Models of alternative life-history strategies, population structure and potential speciation in salmonid fish stocks

1. High Arctic landlocked Arctic charr, Salvelinus alpinus L., are used as a reference system to examine the theory of alternative life history strate gies in Salmonids and their implication for population structure and potent ial speciation. In these closed, autonomous systems, Arctic charr are the o nly fish species present and alternative life history strategies are repres ented by two distinct size modes, commonly known as 'dwarf' and 'normal' ch arr. 2. Simple mathematical models are used to caricature the density-dependent interactions within and between the dwarf and normal components of these bi modal populations. Two different assumptions concerning the genetic relatio nship between the two alternative life history strategies are investigated: complete reproductive isolation, or a single stock which chooses between t he alternative strategies facultatively. 3. In constant environmental conditions, the evolutionarily stable strategy (ESS) is not affected by the assumed genetic relationship, and the ESS pro portions of dwarfs and normals are shown to depend on environmental paramet ers. The ESS proportions vary with these parameters along an evolutionarily stable continuum (ESC) and, in particular, a high proportion of normals is associated with high growth rate as a juvenile, and a 'stunted' or unimoda l population with low growth rate. The predicted ESC for selection of the n ormal strategy is similar to that observed experimentally in the analogous case of smelting in Atlantic salmon, Saline salar L. 4. A difference in the ratio of fecundity to mortality rate between dwarf a nd normal sub-populations is shown to be essential to maintaining bimodal p opulations, and it is argued that cannibalism is one mechanism which could achieve this difference. 5. The model is extended to ask what ecological conditions would favour evo lution toward the speciated or reproductively isolated state. It is conclud ed that short-term environmental fluctuations select against speciation and it is speculated that most natural bimodal Arctic charr stocks will achiev e maximum individual fitness within a 'semispeciated' configuration which i s maintained by incomplete assortative mating.