We analyzed data on the four species that 

 were the most abundant to compare differences 

 in abundance between stations and between 

 day and night tows (fig. 18). Mean total lengths 

 of these species also were connpared between 

 natural and altered areas (fig. 19). 



Spot were much more abundant and slightly 

 longer in the natural than in the altered area. 

 In the natural area, spot concentrated at sta- 

 tions bordered by marsh vegetation and were 

 caught in greatest abundance at the stations 

 farthest from the bay (fig. 18). Catches in the 

 open bay (station 10) were similar to those in 

 the altered area. The largest catches in the 

 altered area were made in a small canal (sta- 

 tion 1) remote from the bay. Differences be- 

 tween day and night catches appeared random 

 when all stations were considered. 



More largescale nnenhaden were taken in the 

 altered than in the natural area, but average 

 lengths were slightly greater in the natural 

 area. Differences in abundance were not con- 

 sistent, however, when we compared catches 

 from individual stations. In general, the sta- 

 tions farthest from the bay--2, 3, 6, and 7-- 

 had the highest catches, regardless of the 

 area. More menhaden were caught at night 

 than during the day at all stations except sta- 

 tion 1. Differences may possibly be explained 

 by the ability of menhaden to escape the trawl 

 during the day. 



Brown shrinap were slightly more abundant 

 in the altered than in the natural area; mean 

 lengths of shrimp in the two areas were simi- 

 lar. Except for the low catches at station 2, 

 differences in abundance were not great be- 

 tween stations. Greater catches were made 

 at night than during the day at 7 of the 10 

 sampling locations. Apparently, the shrimp 

 burrowed in the day and were not as vulnerable 

 to the trawl as they were in the night when 

 they were on or above the bottonn. 



Atlantic croaker were considerably more 

 abundant in the altered than in the natural 

 area, but mean lengths were similar in the 

 two areas. All stations in the altered area 

 produced high catches, whereas in the natural 

 area, only station 6 produced high catches. A 

 tentative explanation for this distribution is 

 that Atlantic croaker prefer areas with a soft 

 bottom. Day catches did not differ from night 

 catches when all stations were considered. 



Environmental comparisons .-- Data for each 

 hydrographic variable measured in this study 

 were compared to determine variations be- 

 tween natural and altered areas, and between 

 stations (fig. 20). 



Water ten-iperature and salinity were similar 

 between areas and stations. 



Average values of oxygen were generally 

 higher, and fluctuations greater, in the natural 

 than in the altered area. These general dif- 

 ferences may reflect the station depths in the 

 two areas. The average sannpling depth was 



1.5 m. (5 ft.) in the altered area and 0,3 m. 

 (1 ft.) in the natural area. Photosynthesis and 

 tidal action probably were the primary causes 

 of the oxygen concentrations in the natural 

 area, whereas tidal action and vertical mixing 

 were the primary causes of the concentrations 

 in the altered area. 



The natural area had slightly higher average 

 values of phosphorus than the altered area. 

 The lowest values recorded in the altered area 

 were at station 5, and the highest values re- 

 corded in the natural area were at station 10. 

 Phosphorus values fluctuated independently 

 between stations and showed no general trend 

 within areas. 



Concentrations of Kjeldahl nitrogen varied 

 between stations and between areas. Average 

 values were lowest at stations 4, 5, and 10, 

 which were near or in the open bay. Inter- 

 mediate values were at stations 1, 2, 3, and 9, 

 and the highest values were at stations 6, 7, 

 and 8. Highest average values observed in the 

 natural area nnay have been caused by fresh- 

 water drainage fronn the adjacent land or the 

 interchange of waters (tides) between bayou 

 and marsh. 



Nitrite values varied between areas and 

 stations; lowest concentrations were at the 

 stations farthest from the bay. The bay was 

 either the major source area for nitrite or 

 basic productivity was lower in the offshore 

 water and the large populations of plankters in 

 the inshore areas utilized more of the avail- 

 able nitrite. 



Carbonate alkalinity varied between areas 

 and stations. Values were highest at the 

 inshore stations of the altered and natural 

 areas and decreased bayward. These concen- 

 trations may have been affected by drainage 

 of fresh water from the surrounding land or to 

 the variations in biological activity between 

 stations. 



Growth of oysters in natural and altered 

 areas . --In February we began a study designed 

 to compare the setting and growth rate of 

 oyster spat, and the growth rate of unattached 

 juvenile oysters in a natural and an altered 

 marsh. For setting and growth studies we 

 placed asbestos spat collectors and trays in 

 the natural area (station 6) and adjacent to a 

 bulkhead (station 1) in the altered area. 



To measure the set and growth of oyster 

 spat, we placed three pairs of plates (32 

 cm. 2 or 5 in.^ ) in the natural area and four in 

 the altered area. These plates were placed at 

 mean low-water level and at 0.3-m. (1-ft.) 

 intervals to the bottom on a suspended rack on 

 February 20, 1969. We replaced the spat col- 

 lectors every 2 weeks, and examined those 

 that were removed for oyster spat. No oysters 

 had set by May 14. 



We determined the growth rates of juvenile 

 oysters in each area by using transplanted 

 oysters from the 1968 spatfall in Galveston 



28 



