Roger J. Zimmerman

Fish predation on juvenile brown shrimp, Penaeus aztecus Ives: The effect of simulated Spartina structure on predation rates

The effect of artificial Spartina structure on the predation rates of four estuarine fish on juvenile brown shrimp (Penaeus aztecus Ives) was examined under laboratory conditions. Vegetative structure reduced predation rates of pinfish and Atlantic croaker but did not affect predation rates of red drum and speckled trout. Pinfish and Atlantic croaker were inefficient predators, needing several strikes before successfully capturing prey. This inefficiency, necessitating repeated detection of prey organisms, probably contributed to the reduced predation rates by these fish in vegetated treatments. Although pinfish and speckled trout appeared to be strictly visual feeders, Atlantic croaker and red drum could apparently detect and feed upon shrimp through other sensory mechanisms. Differences in the mode of feeding among the fish, however, did not appear to be related to the effect of vegetative structure on predation rates. Over all of the experiments, predation rates on shrimp (50–69 mm) ranged between ≈2 and 13 shrimp·fish−1·day−1, and there was a positive relationship between the number of shrimp eaten and the size of the predator.

The importance of edge for natant macrofauna in a created salt marsh

The relationship between marsh edge and animal use was examined in a plantedSpartina alterniflora marsh located in the Galveston Bay system of Texas. A completely randomized block experimental design was used with each of four blocks containing a control and experimental sector. Marsh edge was increased through the construction of channels in experimental sectors. Channel construction had no detectable effect on marsh surface elevation. Effects of these simulated tidal creeks on habitat use were examined by sampling nekton at high tide with drop enclosures both on the marsh surface and within the channels. Crustaceans dominated the nekton, and use of the marsh surface in experimental sectors was significantly higher than in controls; densities of brown shrimpPenaeus aztecus, white shrimpP. setiferus, and daggerblade grass shrimpPalaemonetes pugio were 4.6 to 13 times higher near the channels. Polychaete densities in marsh sediments were also significantly higher near channels, and densities of decapod predators were positively correlated with densities of these infaunal prey. Thus, channel effects on natant decapods may have been related to the distribution of prey organisms. However, increased densities of natant fauna along the channel edge may simply reflect a requirement for departure from the marsh surface at low tide. Marshsurface densities of small bait fishes, bay anchovyAnchoa mitchilli and the inland silversideMenidia beryllina, also increased near channels, but highest densities of these fishes were in the creeks themselves. The abundance and distribution of juvenile blue crabsCallinectes sapidus and gulf marsh fiddler crabsUca longisignalis were not affected by the addition of experimental channels. Overall, the study results indicate that habitat value of created salt marshes can be enhanced by incorporating tidal creeks into the marsh design.

Densities of Penaeus aztecus, Penaeus setiferus, and Other Natant Macrofauna in a Texas Salt Marsh

Habitat-related densities of natant macrofauna were compared between vegetated and nonvegetated areas in aSpartina alterniflora marsh on Galveston Island, Texas. The most abundant macrofauna were crustaceans,Palaemonetes pugio, Penaeus aztecus, Penaeus setiferus, andCallinectes sapidus, and small fish,Gobiosoma bosci, Lagodon rhomboides, Leiostomus xanthurus, Fundulus similis andMicropogonias undulatus. Excluding residentsP. pugio, G. bosci andF. similis, most of the macrofauna were transient juveniles of estuarine-dependent species. Among crustaceans,P. pugio, P. aztecus, andC. sapidus were significantly more dense in vegetated habitat, butP. setiferus was not consistently more abundant in either vegetated or nonvegetated habitat. Of 29 species of fishes, 14 were usually in vegetation, 11 were more often on nonvegetated bottom, and 5 were indifferent to either habitat.Much seasonal variability in abundances ofP. aztecus, P. setiferus, andC. sapidus, but notP. pugio, could be attributed to changes in temperature, salinity and water-level. Strong selection for vegetated habitat byP. aztecus was related to the historical water-level pattern coinciding with seasonal periods of marsh flooding. Apparently, high seasonal tides during the spring and fall facilitated access to vegetated habitat in the marsh and exploitation by transientP. aztecus. In contrast, strong selection for vegetation byP. pugio, abundant year-around in the marsh, was not similarly influenced by seasonal changes in water-level. Overall, habitat-related densities and physical interactions suggest that marsh physiography together with differences in tides may greatly determine the extent to which certain estuarine macrofauna utilize marsh habitats.

