FISHERY BULLETIN: VOL. 82, NO. 1 



were common along with Oikopleura (gammarids, 

 Polyophthalmus pictus, and mysids). The amount of 

 dietary overlap between the two locations was not 

 considered significant. 



Although more abundant at the Point, a significant- 

 ly greater amount of Calanus sp. was eaten by black- 

 smith at the discharge than at the Point. A possible 

 explanation for the high usage of Calanus at the dis- 

 charge could be the increased susceptibility of 

 zooplankton to predation as a result of turbulent out- 

 flow. Entrained Calanus are more accessible to 

 planktivorous fishes, since the mortality rate of 

 copepods passing through a power plant may reach 

 70% (Carpenter et al. 1974). Dead or damaged 

 copepods would appear as viable prey upon dis- 

 charge from the plant and could be easily consumed. 

 Increased mortality from turbulence has also been 

 shown for other zooplankters (Gregg and Ber- 

 gersen 1980). 



There is evidence that alterations in plankton dis- 

 tributions at outfall areas are the result of upward 

 vertical displacement of deep-water organisms. 

 Evans (1981) noted that deeper living zooplankton 

 are carried vertically upward to the turbulent waters 

 over the discharge jets. Although analysis of 

 plankton sampled did not prove the existence of such 

 currents, in a previous study at King Harbor dye 

 injections were carried to the plume from bottom 

 water 20 m away from the discharge. 7 



Large gammarids, polychaetes, and juvenile 

 anemones, all of which were common in stomachs of 

 blacksmith from the discharge, are not normal con- 

 stituents of King Harbor plankton. The force of the 

 swirling effluent is strong enough to detach and 

 entrap these organisms from their normal habitat 

 inside and around the discharge pipe. Once 

 entrapped in the plume, these large invertebrates are 

 accessible to the planktivorous blacksmith. 



Zooplankton avoid predation through escape 

 movements upon detection of suction currents 

 created by predatory fish (Dreeneretal. 1978; Kettle 

 and O'Brien 1978). Once entrained in the effluent 

 plume, the ability of zooplankton to detect these 

 currents becomes impaired (Evans 1981). As a 

 result, fish frequenting the plume have the potential 

 for feeding on a high concentration of zooplankton 

 with limited predator avoidance. The greater 

 stomach weight and stomach fullness of blacksmith 

 feeding at the discharge support this theory. 



Results from other studies examining the feeding 



7 Kinnetic Laboratories, Inc. 1981. Hydrodynamic characteris- 

 tics of offshore intake structures. Field verification studies. Kin- 

 netic Labs., Inc., P.O. Box 1040, 1 Potrero St., Santa Cruz, CA 

 95061. 



habits of blacksmith appear to be similar to those 

 found at the Point. The food items consumed by 

 blacksmth at Santa Catalina Island are (listed in de- 

 creasing abundance) Oikopleura, calanoid and 

 cyclopoid copepods, fish eggs, cladocerans, and 

 other crustaceans (Hobson and Chess 1976). At 

 Naples Reef, off Santa Barbara, Calif., Bray (1981) 

 found the diet of blacksmith to consist of larvaceans 

 (Oikopleura), copepods, cladocerans, chaetognaths, 

 decapods, and polychaetes. In the two above- 

 mentioned studies and from the Point, blacksmith 

 consumed at least twice as many Oikopleura as any of 

 the other food items, while at the discharge, Calanus 

 was the most abundantly consumed prey and gam- 

 marids comprised the greatest volume of prey eaten 

 (Table 2). When Calanus, gammarids, mysids, and 

 the polychaete Polyophthalmus pictus are removed 

 from the analysis of the 10 most abundant prey con- 

 sumed, no significant difference was observed be- 

 tween the two locations (G — 9.4, n.s. atP= 0.05, df = 

 7). 



It has long been recognized that blacksmith forage 

 on plankton in areas where currents are present 

 (Limbaugh 1955, 1964; Feder et al. 1974; Ebeling 

 and Bray 1976; Hobson and Chess 1976; Bray 1981). 

 The tropical species of damselfish (family Pomacen- 

 tridae) also prefer feeding in areas where currents are 

 strong (Hobson and Chess 1978). Blacksmith have 

 been shown to prefer incoming currents (Limbaugh 

 1955, 1964; Ebeling and Bray 1976; Bray 1981), and 

 Limbaugh believed they materially affected the 

 amount of plankton entering the kelp beds. In Bray's 

 (1981) study, stomach fullness was greater in fish at 

 the incurrent end of the reef than in fish at the 

 excurrent end. 



Areas of strong currents are rich in zooplankters 

 (Hobson and Chess 1978) as is the discharge which 

 receives both entrained and entrapped organisms. 

 Although the discharge releases warm water, the 

 current created by the outflow is the major attract- 

 ant. Blacksmith, a species which prefers warm water 

 (mean preferred temperature = 14°-15°C), are found 

 in 26°-32°C discharge plume water, above their 

 upper temperature avoidance limit of 23°-25°C 

 (Shrode et al. 1982). In the presence of food, black- 

 smith will disregard their normal avoidance limits for 

 chlorine, intermittently present in most power plant 

 effluents (Hose and Stoffel 1980). 



It can be concluded that the outflowing effluent and 

 its related phenomena attract blacksmith to the dis- 

 charge. This theory is further supported by 

 documentation of similar attraction and rheotropic 

 behavior by blacksmith at an offshore water intake 

 structure (Helvey and Dorn 1981). 



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