Lawrence P. Rozas

Estimating Densities of Small Fishes and Decapod Crustaceans in Shallow Estuarine Habitats: A review of sampling design with focus on gear selection

Shallow estuarine habitats often support large populations of small nekton (fishes and decapod crustaceans), but unique characteristics of these habitats make sampling these nekton populations difficult. We discuss development of sampling designs and evaluate some commonly used devices for quantitatively sampling nekton populations. Important considerations of the sampling design include the size and number of samples, their distribution in time and space, and control of tide level. High, stable catch efficiency should be the most important grear characteristic considered when selecting a sampling device to quantify nekton densities. However, the most commonly used gears in studies of estuarine habitats (trawls and seines) have low, variable catch efficiency. Problems with consistently low catch efficiency can be corrected, but large unpredictable variations in this gear characteristic pose a much more difficult challenge. Study results may be bised if the varibility in catch efficiency is related to the treatments or habitat characteristics being measured in the sampling design. Enclosure devices, such as throw traps and drop samplers, have fewer variables influencing catch efficiency than do towed nets (i.e., trawls and seines); and the catch efficiency of these enclosure samplers does not appear to vary substantially with habitat characteristics typical of shallow estuarine areas (e.g., presence of vegetation). The area enclosed by these samplers is often small, but increasing the sample number can generally compensate for this limitation. We recommend using enclosure samplers for estimating densities of small nekton in shallow estuarine habitats because these samplers provide the most reliable quantitative data, and the results of studies using these samplers should be comparable. Many kinds of enclosure samplers are now available, and specific requirements of a project will distate which gear should be selected.

Oil Impacts on Coastal Wetlands: Implications for the Mississippi River Delta Ecosystem after the Deepwater Horizon Oil Spill

On 20 April 2010, the Deepwater Horizon explosion, which released a US government—estimated 4.9 million barrels of crude oil into the Gulf of Mexico, was responsible for the death of 11 oil workers and, possibly, for an environmental disaster unparalleled in US history. For 87 consecutive days, the Macondo well continuously released crude oil into the Gulf of Mexico. Many kilometers of shoreline in the northern Gulf of Mexico were affected, including the fragile and ecologically important wetlands of Louisianas Mississippi River Delta ecosystem. These wetlands are responsible for a third of the nations fish production and, ironically, help to protect an energy infrastructure that provides a third of the nations oil and gas supply. Here, we provide a basic overview of the chemistry and biology of oil spills in coastal wetlands and an assessment of the potential and realized effects on the ecological condition of the Mississippi River Delta and its associated flora and fauna.

Hydroperiod and Its Influence on Nekton Use of the Salt Marsh: A Pulsing Ecosystem

The salt marsh surface is not a homogeneous environment. Rather, it contains a mix of different microhabitats, which vary in elevation, microtopography, and location within the estuarine system. These attributes act in concert with astronomical tides and meteorological and climatological events and result in pulses of tidal flooding. Marsh hydroperiod, the pattern of flooding events, not only controls nekton access to marsh surface habitats directly but may also mediate habitat exploitation through its influence on other factors, such as prey abundance or vegetation stem density. The relative importance of factors affecting marsh hydroperiod differ between the southeast Atlantic and northern Gulf of Mexico coasts. Astronomical tidal forcing is the primary determinant of hydroperiod in Atlantic Coast marshes, whereas predictable tides are often overridden by meteorological events in Gulf Coast marshes. In addition, other factors influencing coastal water levels have a proportionately greater effect on the Gulf Coast. The relatively unpredictable timing of marsh flooding along the Gulf Coast does not seem to limit habitat utilization. Some of the highest densities of nekton reported from salt marshes are from Gulf Coast marshes that are undergoing gradual submergence and fragmentation caused by an accelerated rise in relative sea level. Additional studies of habitat utilization are needed, especially on the Pacific and Atlantic coasts. Investigations should include regional comparisons of similar microhabitats using identical quantitative sampling methods. Controlled field experiments are also needed to elucidate the mechanisms that affect the habitat function of salt marshes.

