Gag (Mycteroperca microlepis)

The gag is a popular grouper species (Family:Serranidae) commonly fished from the west Florida shelf. Although this area is the focus of the fishery within the Gulf of Mexico, gag range along the Atlantic seaboard from Massachusetts to south Florida, and throughout the Gulf. Gag can range in size to over 100 cm (40 inches) and in excess of 16 kg (35 lbs). Several age studies have been conducted within the Gulf (McErlean 1963, Hood and Schlieder 1992, Johnson et al., 1993, Johnson and Koenig in Press, Fitzhugh et al. 2001) and South Atlantic (Manooch and Haimovici 1978, Collins et al. 1987, Harris and Collins 2000) and have generally found that the fishery focuses on ages 3 to 10.

Ages older than 10 years are much more rare but individuals with ages in the 20s have been sampled. Sexual maturity for gag is typically attained by about age 4 (over half mature) at sizes of 400-900 mm but gag are protogynous hermaphrodites and change sex with larger and older individuals becoming male. Males are often identified with a pigment change (denoted as "rusty"-or "copperbellys") and typically range in size from 800-1200 mm with age greater than 10 years. Although estimates vary due to sample selectivity and probable fishing effects, males may have historically comprised up to about 15% of the "mature"population and all individuals may be males at old ages (> about age 16). Aging methods and interpretations derived during earlier studies are still being used based on both sectioned- and whole otoliths (McErlean 1963, Hood and Schlieder 1992, Johnson et al. 1993).

Older gag (greater than about 8 annuli present) are difficult to age using whole otoliths and these otoliths are sectioned. Improvements in methods have generally focused on faster processing while maintaining quality control. When sectioning is necessary, the processing method of Cowan et al. (1995) is currently followed for species with larger otoliths including gag. Whole and sectioned otoliths are assigned an age based on the count of annuli (opaque zones observed with reflected light) and the degree of marginal edge completion.

For example, otoliths are advanced a year in age after January 1st if their edge-type was a nearly complete translucent zone. Typically, marine fish in the southeastern U.S. complete annulus formation (opaque zone formation) by late-spring to early summer.

Therefore an otolith with two completed annuli and a large translucent zone would be classified as age 3 if the fish was caught during spring in expectation that a 3rd (opaque) annulus would have soon formed. After June 30, when opaque zone formation is underway or complete for gag in the Gulf of Mexico (Hood and Schlieder 1992, Johnson and Koenig, in Press), all fish were assigned an age equal to the annulus count by convention. By this traditional method, an annual age cohort is based on a calendar year rather than time since spawning (Jearld 1983).

Click on a Picture Below for a Larger Image

distal surface of otolithFig. 2 Distal surface of a whole sagitta with 5 annuli (771 mm TL gag) sectioned sagitta Fig. 3 Sectioned sagitta from an older male (1266 mm TL) with an estimated age of 23 years 7 annuli Fig. 4 Dorsal edge of the distal surface of a whole sagitta exhibiting 7 annuli and a partially completed translucent margin (897 mm TL gag) 6 annuli Fig. 5 Dorsal edge of the distal surface of a whole sagitta exhibiting 6 annuli and having an opaque margin (763 mm TL gag) opaque margin Fig. 6. Sectioned sagitta showing 4 annuli and having an opaque margin (716 mm TL gag)

References

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Collins, M.R., C.W.Waltz, W.A. Roumillat, and D.L. Stubbs. 1987. Contribution to the life history and reproductive biology of gag, Mycteroperca microlepis (Serranidae), in the South Atlantic Bight. Fishery Bulletin, U.S. 85:648-653.

Cowan, J.H. Jr., R.L. Shipp, H.K. Bailey IV, and D.W. Haywick 1995. Procedure for rapid processing of large otoliths. Trans. Am. Fish. Soc. 124:280-282.

Fable, D. 1996. Weekly Trends and Conditions reports for the weeks ending...July 24, October 30, and November 13, 1996. Internal memoranda, National Marine Fisheries Service, Southeast Fisheries Science Center, Miami, FL.

Fitzhugh, G.R. L.A. Lombardi-Carlson, and N.M. Evou. 2001. Gag (Mycteroperca microlepis) age structure from the eastern Gulf of Mexico: 1991-2000. Panama City Laboratory Contribution Series 01-02.

Harris, P.J. and M.R. Collins. 2000. Age, growth and age at maturity of gag, Mycteroperca microlepis, from the southeastern United States during 1994-1995.

Hood, P.B., and R.A. Schlieder. 1992. Age, growth and reproduction of gag Mycteroperca microlepis (Pisces: Serranidae), in the eastern Gulf of Mexico. Bull. Mar. Sci. 51:337-352.

Jearld, A. Jr. 1983. Age determination. pp 301-324 In: L.A. Nielsen and D.L. Johnson (eds.), Fisheries Techniques, American Fisheries Society, Bethesda Md.

Johnson, A.G., L.A. Collins and J. Jeffery Isely. 1993. Age-size structure of gag, Mycteroperca microlepis, from the northeastern Gulf of Mexico. Northeast Gulf Science Vol. 13 (1):59-63.

Johnson, A.G. and C.C. Koenig. In Press. Age and size structure of the fishery and juvenile abundance of gag (Mycteroperca microlepis), from the northeastern Gulf of Mexico.

Manooch, C.S. and M. Haimovici. 1978. Age and growth of the gag, Mycteroperca microlepis, and size-age composition of the recreational catch off the southeastern United States.

McErlean, A.J. 1963. A study of the age and growth of gag, Mycteroperca microlepis (Pisces: Serranidae), on the west coast of Florida. Florida Board of Conservation, Marine Lab. Tech. Ser. No. 41. 29 pp.