LARVAL TRANSPORT AND YEAR-CLASS STRENGTH OF ATLANTIC 

 MENHADEN, BREVOORTIA TYRANNUS 1 



Walter R. Nelson, 2 Merton C. Ingham, 3 and William E. Schaaf 2 



ABSTRACT 



A Ricker spawner-recruit model was developed for Atlantic menhaden, Brevoortia tyrannus, from data 

 on the 1955-70 year classes. The number of eggs produced by the spawning stock was calculated as the 

 independent variable to account for changes in fecundity due to changes in population size and age 

 structure. A survival index was developed from deviations around the Ricker curve and was regressed 

 on several environmental parameters to determine their density-independent effects. The recruit- 

 environment model accounted for over 84% of the variation in the survival index. Zonal Ekman 

 transport, which acts as a mechanism to transport larval menhaden from offshore spawning areas to 

 inshore nursery grounds, was the most significant parameter tested. Ricker functions for good and poor 

 environmental years were developed, indicating the wide range of recruitment that can be expected at 

 different stock sizes. Comparisons of spawner-recruit relations for Pacific sardine and Atlantic 

 menhaden indicated striking similarities. Surplus yield for the Atlantic menhaden fishery was cal- 

 culated from observed and predicted survival, and compared with the actual performance of the fishery. 



One of the more intriguing and important prob- 

 lems in fishery science, that of the relative 

 influence of spawning stock size and environ- 

 mental variation on year-class strength, has 

 resulted in a long-standing controversy among 

 fishery biologists. The two principal reasons for 

 investigating the effects of stock size and en- 

 vironmental change on year-class strength are, of 

 course, to understand what has happened and to 

 predict what will happen. Since environmental 

 conditions will produce varying recruitment at a 

 given stock size, one must determine both the 

 reproductive potential under average en- 

 vironmental conditions, i.e., the density- 

 dependent spawner-recruit curve, and the effect of 

 varying environmental conditions, or the 

 density-independent function. The difficulty 

 comes, as Clark and Marr (1955) point out, in 

 separating the relative influences of the two 

 functions. A prerequisite for such an attempt is a 

 reliable long-term series of data, adequate to 

 estimate the size of the spawning stocks, the 

 number of recruits, the age structure of the 

 populations, the patterns of environmental var- 

 iation, and the rate at which the resource is being 

 harvested. 



"MARMAP Contribution No. 88. 



2 Atlantic Estuarine Fisheries Center, National Marine 

 Fisheries Center, NOAA, Beaufort, NC 28516. 



3 Atlantic Environmental Group, National Marine Fisheries 

 Service, NOAA, Narragansett, RI 02882. 



Biologists are in general agreement that the 

 most critical survival period for many marine 

 fishes is during the time of egg and larval drift. 

 Major factors affecting survival during this period 

 are food (Cushing 1969), cannibalism by filter- 

 feeding parents (Radovich 1962; Murphy 1967), 

 and ocean currents (Sette 1943). The first two of 

 these factors are density dependent and tend to 

 control population growth. Transport by ocean 

 currents to or from areas favorable to survival is 

 density independent and has been used to explain 

 successful year classes of Atlantic mackerel by 

 Sette (1943) and Atlantic haddock by Walford 

 (1938). A relationship between winds and year- 

 class success for the East Anglian herring fishery 

 was reported by Carruthers (1938). Cushing 

 (1969) pointed out that ". . . correlations between 

 recruitment and winds were often successful for a 

 period of years, after which they failed catas- 

 trophically." 



Other density-independent factors, such as 

 temperature, particularly in the sense of long- 

 term climatic change, have been related to 

 changes in spawning success and location. For 

 example, a change in the environment of the Pa- 

 cific sardine over a period of time which resulted 

 in a change in normal distribution patterns and a 

 series of poor year classes was postulated by 

 Radovich (1962). Sissenwine (1974) documented a 

 significant relationship between atmospheric 

 temperature and the recruitment and equilibrium 

 catch of yellowtail flounder, but did not explain 



Manuscript accepted June 1976. 



FISHERY BULLETIN: VOL. 75, NO. 1, 1977. 



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