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

Abstract

Summary High Arctic landlocked Arctic charr, Salvelinus alpinus L., are used as a reference system to examine the theory of alternative life history strategies in Salmonids and their implication for population structure and potential speciation. In these closed, autonomous systems, Arctic charr are the only fish species present and alternative life history strategies are represented by two distinct size modes, commonly known as ‘dwarf’ and ‘normal’ charr. Simple mathematical models are used to caricature the density‐dependent interactions within and between the dwarf and normal components of these bimodal populations. Two different assumptions concerning the genetic relationship between the two alternative life history strategies are investigated: complete reproductive isolation, or a single stock which chooses between the alternative strategies facultatively. In constant environmental conditions, the evolutionarily stable strategy (ESS) is not affected by the assumed genetic relationship, and the ESS proportions of dwarfs and normals are shown to depend on environmental parameters. 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 unimodal population with low growth rate. The predicted ESC for selection of the normal strategy is similar to that observed experimentally in the analogous case of smolting in Atlantic salmon, Salmo salar L. A difference in the ratio of fecundity to mortality rate between dwarf and normal sub‐populations is shown to be essential to maintaining bimodal populations, and it is argued that cannibalism is one mechanism which could achieve this difference. The model is extended to ask what ecological conditions would favour evolution toward the speciated or reproductively isolated state. It is concluded that short‐term environmental fluctuations select against speciation and it is speculated that most natural bimodal Arctic charr stocks will achieve maximum individual fitness within a ‘semispeciated’ configuration which is maintained by incomplete assortative mating.