FISHERY BULLETIN: VOL. 69, NO. 4 



EFFECT OF SALINITY, TEMPERATURE, 



AND TIDE ON THE DISTRIBUTION 



OF YOUNG MENHADEN 



SALINITY 



Larval menhaden, after entering: the lower 

 estuai-y, move upstream into lower salinities to 

 metamorphose. They seek the zone of the river 

 from 1 '/,, salinity to fresh water. This zone ex- 

 tends a short distance upstream from the inter- 

 face between fresh and salt water. Larval and 

 prejuvenile menhaden were most abundant in 

 this zone where metamorphosis occurs (Figure 

 4). They api)arently range into fresh water for 

 only a short distance since they were absent or 

 present only in small numbers in our samples 

 farther upstream. 



After the menhaden have transformed from 

 prejuveniles to juveniles, they appear to seek 

 higher salinity water. In late May when most 

 of the prejuveniles had metamorphosed, we 

 found juveniles in low-salinity water. As the 

 season progressed the juveniles were present in 

 the low-salinity water upstream, but they tended 

 to be more abundant in water downstream. 

 Schools of juvenile menhaden generally moved 

 out of the estuary in the fall. 



Both the position and length of the upstream 

 zone where young menhaden concentrated are 

 influenced by tidal excursion, rainfall, and wind 

 dii'ection and strength. A northeast wind causes 

 unusually high tides and pushes salt water far- 

 ther upstream than during normal flooding and 

 ebbing tides. 



We do not understand why low-salinity water 

 is important to young menhaden, but one expla- 

 nation is that they cannot metamorphose prop- 

 erly in either fresh or high-salinity water. 

 When larvae were held in salinities ranging from 

 15 to 40 ';, , about one-third of the flsh in each 

 salinity group develojied abnormalities of the 

 spine (Lewis, 19(i6). Juveniles may also con- 

 gregate in low-salinity areas at times because 

 food may be more abundant. 



In some estuaries turbidity may vary with sa- 

 linity. We measured the turl)idity at the Swans- 

 boro site in March, April, and May 1968 and at 

 our upstream sites during the summer of 19fi8 to 



determine if it afl^ected the number of young 

 menhaden caught by our net, and to see if there 

 was any relation between salinity and turbidity. 

 At Swansboro, where the water remained rela- 

 tively clear (light transmittance ranged from 89 

 to 96 ' r ) , no correlation existed between turbi- 

 dity and the catch of larvae. At our upstream tow 

 areas, where the water also was clear, light 

 transmittance ranged from 68 to 9.5 ""r . We also 

 found no relation between turbidity and catch 

 of young menhaden. We concluded therefore 

 that turbidity was too low to affect catchability 

 in the White Oak River. As there were no 

 marked differences between up- and downstream 

 turbidities, we concluded also that there was no 

 relation between salinity and turbidity. 



TEMPERATURE 



Larval menhaden are sensitive to low temper- 

 atures, particularly if the salinities are high or 

 low. They have the best chance for survival in 

 an estuary if the temperature remains above 

 4° C and the salinity ranges between 10 and 20 '/,c 

 (Lewis, 1966) . Below 4° C they sui'vive for only 

 a short time. Lewis (1965) determined that the 

 number of hours to 50";^ mortality at 2.0° C 

 varied from 3.2 to 38.5 hr depending on the ac- 

 climation temperature. 



We compared the temperatures in the estuary 

 during the 2-year study with temperature tol- 

 erances of larval menhaden determined in the 

 laboratory by Lewis (1965, 1966). The water 

 temperature in the White Oak River went below 

 4° C for several days in January and December 

 1968 and January 1969. Except for two sam- 

 pling days in 1968 and one in 1969, the water 

 temperature did not get over 10° C from the be- 

 ginning of January to mid-March ; it stayed at 

 2° C for 2 days during this period in 1968, and 

 1 day in 1969. We caught few menhaden larvae 

 during the jjeriods of low water temperature. 



Most larvae that enter the estuary before the 

 lethal cold water temperatures in the winter 

 probal)ly do not survive, while those that enter 

 during the late winter and early spring probably 

 remain in the downstream area because of the 

 colder water upstream. As the water warms 

 in the spring to above 10° C thev move towards 



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