Latitudinal Variation in Reproductive Characteristics of American Shad (Alosa sapidissima): Evidence for Population Specific Life History Strategies in Fish

Abstract

The reproductive characteristics of five populations of American shad (Alosa sapidissima) on the Atlantic coast were studied. The proportion of repeat spawners increased with the latitude of the home river. Relative and absolute fecundities decreased as the proportion of repeat spawners increased. These reciprocal trends in reproductive characteristics are independent of growth parameters. The principal factor influencing reproductive strategies in shad appears to be variability in the thermal regime of the home river which influences egg and larval survival. Northern populations, spawning in environments that are thermally harsh and variable, allocate a greater proportion of their energy reserves to migration thereby ensuring higher postspawning survival. This is accomplished by reducing the energy allocated to gonads. The pattern of reproductive responses of shad to the thermal environment of the natal river is consistent with existing ecological theory concerning the evolution of reproductive strategies in response to differing environmental conditions. Available literature for several other fishes suggests that fine tuning of reproductive strategies to local environmental conditions may be widespread among fish and may be the ultimate basis for the evolution of homing. Key words: American shad, Alosa sapidissima, reproductive strategies, fecundity, frequency of reproduction, energy allocation, latitudinal variation, theoretical ecology

A Theoretical Investigation of the Effect of Latitude on Avian Life Histories

“…Versions of our model incorporate explicit predation and a condition variable. Although we have investigated avian life histories, our approach is completely general and could be used to analyze examples of latitudinal trends in other groups, such as mammals (Lord 1960; Spencer and Steinhof 1968;Cockburn et al 1983;Temte 1993) and fish (Leggett and Carscadden 1978;Fleming and Gross 1990;Johnston and Leggett 2002;Heibo et al 2005). Of course, the details of the models would have to be changed to make them appropriate.…”

A Life-History Model of Human Fitness Indicators

“…This theory posits that that ability to allocate ones resources differentially in response to environmental condition is an adaptation allowing individuals to track and respond to environmental conditions, which will express itself phenotypically as individual within-species variation. Indeed, in a review of key life-history traits Rushton has shown that this sort of within-species variation does exist in diverse species such as; dandelions (Gadgil & Solbrig, 1972), field mice (Krebs, Gaines, Keller, Myers, & Tamarin, 1973), American shad (Leggett & Carscadden, 1978), as well as flies and milkweed bugs (Taylor & Condra, 1980).…”

Criteria for Determining Maturity Stage in Female American Shad, Alosa Sapidissima, and a Proposed Reproductive Cycle.

“…For comparative purposes, we also examined ovarian tissue from six specimens collected in the Edisto river near Jacksonboro, South Carolina, in March 1999, and 30 specimens collected in the Connecticut river at the Holyoke Fish Lift, Holyoke, Massachusetts, in May-June 2000. Populations of American shad in the Edisto river are semeparous while those in the York and Connecticut rivers are iteroparous (LEGGETT and CARSCADDEN, 1978 ;ASMFC, 1999). …”

“…Differences in the predictability of the reproductive environment, through its influence on juvenile mortality, are believed to regulate latitudinal patterns in life history strategies of anadromous fishes (LEGGETT and CARSCADDEN, 1978). Rapid drops in water temperature produced by the passage of cold fronts during the spawning season significantly decrease egg production, hatching success, growth, and survival of larvae (GLEBE and LEGGETT, 1981 ;SECOR and HOUDE, 1995 ;RUTHERFORD and HOUDE, 1995 ;MCGOVERN and OLNEY, 1996).…”

“…Along the Atlantic coast of the United States, spawning runs of American shad exist in approximately 193 rivers from the St Johns river, Florida, north to Atlantic Canada (BIGELOW and SCHROEDER, 1953 ;SCOTT and CROSSMAN, 1973 ;RULIFSON, 1994). Along the latitudinal gradient, populations of American shad may be either semelparous in southern rivers from Florida to North Carolina or predominately iteroparous in more northerly rivers (LEGGETT and CARSCADDEN, 1978).…”