mechanism, if true, would explain the location 

 of many setting areas in tidal rivers above the 

 principal oyster gi-ounds. The theory has stimu- 

 lated a great deal of field observation, but, un- 

 fortunately, the experimental evidence upon 

 which it rests has not been fully documented. 

 Only a few laboratory observations have been 

 made on the effects of changes in current velocities 

 and salinity on the behavior of oyster larvae, and 

 the experiments reported by Nelson and Perkins 

 were performed luider the most primitive con- 

 ditions. Great experimental difficulties were in- 

 volved in conducting this type of study, and the 

 elaborate equipment necessary for recording larval 

 behavior was not available to the investigators. 



Observations on larval distribution in waters 

 other than New Jersey differ from those described 

 by Nelson and his associates. Prytherch (1929) 

 states that in MUford Harbor, Conn., "the oyster 

 larvae were found to be most abundant at the 

 time of low slack water and gradually disappeared 

 as the tide began to run flood." He further states 

 that no larvae could be found swimming in the 

 water when the flood current had reached a veloc- 

 ity of 0.6 foot per second, and supports this state- 

 ment by observations on oyster larvae kept in a 

 tank. Oyster larwie remained swimming in the 

 tank while the water was at a standstill, but 

 dropped to the bottom wlien the current velocity 

 produced by artificial circulation was from 0.3 to 

 0.5 foot per second. The experimental technique 

 was very primitive, and the results cannot be 

 considered convincing. Observations made by 

 Loosanoff (1949) in Long Island Sound do not 

 confirm Prytherch's interpretations. No evidence 

 was found that early and late umbo larvae were 

 common near the bottom. On the contrary, in 

 several instances "their number was greatest mid- 

 way between the high and low water when the 

 tidal current was near the maximum velocity." 

 A similar conclusion that larvae do not descend 

 during periods of ra])id tidal flow was reached by 

 Carriker (1959) from studies of conditions in a 

 salt-water pond on Gardiners Island at the eastern 

 end of Long Island, N.Y. 



Current views of tlic movement of oyster larvae 

 up estuaries were summarized by Carriker (1961). 

 The consensus of opinions of those who studied 

 the problem in typical estuaries indicates that 

 fully developed larvae (pediveligers) luive a tend- 

 ency to remain in lower, more saline strata and 

 are passively convej'ed toward the upper reaches 



370 



of an estuary by the net, nontidal flow of deeper 

 and denser layers of water (circulation in the 

 estuaries is discussed in Chapter XVIII, p. 402). 

 The discrepancy between the observations made 

 in New Jersey waters and in Long Island Sound 

 may be explained by differences in hydrography. 

 Long Island Sound is not an estuary in the strict 

 meaning of the term, but can be regarded as an 

 embayment with several true estuaries, as for 

 instance, the mouth of the Housatonic River, 

 Milford Harbor, New Haven Harbor, and many 

 others. The distribution of the larvae in the 

 vSound is not, therefore, comparable to that ob- 

 served in New Jersey waters. Further, salinity 

 change from surface to bottom is small, rarely 

 exceeding 27oo,and there is considerable exchange 

 of sea water between the Sound and the outside 

 waters. Under these conditions one may expect 

 substantial losses of larvae during a tidal cycle. 

 It is known that abundance of fully grown larvae 

 in the Sound area is so low that quantitative 

 sampling is not reliable. 



The evidence that oyster larvae are actually 

 conveyed by tidal current to the upper part of a C 

 tidal river is provided by the investigations of 

 Dimick, Egland, and Long (1941) on 0. lurida in 

 Yaquina Bay, Oreg. Yaquina Bay and River is 

 a short estuary, about 12 miles, on the coast of 

 Oregon. The natural oyster beds cover only 101.9 

 acres. Plankton samples taken systematically at 

 known distances from the mouth of tlie bay showed 

 that "up-river limit of the free-swimming larvae 

 was . . . approxinuitely 4 miles above the upper 

 limits of the natural oyster beds." No larvae 

 were found in this area at near low tide. There 

 is no doubt that these larvae were carried up- 

 stream by flood tide. 



Lack of agreement on the results of field ob- 

 servations on the relation of larvae to tidal stages 

 is the result of inadequacy of sampling techniques 

 and a lack of understanding the responses of the 

 larva to environmental changes. Changes in 

 temperature, salinity, current velocities, oxygen, 

 and food content of water vary in each estuary, so 

 the occurrence or absence of larvae camiot be 

 related to a given tidal phase unless the major 

 conditions during this stage of tide are fully 

 understootl and their effects on larvae are known. 

 The volume of water transported by ebb flow 

 in estuaries usually exceeds the volume of water 

 le-cntciing at flood, the difference being equal to 

 tiie volume of river discharge at the head. If 



FISH AND WILDLIFE SERVICE 



