Jiangang Luo

Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes

Climate model predictions1, 2 and observations3, 4 reveal regional declines in oceanic dissolved oxygen, which are probably influenced by global warming5. Studies indicate ongoing dissolved oxygen depletion and vertical expansion of the oxygen minimum zone (OMZ) in the tropical northeast Atlantic Ocean6, 7. OMZ shoaling may restrict the usable habitat of billfishes and tunas to a narrow surface layer8, 9. We report a decrease in the upper ocean layer exceeding 3.5 ml l−1 dissolved oxygen at a rate of ≤1 m yr−1 in the tropical northeast Atlantic (0–25° N, 12–30° W), amounting to an annual habitat loss of ~5.95×1013 m3, or 15% for the period 1960–2010. Habitat compression and associated potential habitat loss was validated using electronic tagging data from 47 blue marlin. This phenomenon increases vulnerability to surface fishing gear for billfishes and tunas8, 9, and may be associated with a 10–50% worldwide decline of pelagic predator diversity10. Further expansion of the Atlantic OMZ along with overfishing may threaten the sustainability of these valuable pelagic fisheries and marine ecosystems.

Length-based assessment of sustainability benchmarks for coral reef fishes in Puerto Rico

SUMMARY The sustainability of multispecies coral reef fisheries is a key conservation concern given their economic and ecological importance. Empirical estimation and numerical model analyses were conducted to evaluate exploitation status via resource reference points (or sustainability benchmarks) for coral reef fishes of the snapper-grouper complex in Puerto Rico. Mean size (¯ L, in length) of animals in the exploited part of the population was estimated from fishery-dependent and fishery-independent size composition data and used as an indicator variable of exploitation rates. In application, fishing mortality rates estimated from ¯ L of various data sources were comparable. Of the 25 reef fish species assessed, 16 were below 30% spawning potential ratio (SPR), six were above 30% SPR, and three could not be reliably determined owing to low sample sizes. These findings indicate that a majority ofsnapper-grouperspeciesinPuertoRicoarecurrently fished at unsustainable levels.

An Efficient Sampling Survey Design to Estimate Pink Shrimp Population Abundance in Biscayne Bay, Florida

Abstract We developed an efficient sampling design-based approach using fishery-independent surveys to estimate population abundance of pink shrimp Penaeus duorarum over time in Biscayne Bay, Florida. We initially implemented quarterly stratified random sampling (StRS) using nine habitat strata and determined that average pink shrimp density (numbers/m2) was highest in late fall and lowest in spring and late summer. Coefficient of variation of the quarterly surveys, expressed as percent standard error/mean density, ranged from 5.8% to 14.3%. We found StRS to be more efficient (i.e., with lower variance) than simple random sampling (SRS) in most seasons. Statistical analyses suggested that pink shrimp densities were dependent on the biophysical habitat variables of bottom substrate, depth, and salinity. We also noted ontogenetic shifts in these relationships that were particularly pronounced at the onset of sexual maturation. Poststratification analysis was used to further evaluate several alternative habi...

Benthic Habitat Mapping in the Tortugas Region, Florida

Concern about declining trends in coral reef habitats and reef fish stocks in the Florida Keys contributed to the implementation of a network of no-take marine protected areas in 1997. In support of the efforts of the Dry Tortugas National Park and Florida Keys National Marine Sanctuary to implement additional no-take areas in the Tortugas region in 2001, we expanded the scale of our fisheries independent monitoring program for coral reef fishes in the region. To provide a foundation for the habitat-based, stratified random sampling design of the program, we created a digital benthic habitat map of coral reef and hard-bottom habitats in a geographic information system by synthesizing data from bathymetric surveys, side-scan sonar imagery, aerial photogrammetry, existing habitat maps, and in situ visual surveys. Existing habitat maps prior to 1999 were limited to shallow-water (< 20 m depth) soft-sediment, coral reef, and hard-bottom habitats within Dry Tortugas National Park and did not include deeper areas such as the Tortugas Bank, now partially contained within no-take marine protected area boundaries. From diver observations made during the 1999 survey, we developed a classification scheme based on habitat relief and patchiness to describe nine hard-bottom and coral reef habitats encountered from 1-33 m depth. We provide estimates of area by habitat type for no-take marine protected areas in the Tortugas region. Updated information on the spatial distribution and characteristics of benthic habitats will be used to guide future monitoring, assessment, and management activities in the region. Significant data gaps still exist for the western area of the Florida Keys National Marine Sanctuary and are a priority for future research.

