GRAHAM: RETENTION OF LARVAL HERRING 



NET TIDAL EXCURSION (Kilometers) 



BOTTOM 



B. 



NET TIDAL EXCURSION (Kilometers) 



I OFFSHORE WIND »► INSHORE* 



21012 10123 3210123 7654321012345678 



Figure 4. — A. Vertical profiles of 

 mean tidal excursion for the four 

 sets of buoyed nets in the estuary 

 of the Sheepscot River (after Gra- 

 ham and Davis, 1971) . B. An effect 

 of onshore wind on the net tidal ex- 

 cursions for individual sets of nets 

 in the estuary. 



-10 



i: 



L 15 



Ll 



^20- 



\ 



K 



\ 



\ 



\ 



jfffn 



iiiiiiiiiiiiJiiiiiiiiiiiiiiiiiiiniiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiJiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiitiiiiiiiin^ 



BOTTOM 



would retain the larvae within the channel after 

 they had entered it on the net flood tide near the 

 bottom. 



The system of movements diagrammed in 

 Figure 3 is based on the average two-layer semi- 

 diurnal tidal flow (Figure 4 A) , But the dynam- 

 ics of estuarine flow can be changed by large 

 amounts of freshwater discharge, wind direction 

 and velocity, and the shape of the channel, Gra- 

 ham and Venno (1968) found that an increase 

 in freshwater discharge and the shape of the 

 channel could change considerably the catch of 

 buoyed nets in the Sheepscot estuary. Althoug'h 

 the nets were positioned in the experimental de- 

 sign to lessen such effects, three of the experi- 

 ments had results that could not be attributed 

 to the system of movements; they are unnum- 

 bered in Table 2. All three had interactions be- 



tween depth and tide and in one case (April 27, 

 1966) between location and tide as well. The 

 distribution of larval catches was opposite to 

 that expected. For instance, catch rates were 

 larger on the flood tide near the surface and 

 on the ebb tide near the bottom. Because tide 

 was involved in each case possibly these data 

 resulted from unusual tidal dynamics. On May 

 9, 1967, tidal excursions were modified by winds. 

 Winds blew landward and up the channel during 

 the day preceding the setting of the nets and 

 continued through the flood tide on the night 

 the nets were set. Wind velocities were usu- 

 ally 10 to 20 knots with gusts up to 30 knots. 

 Readings from flow meters showed a typically 

 shaped profile of net tidal excursion at the most 

 landward station (Figure 4B). This was pro- 

 gressively altered seaward until the last station 



303 



