cause of its large size and overall abundance 

 within the area and the latter because of its school- 

 ing behavior. Schools of i?. bonasus often destroy 

 large areas of eelgrass and other habitats in 

 search of clams, their primary food (Orth 1975, 

 1977). Burton (footnote 3) used hog wire fencing to 

 keep schools of cownose rays from his beds of in- 

 ventoried and replanted market size Mercenaria. 

 Because of the suddenness of the disappearance 

 (<2 wk) and the presence of crushed clam shell in 

 this and other plantings, we believe the most 

 likely predator was a school of/?, bonasus. 



Our data indicate that losses, due to such preda- 

 tion, would be unpredictable, but it would be 

 financially devastating to the clam grower. The 

 use of a fence or some other device to protect the 

 clams is essential for successful field culture in 

 areas where large predators occur. These fences 

 can be removed during the winter to prevent ice 

 damage, but along the Virginia coast they should 

 be kept in place and maintained at all times from 

 late March to early November. 



Literature Cited 



KRAEUTER, J. N., AND M. CASTAGNA. 



1977. An analysis of gravel, pens, crab traps, and current 

 baffles as protection for juvenile hard clams (Mercenaria 

 mercenaria). Proc. 8th Annu. Meet. World Mariculture 

 Soc, p. 581-592. 



MUSICK, J. A. 



1972. Fishes of Chesapeake Bay and the adjacent coastal 

 plain. In M. L. Wass i compiler), A check list of the biota 

 of lower Chesapeake Bay, p. 175-212. Va. Inst. Mar Sci. 

 Spec. Sci. Rep. 65. 



Orth, r. j. 



1975. Destruction of eelgrass, Zosfera marina, by the cow- 

 nose ray, Rhinoptera bonasus, in the Chesapeake 

 Bay Chesapeake Sci. 16:205-208. 



1977. The importance of sediment stability in seagrass 

 communities. In B. C. Coull (editor), Ecology of marine 

 benthos, p. 281-300. Belle W. Baruch Lib. Mar Sci. 6. 

 Univ. S.C. Press, Columbia. 

 RICH.ARDS, C. E., AND M. CAST.^GNA. 



1970. Marine fishes of Virginia's Eastern Shore (inlet and 

 marsh, seaside waters). Chesapeake Sci. 11:235-248. 



Tiller, R. e., J. B. Glude, and L. D. Stringer. 



1952. Hard-clam fishery of the Atlantic coast. Commer 

 Fish. Rev 14(10:1-25. 



JOHN N. KRAEUTER 



Michael Castagna 



Virginia Insititute of Marine Science and 

 School of Marine Science 

 College of William and Mary 

 Wachapreague, VA 23480 



A DIRECT METHOD FOR 



ESTIMATING NORTHERN ANCHOVY, 



ENGRAULIS MORDAX, SPAWNING BIOMASS 



Two methods exist for estimating spawning bio- 

 mass, the total weight of mature fish, from abun- 

 dance of spawning products. The first, or direct, 

 method (Saville 1963) consists of dividing an 

 estimate of egg production by the product of batch 

 fecundity and the proportion of females in the 

 mature stock. Saville safely assumed spawning 

 frequency to be unity. The second method is 

 indirect (Murphy 1966; Smith 1972) and utilizes 

 information from two different species. Smith 

 illustrated the second method, using information 

 on the Pacific sardine, Sardinops caerulea, and 

 northern anchovy, Engraulis mordax. Sardine 

 spawner biomass is estimated from landings data 

 and cohort analysis; anchovy spawner biomass is 

 estimated by multiplying the estimated sardine 

 spawner biomass by the product of the anchovy-to- 

 sardine ratio of larval abundance and the sardine- 

 to-anchovy ratios of fecundity, and spawning 

 frequency. Computation was facilitated by assum- 

 ing the unknown spawning frequencies to be 

 equal, making the ratio of spawning frequencies 

 unity. Up to the present only the second method 

 has been used for the northern anchovy. This 

 paper presents estimates derived from the first. 



Computation of spawning biomass is simplified 

 for the direct method when spawning occurs but 

 once and for the indirect method when both 

 species spawn with equal frequency. Difficulties 

 arise when spawning is continuous and when it 

 cannot be safely assumed that all mature fish 

 spawn with the same frequency. This is the case 

 with the northern anchovy. Spawning products 

 are present all year, with a maximum abundance 

 occurring in the late winter and early spring and 

 a minimum during late summer and early fall. 

 Abundance of and seasonal pattern of spawning 

 products give no clue as to the number of spawn- 

 ings by size and age, or even to the average 

 number of spawnings. 



Under the following conditions spawning fre- 

 quency can be estimated from examining the 

 spawming condition of females: 1) females can be 

 examined for a characteristic that indicates when 

 spawning takes place; 2) the length of tim.e such 

 a characteristic remains detectable can be esti- 

 mated; 3) the spavming rate remains relatively 

 constant over the sampling interval. 



The spawning fraction, or frequency, is the 



FISHERY BULLETIN: VOL. 78. NO. 2. 1980. 



541 