A Comparison of Spatial and Movement Patterns between Sympatric Predators: Bull Sharks (Carcharhinus leucas) and Atlantic Tarpon (Megalops atlanticus)

Background Predators can impact ecosystems through trophic cascades such that differential patterns in habitat use can lead to spatiotemporal variation in top down forcing on community dynamics. Thus, improved understanding of predator movements is important for evaluating the potential ecosystem effects of their declines. Methodology/Principal Findings We satellite-tagged an apex predator (bull sharks, Carcharhinus leucas) and a sympatric mesopredator (Atlantic tarpon, Megalops atlanticus) in southern Florida waters to describe their habitat use, abundance and movement patterns. We asked four questions: (1) How do the seasonal abundance patterns of bull sharks and tarpon compare? (2) How do the movement patterns of bull sharks and tarpon compare, and what proportion of time do their respective primary ranges overlap? (3) Do tarpon movement patterns (e.g., straight versus convoluted paths) and/or their rates of movement (ROM) differ in areas of low versus high bull shark abundance? and (4) Can any general conclusions be reached concerning whether tarpon may mitigate risk of predation by sharks when they are in areas of high bull shark abundance? Conclusions/Significance Despite similarities in diet, bull sharks and tarpon showed little overlap in habitat use. Bull shark abundance was high year-round, but peaked in winter; while tarpon abundance and fishery catches were highest in late spring. However, presence of the largest sharks (>230 cm) coincided with peak tarpon abundance. When moving over deep open waters (areas of high shark abundance and high food availability) tarpon maintained relatively high ROM in directed lines until reaching shallow structurally-complex areas. At such locations, tarpon exhibited slow tortuous movements over relatively long time periods indicative of foraging. Tarpon periodically concentrated up rivers, where tracked bull sharks were absent. We propose that tarpon trade-off energetic costs of both food assimilation and osmoregulation to reduce predation risk by bull sharks.

An Extensive Deep Reef Terrace on the Tortugas Bank, Florida Keys National Marine Sanctuary

While most of the mapped area (137.5 km 2 ) of the Tortugas Bank consists of low-relief hard-bottom (105.5 km 2 or 77%) and scattered, rocky outcrops (16.6 km 2 or 12%), a sizeable portion of the western rim or platform edge (24 o 42.30’ N, 83 o 02.64’ W) is a well-developed reef terrace community (top panel). The topography of the substratum is very complex, owing to the numerous undercuts and caverns, as well as mushroom-shaped and plating corals up to 2 m in height. Coral cover is high (26.3 to 28.3% among three sites visited) relative to offshore reefs in the Florida Keys and is dominated by Montastraea faveolata, M. franksi, M. cavernosa, and Siderastrea siderea. The terrace community is a deeper version (22-27 m) of the reef terraces near Loggerhead Key 15 km to the southeast in Dry Tortugas National Park (Davis 1982). Anecdotal observations suggested that the western edge of the Tortugas Bank, locally named Sherwood Forest because of the predominance of mushroom-shaped corals (bottom panel), was spatially extensive. Using a combination of side-scan sonar, diver surveys, and bathymetry data, we estimate that the reef terrace is approximately 15 km 2 or about 10% of the mapped area of the Tortugas Bank. The “discovery” of this area by our science team was clearly preceded by the knowledgeable and active fishers of the region. Despite the remoteness of the Dry Tortugas relative to the Florida Keys, and the well developed reef structure, preliminary data indicate evidence of overfishing by a general absence of large species and individuals among exploited species, especially grouper (Serranidae) and snapper (Lutjanidae). Also, surprisingly few shark and barracuda were observed, and there was evidence of shrimp trawl damage to hard-bottom habitat. Because remoteness does not guarantee protection, the implementation of marine reserves in this region holds promise for restoration of fish stocks and protection of one of the largest areas of well-developed and previously undescribed coral reefs in Florida.

