Graduate Thesis Or Dissertation
 

Life history patterns of two allopatric clupeids : Atlantic menhaden (Brevoortia tyrannus) and gulf menhaden (B. patronus)

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  • Atlantic menhaden, compared to gulf menhaden, exhibit a life history pattern that appears to be adapted to a more unpredictable reproductive environment. From experiments on laboratory-reared eggs and larvae, Atlantic menhaden have larger eggs with larger yolk volume. They are larger at hatching, utilize their yolk faster, begin feeding at an earlier age, and are larger at onset of feeding. Egg size and the rate of morphogenesis from hatching to first feeding appeared to influence future larval size. When given ample food and reared at moderate (20°C) to high (24°C) temperatures, Atlantic menhaden were larger than gulf menhaden at 10-days past first feeding even though growth of both larvae was similar. As Atlantic menhaden undergo their first apparent transformation at a relatively smaller size, they will spend relatively less time in the earliest life history stage. A synthesis of published adult life histories also indicated that Atlantic menhaden life histories are adapted to a more unpredictable reproductive environment. They have a larger maximum body size and are larger than gulf menhaden at any specific age. They are older than gulf menhaden at the onset of sexual maturity, but have a longer reproductive span. Stock-recruitment models indicated that variations in environmental factors play a major role in determining the number of Atlantic menhaden recruits, whereas environmental factors play a lesser role in gulf menhaden recruitment. Atlantic menhaden populations, unlike gulf menhaden, exhibit density-dependent growth suggesting that food resources for pre-recruit Atlantic menhaden may be limiting when numbers are high. Exploitation of Atlantic menhaden populations has resulted in a truncation of the population's age structure. This has resulted in: a contraction of spawning in time and space; a truncation of their reproductive span, thus altering the net reproductive rate; a loss in fecundity, as fecundity is age-specific; and if egg size is related to age of fish, a modification of the egg size distribution within the population. Overexploitation of gulf menhaden resulting in alteration of life history traits could have serious consequences because the reproductive span of this menhaden is so brief.
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