Salt Marsh Linkages to Productivity of Penaeid Shrimps and Blue Crabs in the Northern Gulf of Mexico

Secondary production derived from coastal marshes of the northern Gulf of Mexico exceeds that of other regions in the United States and is exemplified by large fishery catches of penaeid shrimps (Farfantepenaeus aztecus, F. duorarum, and Litopenaeus setiferus – 66% of U.S.) and blue crabs (Callinectessapidus −25% of U.S.). We believe that this production arises from coastal wetlands, and is driven by wetland geomorphology and hydrology resulting from the delta building and wetland loss cycles of the Mississippi River. Quantitative surveys document that high densities of shrimps and blue crabs directly use northern Gulf marsh surfaces. Manipulative experiments demonstrate that such marshes provide these fishery species with increased resources for growth and with protective cover to reduce predator-related mortality. Thus, access to the marsh surface is an important component in controlling the link between secondary productivity and coastal wetlands. Marsh access is influenced by tidal flooding patterns, amount of marsh/water edge, and extent of connections between marsh systems and the Gulf. Low-elevation Gulf marshes are flooded nearly continually during some seasons and are extensively fragmented; such characteristics provide maximum access. By contrast, U.S. Atlantic coast marshes have less fragmentation and less flooding. These geomorphic and hydrologic differences coincide with differences in secondary production between the regions, e.g., marsh-derived fishery production is lower on the Atlantic coast. Despite the linkage between coastal wetlands and secondary production, the current rapid loss of wetlands in the Gulf does not appear to be causing a decline, but instead is associated with an increase in fishery productivity. This paradox may be explained by changes in access and habitat function during areal loss of wetlands. Wetland loss is accompanied by increased marsh inundation and fragmentation, expansion of saline zones, and shortened migratory routes. These processes extend the utilization of remaining marsh and support temporary increases in secondary production.

A comparison of the diets of Gulf killifish,Fundulus grandis Baird and Girard, entering and leaving a Mississippi brackish marsh

We examined the diets of Gulf killifish,Fundulus grandis Baird and Girard, collected monthly from March through July 1988 with unbaited minnow traps during two sampling periods: (1) on flood tides before they reached the marsh surface, and (2) on ebb tides as they left the marsh. Thirty-five prey taxa, plant parts, and detritus were identified from the stomach contents of 110 Gulf killifish (mean SL = 55 mm, range = 30−82 mm). Fiddler crabs,Uca longisignalis Salmon and Atsaides; amphipods, mostlyCorophium louisianum Shoemaker; tanaidaceans,Hargeria rapax (Harger); and hydrobiids,Littoridinops palustris Thompson, were their most important prey. Killifish diets differed both quantitatively and qualitatively relative to the habitat in which they were feeding. Fiddler crabs and polychaetes were consumed more frequently and in greater numbers in the intertidal zone, whereas more amphipods were eaten by killifish feeding in subtidal and low intertidal areas. Gulf killifish consumed a greater volume of food when they had access to the marsh surface than when they were confined to subtidal areas.

