VARIATIONS IN THE AGE AND GROWTH OF YELLOWFIN TUNA LARVAE, THUNNUS-ALBACARES, COLLECTED ABOUT THE MISSISSIPPI RIVER PLUME

Eight hundred and one yellowfin tuna larvae ranging from 2.57-7.48 mm SL were collected near the Mississippi River discharge plume in the Gu lf of Mexico during July and September, 1987. Larvae were most abundan t at intermediate salinities (i.e. frontal waters) where chlorophyll a and macrozooplankton displacement values were also highest. Using sag ittal otolith microstructure, we estimated larval ages ranging from 3- 14 d. These ages were used to back calculate spawning dates from 13-24 July and 22-31 August. Mean absolute individual growth rate (length a ge-1) was 0.47 mm d-1, with the least squares linear regression SL = 1 .67 + 0.47 AGE (r2 = 0.60, Pr > F = 0.0001) representing the best grow th curve. Highest growth occurred at intermediate salinities near 31 p arts per thousand, and temperatures near 29-degrees-C. There was signi ficant temporal variation in growth, with larvae collected in July gro wing slower than those from September (0.37 and 0.48 mm d-1, respectiv ely). The pooled instantaneous daily mortality rate (Z) of the larvae was estimated to be 0.33 d-1 (0.16 d-1 in July and 0.41 d-1 in Septemb er). These results show that significant spawning of yellowfin tuna ma y occur in the northern Gulf of Mexico in the vicinity of the Mississi ppi River discharge plume, and suggest that larval growth and survival may be enhanced in the plume frontal waters.