Digital echo visualization and information system (DEVIS) for processing spatially-explicit fisheries acoustic data

Abstract Spatially explicit analysis of fisheries acoustic data preserves heterogeneity observed in spatial distributions of fish. A software system — Digital Echo Visualization and Information System (DEVIS) — has been developed to process digital underwater acoustic data for spatially-explicit fisheries acoustic research. This system can be used to obtain spatial and temporal distributions of fish density, fish abundance, and fish lengths for management applications and for ecological modeling. DEVIS first reads digital data, corrects the data according to the sonar equation, discriminates individual targets, and vertically and horizontally integrates the data into a two-dimensional array of mean volume backscattering strength. Individual target information (TS, spatial location) is meshed with the volume backscattering array, and representative acoustic sizes are estimated in array cells with missing target information. Estimation methods for acoustic sizes and potential biases in abundance estimates are introduced and discussed. The final output is the spatial distribution of numeric density and fish length by length classes and for all fish. Data obtained on Lake Erie in September 1994 and on Chesapeake Bay in July 1995 were processed using DEVIS and are shown graphically. Steps required to process digital data are described and how these data can be applied to fish ecology is shown.

Transport and connectivity modeling of larval permit from an observed spawning aggregation in the Dry Tortugas, Florida

Large aggregations of adult permit (Trachinotus falcatus) were consistently observed since 2004 by divers in a collaborative fishery-independent reef fish visual census survey during May and June on the western-most edge of the Dry Tortugas Bank, Florida, in coral reef habitat, indicating proximal spawning sites. We investigated the possible fate and connectivity of larvae spawned at this location in the Dry Tortugas and two other published aggregation sites through a drift analysis using the ocean circulation and transport dynamics simulator HYCOM (Hybrid Community Ocean Model). New age-length data facilitated the determination of larval durations and rates of juvenile growth. Modeled larval transport data from spawning sites in the Dry Tortugas, Belize and Cuba were evaluated and compared to a spatially-extensive empirical juvenile permit data set from Florida. Our study revealed that unique oceanographic processes provided pathways for both downstream larval transport and juvenile retention, to and from Florida waters. These simulation results indicated that the Dry Tortugas region is a key source of permit recruits to southeast Florida stretching from the Florida Keys and up Florida’s east coast, and to a much lesser extent the west Florida shelf. Simulations from Belize and Cuba spawning sites revealed high local retention with low connectivity to Florida, emphasizing the importance of local resource management throughout the permit’s range.

Ocean Heat Content Reveals Secrets of Fish Migrations

For centuries, the mechanisms surrounding spatially complex animal migrations have intrigued scientists and the public. We present a new methodology using ocean heat content (OHC), a habitat metric that is normally a fundamental part of hurricane intensity forecasting, to estimate movements and migration of satellite-tagged marine fishes. Previous satellite-tagging research of fishes using archival depth, temperature and light data for geolocations have been too coarse to resolve detailed ocean habitat utilization. We combined tag data with OHC estimated from ocean circulation and transport models in an optimization framework that substantially improved geolocation accuracy over SST-based tracks. The OHC-based movement track provided the first quantitative evidence that many of the tagged highly migratory fishes displayed affinities for ocean fronts and eddies. The OHC method provides a new quantitative tool for studying dynamic use of ocean habitats, migration processes and responses to environmental changes by fishes, and further, improves ocean animal tracking and extends satellite-based animal tracking data for other potential physical, ecological, and fisheries applications.

Length-based risk analysis for assessing sustainability of data-limited tropical reef fisheries

&NA; This study extended a “data‐limited” length‐based stock assessment approach to a risk analysis context. The estimation‐simulation method used length frequencies as the principal data in lieu of catch and effort. Key developments were to: (i) incorporate probabilistic mortality and growth dynamics into a numerical cohort model; (ii) employ a precautionary approach for setting sustainability reference points for fishing mortality (FREF) and stock reproductive biomass (BREF); (iii) define sustainability risks in terms of probability distributions; and, (iv) evaluate exploitation status in terms of expected length frequencies, the main “observable” population metric. This refined length‐based approach was applied to six principal exploited reef fish species in the Florida Keys region, consisting of three groupers (black grouper, red grouper, and coney), two snappers (mutton snapper and yellowtail snapper), and one wrasse (hogfish). The estimated sustainability risks for coney were low (<35%) in terms of benchmarks for fishing mortality rate and stock reproductive biomass. The other five species had estimated sustainability risks of greater than 95% for both benchmarks. The data‐limited risk analysis methodology allowed for a fairly comprehensive probabilistic evaluation of sustainability status from species and community perspectives, and also a frame of reference for exploring management options balancing sustainability risks and fishery production.