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.

Carnivory vs. herbivory in juvenile Penaeus setiferus (Linnaeus) and Penaeus aztecus (Ives)

Abstract Postlarval Penaeus setiferus (Linnaeus), the white shrimp, and Penaeus aztecus (Ives), the brown shrimp, were reared for 24 days on vegetal, animal, and combination diets. Incremental weight change was used to compare shrimp growth rates among dietary treatments. Both species grew most quickly when fed a combination diet. Shrimp fed an animal diet also grew, but those fed vegetal diets either did not survive or grew very little. There were significant differences between the relative responses of the two species. Brown shrimp fed an animal diet of Artemia grew significantly more than did the white shrimp. Furthermore, the diatom Skeletonema was capable of producing growth in white shrimp that was similar to that produced by an Artemia diet until Day 20, but shrimp fed this vegetal diet did not survive until Day 24. There was no difference in the growth between white and brown shrimp fed a combination diet. The differences between the two species in their ability to utilize plant and animal material can be related to the resources available at their times of immigration.

The use of infauna by juvenile Penaeus aztecus Ives and Penaeus setiferus (Linnaeus)

Penaeus aztecus Ives, the brown shrimp, and Penaeus setiferus (Linnaeus), the white shrimp, co-occur in Texas salt marshes as juveniles. Although their life cycles are similar, evidence indicates that the species utilize different resources for the primary faunal element of their diets. Prey selection and growth studies have shown that brown shrimp successfully remove infauna from natural sediment. Further, a diet of polychaetes, whether alone or in combination with algae, produced growth in the species. Brown shrimp appear to be trophically linked to infaunal populations, thus the structure and dynamics of the benthic community may directly affect local brown shrimp productivity. Areas dominated by surface-dwelling polychaetes as opposed to deep burrowers may provide more accessible foraging opportunities for juvenile brown shrimp. By contrast, white shrimp neither removed infauna nor grew to a significant degree when provided polychaetes or amphipods as food. White shrimp are omnivorous but do not rely on infaunal material to the same extent as brown shrimp. The primary faunal element in the diet of white shrimp has not yet been identified. The dietary differences between the two species may play a role in determining which species dominates in regions with varying marsh accessibility. Although not the only factors influencing penaeid survival and growth, prey choice and availability may greatly affect production and local success of penaeid populations. *** DIRECT SUPPORT *** A01BY081 00013

Herbivory potential of postlarval brown shrimp associated with salt marshes

Abstract Laboratory feeding experiments were used to study the herbivory potential of postlarval brown shrimp ( Penaeus aztecus Ives). Plant materials fed to shrimp included Skeletonema costatum (Greville) Cleve, Isochrysis sp., Spartina detritus, and Spartina epiphytes. A total of 16 treatments were derived from all possible combinations of the four materials. After 16 days the greatest increases in length and weight occurred in those treatments containing Skeletonema costatum ( Skeletonema group) followed by Spartina epiphytes exclusive of the presence of Skeletonema (epiphyte group). Shrimp growth did not occur in beakers with (1) no food, (2) Isochrysis sp. or Spartina detritus alone or (3) Isochrysis and detritus in combination ( Isochrysis -detritus-no food group). Ecdysis occurred in all treatments, except where no food was present, including those in which there was no measurable growth. Survival and growth were significantly higher in the Skeletonema and epiphyte groups compared to the Isochrysis -detritus-no food group. These results indicate that planktonic diatoms, such as Skeletonema , and epiphytes of Spartina are potential sources of nutrition for postlarval Penaeus aztecus in Spartina marshes.