AGING OF GULF MENHADEN, BREVOORTIA PATRONUS 



William R. Nicholson and William E. Schaaf^ 



ABSTRACT 



Length-frequency distributions, returns of tagged juveniles, and scale annuli indicate that over 97% of 

 Gulf menhaden, Brevoortia patronus, caught in the purse seine fishery are ages 1 and 2. Few fish 

 survive to age 3. About 50% of the fish examined for the years 1971-73 could be aged by scale annuli. 

 Those with no scale annuli or with indistinct or false annuli could be assigned to age 1 or younger or age 

 2 or older on the basis of length. 



When large numbers offish are routinely sampled 

 for age and size distributions, they must be aged 

 by some technique that consumes relatively little 

 time. One common method is to count scale annuli, 

 another is to group the fish by length frequencies. 

 Counting otolith rings usually is impractical be- 

 cause of the large amount of time and effort it 

 takes to collect, prepare, and observe the otoliths. 



As pointed out by Struhsaker and Uchiyama 

 ( 1976), "Attempts to age tropical fishes by conven- 

 tional methods have generally been thwarted by 

 the absence of well-defined annuli in calcarious 

 structures and protracted spawning periods which 

 make length-frequency mode progression analysis 

 difficult." In temperate regions where the winter 

 water temperature may not fall low enough to 

 cause a cessation of fish growth for an extended 

 period, aging fish by counting scale annuli may 

 also be difficult. 



Gulf menhaden, Brevoortia patronus, range 

 along the coasts of the United States and Mexico 

 from Florida to Yucatan. They spawn offshore in 

 the Gulf of Mexico from about October to April 

 (Suttkus 1956; Turner 1969). The eggs hatch in 

 about 48 h and the larvae are transported by on- 

 shore currents to estuaries, where they metamor- 

 phose into adult form (Fore and Baxter 1972). In 

 late summer the juveniles, ranging from about 45 

 to 120 mm fork length (FL), congregate in the 

 lower estuaries before moving to offshore waters 

 (Kroger and Pristas 1975). 



Gulf menhaden, the basic resource for a large 

 meal and oil industry, are caught exclusively in a 

 purse seine fishery extending from western 



'Southeast Fisheries Center Beaufort Laboratory, National 

 Marine Fisheries Center, NOAA, P.O. Box 570, Beaufort, NC 

 28516. 



Florida to eastern Texas. Processing plants, now 

 operating only in Mississippi and Louisiana, for- 

 merly operated in Florida and Texas also. During 

 routine sampling of the catch during the fishing 

 season, usually lasting from late April to October, 

 scales have been removed from, and weights and 

 fork lengths recorded for, about 13,000 fish annu- 

 ally since 1964. 



Aging these fish by conventional methods has 

 been a problem. Although some fish had well- 

 defined rings that appeared to be annuli, others 

 had no rings, or rings that were unclear or oddly 

 spaced. Length-frequency distributions indicated 

 two major age-groups with overlapping lengths, 

 and a third group that appeared in late summer. 



Since neither length frequencies nor scale rings 

 alone were satisfactory for aging all fish, ages sub- 

 sequently were based on a combination of factors: 

 appearance of scales, number and spacing of visi- 

 ble rings, and length of fish at the time it was 

 caught. This method of aging could be criticized as 

 being too subjective. But until returns offish tag- 

 ged at a known age, such as juveniles, were avail- 

 able there were no distributions of known ages to 

 which distributions of estimated ages based on 

 scale rings and lengths could be compared. 



A study to resolve the problem was not begun 

 until returns of juveniles tagged in late summer 

 and early fall 1970-73 became available. In 1975 

 we began a study of fish collected from 1971 to 

 1973. We choose those years because returns of 

 tagged juveniles of the 1970-73 year classes were 

 available, and we limited our material to 3 yr to 

 keep it manageable. Age-0 fish were defined as 

 young-of-the-year that would have no scale ring, 

 age 1 as those in their second year that should 

 have one ring, and age 2 as those in their third 

 year that should have two rings. 



Manuscript accepted August 1977. 

 FISHERY BULLETIN: VOL. 76. NO. 2, 1978. 



315 