NEKTON IN GULF COAST WETLANDS: FINE-SCALE DISTRIBUTIONS, LANDSCAPE PATTERNS, AND RESTORATION IMPLICATIONS

haany decapod crustaceans and fishes are common inhabitants of flooded salt marshes in the northwestern Gulf of Mexico, but spatial distributions are uneven, and pop- ulation sizes are difficult to estimate. We measured fine-scale (1-10 m) distributions of nekton on the vegetated marsh surface using enclosure samplers in Galveston Bay, Texas, and used these patterns to estimate population size. Natant decapod crustaceans were abundant in the marsh; densities of juvenile brown shrimp Farffiantepenaeus aztecus, white shrimp Litopenaeus setiferus, and blue crabs Callinectes sapidus were highest 1 m from the waters edge and declined rapidly to 10 m from the edge. We developed regression models to describe these fine-scale density patterns and validated the models with independent data on density dis- tributions from two other marsh systems. We used a Geographic Information System to transfer the density models to a natural marsh landscape; the highly fragmented Elmgrove Point marsh was composed of shallow nonvegetated bottom (37.4% of the area) and Spartina alternifora vegetation (62.6No) with 15No of the vegetated area within 1 m of the marsh-water interface. We estimated that this 437-ha salt marsh complex supported populations of 16.2 million brown shrimp, 15.5 million white shrimp, and 11.3 million blue crabs. We divided the marsh complex into 39 sectors and examined relationships between nekton populations and land- scape-scale patterns of marsh fragmentation. The amount of edge and the population estimates in a sector increased consistently with the amount of water up to 20-25No. Nekton population declines were not apparent until the late stages of marsh disintegration (>70No open water). We also used our fine-scale density models to simulate the effects of adding creeks to a 1- ha created salt marsh of solid vegetation. For shrimp, 1-2 creeks/ha (or a comparable amount of edge) were required to reach populations equivalent to shallow nonvegetated bottom, and up to 6 creeks/ha were required to reach populations similar to those estimated for the natural marsh complex. Simulated populations of blue crabs reached levels in the natural marsh complex with the addition of fewer creeks (2-3 creeks/ha).

The Impacts of Pulsed Reintroduction of River Water on a Mississippi Delta Coastal Basin

Abstract During the twentieth century about 25% of the wetlands of the Mississippi delta was lost, partially a result of isolation of the river from the delta. River diversions are being implemented to reintroduce river water to the delta plain. We synthesize here the results of extensive studies on a river diversion at Caernarvon, Louisiana, one of the largest diversions in the delta.

Nekton use of submerged aquatic vegetation, marsh, and shallow unvegetated bottom in the Atchafalaya River delta, a Louisiana tidal freshwater ecosystem

We sampled nekton (fishes and decapod crustaceans) in submerged aquatic vegetation (SAV) (Potanogeton nodosus, Najas guadalupensis), in emergent marsh vegetation (Sagittaria spp. andScirpus americanus), and over unvegetated bottom associated with three islands in the Atchafalaya River Delta, Louisiana. The purpose of our study was to quantify nekton densities in these major aquatic habitat types and to document the relative importance of these areas to numerically dominant aquatic organisms. We collected a total of 33 species of fishes and 7 species of crustaceans in 298 1-m2 throw trap samples taken over three seasons: summer (July and August 1994), fall (September and October 1994), and spring (May and June 1995). Fishes numerically accounted for >65% of the total organisms collected. Vegetated areas generally supported much higher nekton densities than unvegetated sites, although bay anchoviesAnchoa mitchilli were more abundant over unvegetated bottom than in most vegetated habitat types. Among vegetation types, most species showed no apparent preference between SAV and marsh. However, inland silversidesMenidia beryllina and freshwater gobiesGobionellus shufeldti were most abundant inScirpus marsh in summer, and blue crabsCallinectes sapidus were most abundant in SAV (Potamogeton) in spring. Several species (sheepshead minnowCyprinodon variegatus, rainwater killifishLucania parva, and blue crab) apparently selected the vegetated backmarsh of islands (opposite of riverside) over stream-sideScirpus marsh. Freshwater gobies, in contrast, were most abundant in streamsideScirpus marsh. Densities of juvenile blue crabs were high (up to 17 m−2) in vegetated delta habitat types and comparable to values reported from more saline regions of Gulf Coast estuaries. Shallow vegetated habitat types of the Atchafalaya River Delta and other tidal freshwater systems of the Gulf Coast may be important nursery areas for blue crabs and other estuarine species.

