. By accounting for empirical relationships instead of using linear catch curves or classical assumptions of natural population decay based on lifespan, we demonstrate that surgeonfishes have turnover times that are two to four times faster than previously believed. Faster turnover times may suggest a higher level of sustainability for surgeonfish fisheries throughout the global tropics than previously estimated.

Abstract Home range size is a fundamental trait that can affect the probability of fish being harvested and, at the same time, may be affected by fishing. The relationship between home range size and fishing will impact the effectiveness of fully protected areas (FPAs), as it will influence the number of fish moving into fished areas, affecting both spillover and edge effects. One hypothesis is that individuals within FPAs will present reduced home range size relative to individuals in fished areas. This pattern can be driven by demographic selection (e.g. fishing of individuals with large home ranges leaving the FPAs), improved habitat requiring less foraging movements, or behavioural changes associated with reduced fishing threats. To test the relationship between home range size and protection, we compiled 1143 individual‐level home range sizes based on acoustic tracking, covering 17 species from 11 FPAs in 7 countries, with information on distance from FPA borders. A dichotomic analysis (in/out of FPAs) did not support a significant change in the home range size between FPAs and fished areas. However, continuous analysis across the FPA borders demonstrated reduced home range size within the FPAs. We did not find an effect of FPA age or size on this pattern. While we cannot pinpoint the underlying mechanism for the pattern revealed, we suggest behavioural changes as the main driver for reduced home range within FPAs. This mechanism will lead to more resident populations within FPAs, reducing fishing mortality within FPAs yet limiting spillover benefits to adjacent fisheries.

Understanding how environmental stressors impact fisheries is imperative for the sustainable management of our marine resources. Synchrony in inter-annual growth patterns among individuals and populations has been identified across large spatial scales, both within and among species. This synchrony indicates a detectable sensitivity to changes in climatic or environmental conditions. We explored within-region effects of environmental and climatic variability using inter-annual otolith growth rates (increments) in a tropical coral reef fish, Naso unicornis . Dendrochronology techniques were applied to remove age-specific growth effects and extract a high-frequency variability signal indicative of short-term environmental change. Using linear mixed-effects models, we identified best predictors of the variation in growth at two adjacent latitude subsets in the Commonwealth of the Northern Mariana Islands. We found different latitude subset growth responses for the years 2000–2013 and 2005–2017, indicating independent fluctuations in growth across latitude however, synchrony was present among individuals within a latitude range. Local environmental processes were more important than regional climatic processes for explaining N. unicornis growth in the north, but in the central islands, neither process had a clear effect. Otolith growth in fish inhabiting the north had a positive response to increased annual average sea surface temperature (SST). In adjacent central islands, otolith growth responded negatively to warmer winter SST. Baseline information for most fisheries on the direct impact of external forcings on fish, especially in tropical coral reef fisheries, remains sparse. We provide information on how climate and environment have impacted past growth with implications for future fisheries productivity monitoring.

ABSTRACT Recreational fisheries in remote regions often operate under limited regulation and data availability, presenting challenges for sustainable management. Diego Garcia, located within the Chagos Marine Protected Area, hosts a unique recreational reef fishery embedded within a highly protected environment. We assessed the stock status of five key species— Lutjanus bohar , L. gibbus , L. monostigma , Caranx melampygus , and Variola louti —using the Length‐Based Spawning Potential Ratio (LB‐SPR) approach and length composition data collected through stereo‐video surveys. The average spawning potential ratio (SPR) across all species appears to be operating above 30%, a common threshold below which a stock can be considered overfished. The SPR of L. bohar and L. gibbus remained above 30% across all bootstrap iterations indicating confidence that these species are exploited at a sustainable level. L. monostigma , C. melampygus , and V. louti exhibited greater uncertainty in their estimated SPR, spanning all possible values with at least 50% of bootstrap iterations remaining above 30% SPR; for these species, caution should be applied until additional sampling can improve the resolution of these estimates. These findings suggest that the recreational fishery is operating within sustainable limits, likely due to access restrictions and diffuse fishing pressure across species. However, uncertainty in model outputs for data‐limited species underscores the need for continued biological sampling and catch monitoring.

Accurate life-history information is essential for conducting data-limited stock assessments of coral reef fishes, yet this information is often lacking in small-scale fisheries. For the commercially important longnose emperor, Lethrinus olivaceus , such information is scarce in Micronesia, limiting the ability to manage this species effectively. To fill this gap, age, growth, and reproductive parameters were estimated for L. olivaceus in Palau using samples collected at fish markets, supermarkets, independent fishers, and targeted sampling in 2017–2019 and 2022–2025 (total n = 528). Information on growth, lifespan, reproduction, and mortality was obtained through analysis of sagittal otoliths and gonad samples. The sex-combined von Bertalanffy growth parameters for this species were estimated to be L ∞ = 57.2 cm, K = 0.387 year −1 , and t 0 = −0.324 years (unconstrained) and L ∞ = 56.4 cm, and K = 0.443 year −1 (constrained to t 0 = 0). The maximum observed age was 14 years. Longnose emperors were found to spawn in many months of the year including March to June, August, September, November, and December. There was also evidence of functional protogynous hermaphroditism, with several gonad samples containing both female and male reproductive tissues. Furthermore, males were larger (mean length = 53.5 cm) and older (mean age = 7.4 years) than females (mean length = 45.8 cm and mean age = 4.3 years), with no immature males observed. Females were estimated to reach maturity at 43.2 cm and 3.5 years, and sex change to males occurred at 55.1 cm and 7.1 years. Total mortality for both time periods combined was estimated to be 0.438 year −1 and mean natural mortality was estimated to be 0.358 year −1 . When compared to other locations at higher latitudes, including Australia, Japan, and French Polynesia, longnose emperors in Palau had faster growth rates, shorter lifespans, and reached smaller sizes. The high diversity of life-history traits among locations highlights the importance of collecting locally-derived information. These findings provide comprehensive life-history estimates for L. olivaceus in Palau, which can be used to improve data-poor assessments and management of this species in the future.