FISHERY BULLETIN; VOL. 69, NO. 4 



the low-salinity freshwater zone. Those that 

 enter later in the season, after the water has 

 warmed, move upstream in a shorter time. Once 

 menhaden had moved upstream, salinity and food 

 supiily probably affected their distribution more 

 than temperature. 



Since dead menhaden larvae do not float, we 

 probably would not notice if kills occurred when 

 the temperature droijped below 4° C. However, 

 we did observe many dead young and adult bay 

 anchovy (Anchoa mitchiUi) and pinfish (Lagod- 

 on rhomboides) floating on the surface on Jan- 

 uary 10 and 12, 1968, when the water temper- 

 ature was 2° C. Some of the floating fish revived 

 when placed in warmer water, but most did not. 

 We assume that many of the species present in 

 the estuary either died from or were subject to 

 cold stress. Thus any cold weather that occurs 

 during the time larval fish are present in the 

 estuary can have an important effect on the num- 

 ber of individuals surviving in the ])opulation. 



Although high water temperature does kill 

 juvenile menhaden, it did not appear to cause 

 any mortality in the White Oak River estuary. 

 In laboratory tests, juvenile menhaden died in 

 water temperatures above 33° C (Lewis and 

 Hettler, 1968). In the White Oak River the 

 temperature remained below 33° C except for a 

 short period when it rose to 34.1° C. 



TIDE 



Velocity of water current affects the abun- 

 dance and distribution of fish larvae in an estu- 

 ary. Bishai (1959) found that herring larvae 

 (6-8 mm total length) maintained themselves 

 in a current of 0.58 to 1.03 cm/sec and that at 

 higher velocities they drifted with the current 

 but at a rate less than the current. 



During our sampling in the field we noticed 

 that larval menhaden also held their jiositions 

 only in weak currents. The menhaden larvae 

 collected at the Swansboro bridge were larger 

 (10-30 mm total length) than the herring tested 

 by Bishai and seemed able to maintain their 

 position at velocities less than 10 cm/sec. Above 

 this velocity they were carried by the current. 

 One would expect, therefore, to obtain large 

 larval indexes during peak tidal flows at midflood 



Table 2. — Number of menhaden larvae per 100 m' of 

 water at hourly flood and ebb-tide stages, Swansboro, 

 N.C., February-March 1969. 



The ebb-tide stoge generolly lasted about 2 hr longer than flood. 



and at early ebb. Larval indexes during these 

 periods varied considerably, but in general were 

 larger than indexes during late ebb and early 

 flood tide stages (Table 2). The variability in 

 abundance indexes arises, in part, from day-to- 

 day changes in menhaden distribution in the low- 

 er estuary during the 2-month period. A 24-hr 

 study in March 1968 at Beaufort, N.C., showed 

 that larval abundance varied with the tide, cur- 

 rent, and time of day (Lewis and Wilkens, 1971). 



Tides affect the movement of larvae in and out 

 of an estuary as well as within an estuary. 

 Flooding currents carry larval menhaden into 

 the estuary where, before heading upstream, 

 they move back and forth with the changing 

 tides. At the Swansboro bridge station more 

 larvae were caught on flood tide than on ebb. 

 During February and March 1969, larval indexes 

 greater than 10 occurred in 81 'r of the sets made 

 on flood tide but only in 51 ""f of the sets on ebb 

 tide. June and Chamberlin (1959) reported sim- 

 ilar results at Indian River, Del. Some of the 

 larger catches of the season occurred on late 

 flood. The early hours of ebb tide had higher 

 larval indexes than late ebb. As more larvae 

 enter than leave, the number of larvae in the 

 estuary reaches a maximum by midsiiring. 



Tidal stage and current velocity affected the 

 catchability of larvae. In the lower estuary these 

 forces either concentrated the larvae in one lo- 

 cation or spread them over a large area. In the 

 u)iper e.stuary the location and width of the low- 



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