DURBIN ET AL.: SWIMMING SPEEDS AND RESPIRATION RATES OF ATLANTIC MENHADEN 



period was greater with larger ration sizes (-4.5 

 h, experiment 4); at low food levels (e.g., experi- 

 ments 7, 9) the respiration rate had essentially 

 returned to baseline in <2 h (Figure 2). 



The mean swimming speed of the first two post- 

 feeding measurements (no. 6, 7) usually fell 

 within the range of the routine swimming speeds; 

 however, the respiration rates tended to be ele- 

 vated above the routine rates (Figure 5). There 

 was a linear relationship between mean swim- 

 ming speed and logj^ respiration rate in these 

 postfeeding measurements (Figure 5, Table 2). 



DISCUSSION 



These results demonstrate that the voluntary 

 swimming speed of adult Atlantic menhaden is 

 related to the availability of plankton food in the 

 water. If food is not present, the voluntary swim- 

 ming speed and respiratory rate of Atlantic 

 menhaden are low. Swimming speed increases 

 during feeding, following an approximately hyper- 

 bolic relationship with increasing plankton den- 

 sity in the water. Over a wide range of plankton 

 concentrations the characteristic swimming speed 

 of the menhaden is about 1.60 BL/s, with a res- 

 piratory rate of about 0.48 mg 02/g per h. This 

 approximately 5-fold increase in respiratory rate 

 above the routine implies that the energy expendi- 

 tures during feeding will be a major element in the 

 energy budget of menhaden. 



Previous descriptions of menhaden feeding 

 behavior (Durbin and Durbin 1975), based on 

 short-term experiments in which the plankton 

 concentration decreased during the course of each 

 experiment, were generally confirmed by the 

 present study. However, respiration rates mea- 

 sured in the present study indicate that the feed- 

 ing frenzy, which was observed in the earlier study 

 after a large amount of zooplankton was added to 

 the tank, is not likely to persist during prolonged 

 feeding in nature. Swimming speeds during the 

 frenzy were estimated to be about 2-2.5 BL/s. The 

 energy cost of swimming at these speeds, as esti- 

 mated from Equation 3 (Table 2), would be high 

 (10-23 times the routine metabolic rate) and would 

 not appear to be bioenergetically profitable over 

 prolonged periods. 



The same general behavior patterns have been 

 observed in five groups of Atlantic menhaden col- 

 lected during three summers. Such consistency 

 indicates that the behavior of the Atlantic menha- 



den in the laboratory can provide insight into 

 their behavior in the field. However, these labora- 

 tory based predictions of menhaden swimming 

 speeds need to be tested in the field. 



Satiation has often been observed to be an im- 

 portant feature which affects the feeding behavior 

 of fishes (i.e., Ivlev 1961). This is most evident for 

 "macrophageous" fishes (those which take their 

 food in large particles). In the present study, how- 

 ever, there was no evidence of satiation even with 

 the largest ration, when the fish consumed the 

 equivalent of 8% of their body weight during a 7-h 

 period. Evidence of satiation would include a de- 

 crease in their swimming speed during the exper- 

 iments or a switch to intermittent feeding. In con- 

 trast, the fish fed continuously at a constant rate 

 as long as food was available, and they continued 

 to search for food after it stopped coming into the 

 tank. Plankton densities sufficient to saturate the 

 physical capacity of menhaden to handle and pro- 

 cess food may not be of much ecological signifi- 

 cance. This is because natural plankton popula- 

 tions in the size range which can be filtered by 

 menhaden are seldom found in concentrations 

 greater than those used in the present experi- 

 ments. The high concentrations of chlorophyll a 

 which occur in coastal and estuarine waters dur- 

 ing the summer are primarily small flagellates 

 (e.g., Durbin et al. 1975), which are too small to be 

 retained on the gill rakers of the menhaden (Dur- 

 bin and Durbin 1975). 



There are comparatively few studies which have 

 simultaneously measured routine respiration rate 

 and activity. At very low swimming speeds, respi- 

 ration rate has been found to be linearly (Spoor 

 1946) and log linearly (Smit 1965; Muir et al. 

 1965) related to activity. In the present study, the 

 routine swimming speeds were clustered within a 

 narrow range (0.36-0.59 BL/s), and there was no 

 detectable relationship between respiration rate 

 and swimming speed. 



In all of the nonfeeding (initial, final, postfeed- 

 ing) measurements, the respiration rates were 

 higher than those which would have been pre- 

 dicted from the observed swimming speeds and an 

 extrapolation of Equation (3) (Table 2, Figure 5). 

 Thus the respiration rates of the fish when they 

 were not feeding, and therefore swimming slowly, 

 were higher per unit swimming speed than when 

 the fish were feeding and swimming more rapidly. 

 These results were consistent with previous 

 studies, in which routine metabolic rates also 

 tended to be variable, and elevated above those 



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