DIEL FORAGING ACTIVITY OF 



AMERICAN EELS, 



ANGUILLA ROSTRATA (LESUEUR), 



IN A RHODE ISLAND ESTUARY 



Although the American eel, Anguilla rostrata 

 (LeSueur), is abundant and commercially exploited 

 along the entire Atlantic coast of North America, 

 its basic biology is not well understood (Tesch 1977; 

 Fahay 1978; Helfman et al. 1984). Foraging activity 

 has not been studied. Helfman et al. (1983) examined 

 daily movement patterns in an estuary and found, 

 as had laboratory studies (Bohun and Winn 1966; 

 Edel 1976; van Veen et al. 1976; Westin and Nyman 

 1979), that American eel locomotor activity is noc- 

 turnal and suggested that American eel foraging 

 activity is also nocturnal. This study sought to 

 describe the diel foraging patterns of wild estuarine 

 American eels by monitoring capture rates in baited 

 eel traps on a 24-h basis. 



Eight eel traps were set 10 m apart along a 

 transect in a tidal portion of the Pettaquamscutt 

 River estuary, R.I. The water was turbid (the bot- 

 tom could not be seen at midday in areas <1 m deep) 

 and the salinity ranged from 20 to 30%u, depend- 

 ing on the tide. Cylindrical traps are commercially 

 constructed of 0.64 cm 2 wire mesh and are 78 cm 

 long and 20 cm in diameter with two single funnel 

 openings of 5 cm in diameter. The traps were baited 

 with 500-700 g pieces of freshly killed horseshoe 

 crab, Limulus polyphemus, an effective eel bait 

 (Bianchini et al. 1981). 



Capture rates probably reflected contempor- 

 aneous foraging because the traps were thought to 

 have a high escape rate. A high escape rate was 

 suspected for two reasons: 1) When we changed 

 from checking the traps once every afternoon to 

 once every 3 h, the daily capture rate increased near- 

 ly 50 fold; and 2) when 40 eels were placed into 4 

 unbaited traps in the river, only 1 eel remained 24 

 h later. Feeding activity in the traps was evidenced 

 by several factors: an examination of the gut con- 

 tent of 10 captured eels found 6 to contain horse- 

 shoe crab and the rest to be empty, anesthetized 

 animals frequently regurgitated bait, eels were often 

 found burrowing in the bait, and unbaited traps rare- 

 ly caught anything. 



Starting at 1200 e.d.t., traps were checked and 

 rebaited at 3-h intervals for six 24-h periods evenly 

 spaced over a 15-d span in early September 1982. 

 This design removed any possible tidal influence 

 because the lunar tidal period is 14.8 d. Within 10 

 min of their capture, eels were released 10 m to one 

 side of the transect' s center point. Traps were 



rebaited every 6 h or whenever the bait was found 

 to have been consumed (which rarely occurred). 

 Baiting schedules were designed so that every other 

 trap was rebaited at each 3-h check, and all portions 

 of the crabs (heads and tails of both males and 

 females) were equally distributed with respect to 

 time and location. A total of 322 American eels were 

 captured (some were probably recaptures): 178 (55% 

 of the total) were caught just after sunset at 2000 

 e.d.t., 140 (44%) were caught during the remainder 

 of the night, and 4 (1%) were caught during daylight 

 (Fig. 1). Although daily capture rates were variable 

 and ranged from 113 to 22, all exhibited this pattern. 

 To determine when foraging activity commenced, 

 the traps were checked and rebaited at 30-min inter- 

 vals between 1715 e.d.t. (40 min before sunset) and 

 2015 e.d.t. for 6 evenings during a 15-d period in 

 early October. Eels were consistently first captured 

 just after sunset, with captures peaking 1 h after 

 sunset and declining thereafter (Fig. 2). Daily cap- 

 ture totals varied considerably but all exhibited this 



60- 



50- 



X 



o 



I- 



< 40 

 O 



_J 



< 



h- 30H 



I- 

 z 



w 20 



o 



UJ 



a. 



10- 



N = 322 



11 14 17 20 23 2 

 TIME OF DAY 



-1 

 11 



Figure 1.— Percentage of total catch of American eels by time 

 of day for the 24-h experiment. The histograms cover the time 

 between checks; i.e., their right boundaries mark the times when 

 traps were checked. The bold section of the x-axis denotes the 

 period between sunset and sunrise. 



746 



FISHERY BULLETIN: VOL. 84, NO. 3, 1986. 



