658 



Fishery Bulletin 98(3) 



island points in shallow waters from 2 to 20 m deep on 

 Montague Island (Jewett et al., 1995). According to the 

 daily ration calculated above, daily consumption of her- 

 ring eggs by greenling would be 1065 eggs per m-. In 1995, 

 the incubation period for herring eggs was 21 d in Prince 

 William Sound; therefore total removal during incubation 

 would have been 22,373 eggs per m^. In 1995, the average 

 number of eggs per m'^ was 606,831, on Montague Island.'' 

 Therefore, using the daily consumption above, we esti- 

 mated that 3.77f of the eggs deposited were consumed by 

 greenling over the course of incubation. 



The weight of the estimated daily ration (11,984 eggs) for 

 greenling was 38.3 g. or weight of the eggs in a greenling 

 stomach multiplied by 1.29. From this conversion factor, 

 daily ration as a percentage of greenling body weight was 

 estimated as Q'/c per d. Results from dive surveys from 

 Rosenthal ( 1980) have shown a greenling biomass at Zaikoff 

 Point on Montague Island of approximately 35,000 kg/km-. 

 Multiplying this biomass estimate by the daily ration as a 

 percentage of body weight and by the number of incuba- 

 tion days in 1995, yields an egg consumption estimate of 

 44,100 kg/km2. In 1995, an estimated 5,922,673 kg of eggs 

 were deposited over a 3.05-km2 area on Montague Island.- 

 If one assumes that all greenling move into subtidal and 

 intertidal areas to feed on eggs deposited there, then green- 

 ling would have consumed 2.3'^ of the total eggs deposited. 



Discussion 



Calculation of daily ration by the Elliot-Persson method 

 presented here assumes that greenling maintain a full 

 stomach throughout the entire daylight period. The expo- 

 nential increase in eggs per stomach with greenling length 

 suggests that our assumption that greenling maintain 

 a constant state of gut fullness is correct because green- 

 ling stomach volume should increase exponentially with 

 body length. Our method for estimating total consump- 

 tion of herring eggs by greenling also assumes that there 

 is no numerical response to availability of eggs. If green- 

 ling move from deeper water to the band of herring eggs, 

 the estimate of consumption may be higher Migration 

 of greenling to the spawning beds from areas where no 

 spawn was deposited would also have increased the con- 

 sumption estimate. 



By sampling during slack tide periods, total consumption 

 of eggs may have been underestimated. If greenling were 

 actively feeding during the time when they encountered 

 gill nets, their stomach contents may not have been repre- 

 sentative of the entire daylight period. Greenling stomach 

 contents may have been less during times of active feeding 

 than during the rest of the day. Gill nets are also known 

 to be both size selective and species selective (Hay et al., 

 1986; Methven and Schneider, 1998). Selectivity, therefore, 

 probably influenced our results both on account of the nets 

 selecting for a small range of greenling sizes and selecting 

 only species that exhibited behaviors that made them sus- 

 ceptible to capture. If the average length of greenling cap- 

 tured during sampling was larger than the average length 

 of greenling in the total population, we may have over- 

 estimated the total consumption of eggs. However, the esti- 

 mate of total consumption of herring eggs is likely to have 

 been underestimated for Prince William Sound, because 

 the estimate is only for two species offish. Because green- 

 ling make up only 56*%^ by number and 599f by biomass 

 of the fish species at Montague Island (Rosenthal, 1980), 

 many other species inhabiting the zone covered by herring 

 eggs would have access to the rich food source the eggs 

 provide. Our results are similar to other studies in the 

 Atlantic where predation on herring eggs by fish has t3rpi- 

 cally been estimated at less than 10'7f of the total herring 

 eggs (Tibbo et al., 1963; Toreson. 1991). 



A concurrent study of egg loss in Prince William Sound 

 has shown that herring egg loss from spawning beds 

 increases at shallower depths (Rooper et al., 1999). Con- 

 sumption of eggs by bird species indicate avian predators 

 may be responsible for removals of large numbers of eggs 

 (27'7f ),'' most of which are lost in the intertidal zone. The 

 different levels of predation in the intertidal and subtidal 

 zones by birds and fish may be the underlying cause of 

 the higher egg loss rates observed at shallower depths. 

 If predation is an important factor regulating survival of 

 eggs to time of hatching, then herring behavior may lend 

 itself to depensatory mortality. Concentration of herring 



Wilcock, J., and K. Ilycr. 1998. Pensonal conimun. Alaska 

 Department of Fish and Game, RO. Box 669. Cordova, AK 99754. 



Bishop, M. A., and P. Green. 1998. Unpubl. data. 

 CRDI, PO Box 1460, Cordova AK 99574. 



USPS- 



