TRENT ET AL.: ABUNDANCE OF MACROCRUSTACEANS IN MARSHES 



Jones (1963) caught grass shrimp in greatest 

 numbers during late winter and early spring and 

 pink shrimp in greatest numbers during spring 

 and early fall. Seasonal abundance patterns 

 reflected by catches in this study were similar to 

 those reported in Redfish Bay: for grass shrimp in 

 the marsh area; and for pink shrimp in the bay- 

 ward canal, marsh and bay areas during late 

 summer and early fall. 



Seasonal abundance of brown shrimp, white 

 shrimp, blue crabs, and pink shrimp deviated 

 from what we expected in the upland canal area. 

 These deviations were probably caused by low dis- 

 solved oxygen. During the period of low dissolved 

 oxygen (below 3.0 ml/liter; from 20 May to 12 

 August) in the upland canal area, mean catches of 

 brown shrimp dropped and remained below the 

 mean catches of brown shrimp in the other three 

 areas; mean catches of white shrimp and blue 

 crabs remained below mean catches of white 

 shrimp and blue crabs in the bayward canal and 

 marsh areas after 3 June. The abundance of pink 

 shrimp increased on 29 July in all areas except the 

 upland canal area and remained higher than in 

 the upland canal area until 7 September Grass 

 shrimp were not caught in large numbers in any 

 area except the marsh and therefore were not used 

 to evaluate the effects of low dissolved oxygen. 



DISCUSSION AND SUMMARY 



Indices of abundance revealed differences in 

 day-night distribution of brown shrimp, white 

 shrimp, blue crabs, and pink shrimp in the study 

 area. Assuming that our catch per unit effort data 

 provided an index which unbiasedly represented 

 density, migration of individuals of all four 

 species into the more shallow areas of the marsh 

 at night best explains these distributional differ- 

 ences. Inherent in the assumption that catch per 

 unit effort unbiasedly estimates density is the 

 equal vulnerability of the animals to capture dur- 

 ing both day and night. Factors which could make 

 this assumption invalid include: 1) burrowing or 

 swimming above the trawl by the animals during 

 one but not the other time period, and 2) avoid- 

 ance of the trawl during the day or night. Re- 

 gardless of the correctness of our assumption, the 

 importance of sampling during both day and 

 night to determine differences in abundance be- 

 tween areas was clearly shown. 



All five species were more abundant in the 



marsh than in the upland canal area during both 

 day and night. Brown shrimp, white shrimp, blue 

 crabs, and pink shrimp were more abundant in 

 the bayward canal area than in the upland canal 

 area. The distributional patterns of pink shrimp 

 and blue crabs in this study were similar to those 

 reported by Lindall et al. (1975), who provided 

 data showing that catches of blue crabs and pink 

 shrimp were highest in the bayward portion of an 

 upland canal in a housing development in Tampa 

 Bay, Fla. 



Four factors probably account for most of the 

 differences observed in abundance of shrimps be- 

 tween areas. Intertidal vegetation was perma- 

 nently eliminated by alteration of the natural 

 area for the housing development. Detrital mate- 

 rials and abundance of benthic macroinverte- 

 brates were lowest in the open bay area, low in 

 the upland canal area, and highest in the bay- 

 ward canal and marsh areas (Gilmore and Trent 

 1974). Eutrophic conditions observed represent 

 the fourth factor. 



Eutrophic conditions, indicated by the observed 

 low values of dissolved oxygen in the upland ca- 

 nals of the housing development during the 

 summer, probably account for the comparatively 

 low catches of brown shrimp, white shrimp, pink 

 shrimp, and blue crabs during that period. 

 Further evidence of eutrophication in this area 

 was provided by studies on: the American oyster, 

 Crassostrea virginica, in which setting, survival, 

 and growth rates were less in the upland canal 

 area than in the marsh area (Moore and Trent 

 1971); phytoplankton in which production was 

 higher in the upland canal area than in the 

 marsh or bay areas (Corliss and Trent 1971); and 

 benthic macroinvertebrates in which the abun- 

 dance of the organisms declined drastically dur- 

 ing the summer months in the upland canal area 

 (Gilmore and Trent 1974). Symptoms of eutrophic 

 conditions in the upland canals of the housing 

 development include inadequate water exchange 

 and high nutrient levels. These factors were dis- 

 cussed in detail by Moore and Trent (1971). 



Alteration of estuaries by dredging and filling 

 for housing developments and boat basins results 

 in an environment highly susceptible to recur- 

 rent low dissolved oxygen levels. This probelm of 

 low oxygen has been shown also in Forida (Taylor 

 and Saloman 1968; Lindall et al. 1973) and 

 California (Reish 1961). Stresses resulting from 

 low dissolved oxygen reduce the abundance of 

 crustaceans and other animals in the stressed 



199 



