656 



Fishery Bulletin 98(3) 



Valdez 



Figure 1 



Study transects on Montague Island in Prince William Sound, 

 Alaska, where gill nets were used to collect fish predators of 

 herring spawn in 1995. 



number of eggs per g. By multiplying the wet weight of 

 herring eggs contained in each stomach by the number of 

 eggs per g, the total number of herring eggs in each stom- 

 ach was estimated. 



Based on stomach-content analysis, estimates of daily 

 ration were calculated only for greenling species. Because 

 of the small number of each greenling species caught, these 

 species were combined to estimate herring egg consump- 

 tion. Estimates of daily ration were calculated by using 

 the Elliot and Persson ( 1978) model. 



C, 



{S,-S,e-'")xRt 

 l-e"' 



(1) 



where C, 

 R 



t 

 S,, and S, 



= food consumption during daylight hours; 

 = the calculated gut clearance rate; 

 = the number of daylight hours; and 

 = average stomach contents at time and time 

 t, respectively. 



Estimates of stomach contents were obtained by examin- 

 ing fish caught during gillnet sampling and were assumed 

 to be constant over daylight hours. The gut clearance rate 

 iB) was calculated from a relationship of species evacua- 

 tion rate versus temperature for marine and freshwater 

 fish: 



/? = 0.01757-0.0442 (Worobec, 1984). (2) 



The average temperature tT) over the incubation period at 

 a Montague Island transect was used in Equation 2. We had 

 to extrapolate the equation in Worobec ( 1984) because their 

 temperature range was warmer than ours. Initial consump- 



tion of food at the onset of daylight was calculated accord- 

 ing to the Elliot-Persson model by using the equation 



n - Q _ c p-Rt 



^initial ~ "^average '^O'^ 



(3) 



where S^^,^, = the average eggs per stomach from gill- 

 net samples; and 



/ 



the nighttime hours. 



Adding C„„,,^,, and C, gives an estimate of the daily con- 

 sumption (Cy) of herring eggs over 24 h. Total consump- 

 tion over the incubation period was then calculated by 



C,o,al = C,xDxl 



(4) 



where C, = the calculated daily consumption in numbers 

 of eggs; 

 D = predator density; and 

 / = length of the incubation period in d. 



Two previous studies estimated nearshore fish abundance 

 on Montague Island. The first study estimated greenling 

 density by using SCUBA surveys (Jewett et al.-) and the 

 second study estimated nearshore fish biomass (Rosen- 

 thal^). We used both these estimates in separate calcula- 

 tions of Equation 4 to derive consumption estimates for 

 greenling. 



In the first calculation we used greenling density esti- 

 mates in the subtidal zone from Jewett et al. ( 1995); total 

 consumption per m-^ was compared directly with the aver- 

 age number of eggs per m^ estimated in 1995. To use 

 the biomass estimates from Rosenthal (1980), we changed 

 the daily ration in egg numbers to a daily ration in egg 

 biomass. Assuming isometric growth for greenling and 

 using the end points of greenling length and weight ranges 

 reported in Rosenthal (1980), we calculated the weight of 

 each greenling caught during gillnet sampling. Using these 

 two calculations for each fish sampled, we then estimated 

 the daily ration as a percentage of body weight. Then, 

 incorporating biomass estimates for greenling in Prince 

 William Sound from Rosenthal (1980) and the number of 

 incubation days for herring eggs in 1995, we calculated 

 the total weight of eggs consumed per km- by converting 

 Equation 4 to 



C,=BgX C,^, X /, 



(5) 



- Jewett, S. C.,T. A. Dean, R. O. Smith, M. Stekoll, L. J. Haldorson, 

 D. R. Laur and L. McDonald. 1995. The effects of the Exxon 

 Valde:: oil spill on shallow subtidal communities in Prince Wil- 

 liam Sound, Alaska 1989-93. Exxon Valdez Oil Spill Restora- 

 tion Project Final Report (Restoration Project 93047; subtidal 

 study number 2Ai, Alaska Dept. of Fish and Game, Habitat and 

 Restoration Division, Anchorage, Alaska. 178 p. 



■' Rosenthal, R. J. 1980. Shallow water fish assemblages in the 

 northeastern Gulf of Alaska: habitat evaluation, species com- 

 position, abundance, spatial distribution and trophic interac- 

 tion. In Environmental assessment of the Alaskan continental 

 shelf p. 451-540. Final reports of principal investigators, 17: 

 biological studies, NOAA/NOS, Office of Oceanography & Marine 

 Services. 



