Given that the amount and quality of available 

 information varies by species, by life stage, between 

 estuaries, and even within an estuary, considerable 

 scientific judgment is required to derive or infer 

 spatial and temporal distributions from existing data 

 and available literature. Unfortunately, even the 

 most informed judgment is far from perfect due to the 

 complexity of estuarine systems. Consequently, 

 information on the level of certainty associated with 

 each data element must be presented when 

 synthesizing multiple data sets (Table 4). Appendices 

 2, 3, and 4 provide a complete summary of the 

 personal communications and primary references 

 used so that readers can track and obtain additional 

 information efficiently. 



Variability in Space and Time. Species data were 

 organized according to the salinity zone boundaries 

 developed for each estuary in the NEI data atlas- 

 Volume 1 (NOAA 1985) and NEI Supplement 3 

 (Shirzad et al. 1 989) . However, division of an estuary 

 on the basis of salinity is highly variable due to the 

 many interacting factors that affect salinity, such as 

 variations in freshwater inflow, wind, and tides. To 

 compile information on species distribution according 

 to these zones, it is assumed that if a particular 

 salinity zone expands or contracts, the distribution of 

 a mobile species in that zone will correspond to the 

 shift. For example, if increased freshwater inflow 

 shifts the tidal fresh zone further down the estuary, 

 the distribution of a species confined to that zone 

 increases to include the new area. If a species 

 occurs over a wide salinity range, a shift may or may 

 not occur. This information was combined with 

 additional habitat parameters, such as bottom type, 

 to develop species distributional data. The final 

 placement of species in a salinity zone was ultimately 

 determined by where they have actually been 

 observed or captured. 



Temporal distributions are often dependent on annual 

 climatic conditions and water currents. Monthly 

 distributional patterns were derived based on the 

 consistent presence of a life stage within a particular 

 month. If a species is only present in an estuary in 

 unusual years (e.g. , drought), it was not portrayed as 

 part of that species' spatial or temporal distribution. 

 However, if a species is usually there, even during a 

 restricted time period, it was considered present for 

 the specific month(s). Greater temporal resolution, 

 such as on a biweekly rather than on a monthly basis, 

 was not possible. 



Abundance Data. Except for a relatively few important 

 commercial or recreational species, little or no 

 quantitative information is available to determine 

 relative species abundance for a large number of 



organisms across estuaries. Therefore, an attempt 

 was made to determine only the relative abundance 

 of a species compared to other species within an 

 individual estuary. For well studied species, e.g., 

 juvenile sciaenids or juvenile penaeids, quantitative 

 data were used to estimate abundance within an 

 estuary. However, in most cases the final level of 

 abundance assigned to a species was determined 

 by asking regional and local experts for opinions 

 based on their knowledge of individual species within 

 an estuary. This effort complemented the quantitative 

 studies, and greatly increased the relialDility of the 

 abundance information. It is important to note that 

 the LDWF has a quantitative computerized data 

 base on the distribution and abundance of several 

 species found in Louisana estuaries. The published 

 information from this data base was a component 

 used to develop the relative abundance information 

 shown in this report. The relative abundance 

 information shown in the data summaries of this 

 report is the best that could be synthesized from the 

 LDWF reports, other studies, and expert reviews. 



Salinity and Species Abundance. The Mississippi 

 Riverdivides the Central Gulf of Mexico boXh physically 

 and biologically. The estuaries east of the Mississippi 

 River (Mississippi Sound and Breton/Chandeleur 

 Sounds) generally have less freshwater inflow, higher 

 salinities, lowerturbidity, more seagrass habitat, and 

 are moreopentothe Gulf of Mexicothan the Louisiana 

 estuaries from theMississippi Riverwestward (Perrett 

 et al. 1 971 ). The lowest salinities and highest turbidity 

 are found in the estuaries directly affected by the 

 Mississippi and Atchafalaya Rivers. However, allot 

 the Louisiana estuaries have relatively low salinities 

 because of the enormous amount of freshwater 

 inflow from the large continental drainage. The effect 

 of varying salinity regimes is often reflected in the 

 distribution and abundance of estuarine and marine 

 species. 



Life History Notes. Because of the complex life 

 histories of some species, the following comments 

 are provided to clarify and supplement the infomnation 

 presented in the data summary tables. 



invertebrates. Sessile invertebrates, such as clams 

 and oysters, usually have a patchy rather than a 

 uniform distribution. Therefore, the areal distribution 

 of these organisms may be overestimated, but the 

 salinity zones of colonization are identified. Specific 

 areas may contain acceptable salinities, but suitable 

 bottom habitat forcolonization may not exist. Specific 

 habitat requirements and life history characteristics 

 of a number of invertebrate species are provided 

 below; 



12 