BROWN SHRIMP ON THE EDGE: LINKING HABITAT TO SURVIVAL USING AN INDIVIDUAL-BASED SIMULATION MODEL

In many coastal areas, natural habitats are being fragmented and lost to encroaching human development. These landscape changes can affect the production of recreationally and commercially important fisheries because many exploited species of fish and shellfish are estuarine dependent and utilize coastal marshes as nursery grounds. Brown shrimp are an example of a commercially exploited species that may be highly affected by changes in the spatial distribution of habitat types. We used a spatially explicit, individual-based simulation model to explore the role of marsh vegetation and edge habitat in brown shrimp survival. The model simulated shrimp movement, mortality, and growth of individual shrimp from arrival as postlarvae to 70-mm body length, when they emigrate offshore. Simulations were performed on 100 × 100 m spatial grid of 1-m2 cells, with each cell labeled as “water” or “vegetation”. Predation mortality was influenced by shrimp size, movement, and habitat. Simulated shrimp growth depended on temperature, habitat, and local shrimp density. We examined the relationships between shrimp survival and marsh attributes (amount of vegetation and edge habitat) by simulating a series of four habitat maps that we created from aerial photographs. Biological parameters were derived from published estimates and from field data. We corroborated the model by comparing the simulated shrimp abundance with summary statistics from long-term monitoring data, by comparing the simulated density with fine-scale patterns observed in field studies, and by comparing simulated and measured stable-isotope values. Surviving shrimp grew faster, moved less, spent more time in vegetation, and experienced slightly higher local densities than shrimp that died during the simulation. Habitat maps with more edge habitat invariably produced higher simulated shrimp survival rates. High-edge habitats increased survival by providing shrimp more direct access to vegetation without additional movement-related mortality and density-dependent growth costs associated with low-edge habitats. Model predictions were robust to higher numbers of initial postlarvae and to alterations to the movement rules. The results of this study suggest that the management of brown shrimp should be extended from protecting the spawning stock through catch regulations to also protecting the estuarine life stages through habitat conservation and restoration.

MARSH TERRACING AS A WETLAND RESTORATION TOOL FOR CREATING FISHERY HABITAT

Terracing is a relatively new wetland-restoration technique used to convert shallow subtidal bottom to marsh. This method uses existing bottom sediments to form terraces or ridges at marsh elevation. A terrace field is constructed by arranging these ridges in some pattern that maximizes intertidal edge and minimizes fetch between ridges; the intertidal area is planted with marsh vegetation. We examined the habitat value of terracing for fishery species at Sabine National Wildlife Refuge, Louisiana (USA) in spring and fall 1999 by quantifying and comparing nekton densities in a 9-yr-old terrace field and nearby reference area using a 1-m2 drop sampler. Decapod crustaceans were more abundant than fishes, composing 62% and 95% of all organisms we collected in spring and fall, respectively. White shrimp Litopenaeus setiferus, daggerblade grass shrimp Palaemonetes pugio, blue crab Callinectes sapidus, and brown shrimp Farfantepenaeus aztecus accounted for 94% of all crustaceans, whereas 60% of all fishes were gulf menhaden Brevoortia patronus. Mean densities of white shrimp (fall), daggerblade grass shrimp, blue crab, and brown shrimp (spring) were significantly greater in terrace marsh than on non-vegetated bottom in the reference pond. Densities of most nekton on non-vegetated bottom were similar in the terrace field and the reference pond, but gulf menhaden and white shrimp had higher densities at terrace pond sites and brown shrimp (spring) were more abundant at reference pond sites. The pattern for biomass was similar to that for density in that the mean biomass of most species was significantly greater at terrace marsh sites than reference pond sites and similar at terrace and reference pond sites. Terrace marsh, however, was not functionally equivalent to natural marsh, as mean densities of daggerblade grass shrimp (fall), brown shrimp (spring), and blue crab and mean biomass of white shrimp (fall), striped mullet Mugil cephalus (spring), and spotted seatrout Cynoscion nebulosus (fall) were greater at reference marsh sites than terrace marsh sites. Using these density and biomass patterns and the percentage of marsh and pond area in the terrace field, we concluded that terrace fields support higher standing crops of most fishery species compared with shallow marsh ponds of similar size. Future restoration projects could include design changes to increase the proportion of marsh in a terrace field and enhance the habitat value of marsh terraces for fishery species.