The Great Lake Sturgeon

Recognizing the Genetic Population Structure of Lake Sturgeon Stocks

n 79 AMY WELSH Recognizing the Genetic Population Structure of Lake Sturgeon Stocks Migration patterns and the presence of corridors play a large role in defining different stocks of fish. Groups of fish may be frequently migrating between different locations and may even be reproducing with the fish they encounter at the new location. On the other hand, fish may move around quite a bit when not spawning , but when it comes time to reproduce, they go to the same spot each time. It can often be hard to detect migration (with resulting reproduction at the new location) through field techniques. That’s where theories of population genetics can help. If populations are not connected through migration, then genetic differences can accumulate between them. This can occur in two primary ways. First, genetic differences may result from chance. Picture a bowl of M&M candies. The colors represent different variants of the same gene (called alleles). If you picked 20 M&Ms from the bowl to represent Population 1, calculated the allele frequencies, and then repeated the process for Population 2, you’d probably find that the allele frequencies would be different for the two populations. And this was due purely to chance. Now if your two M&M populations never mated with each other, then those allele frequencies would remain different throughout the generations. However, if they The Genetic Structure of Stocks 80 n Amy Welsh did mate with each other, eventually the allele frequencies would be the same in the two populations. Another way differences between populations can result is through natural selection. Certain alleles may produce a trait in a fish that is beneficial in terms of survival or reproduction. For example, in many bird species, selection will favor alleles that result in bright colors because that trait gives the individual reproductive advantages. If different traits are favored in different environments, then natural selection may result in allele frequency differences between populations. Migration would have little effect on these traits because migrants with a trait not suited for its new environment would feel the wrath of natural selection. So why do we care whether or not sturgeon move between spawning locations for reproduction? The main reason is genetic diversity. It is a good thing to have high genetic diversity. With high genetic diversity, natural selection has many genes at its disposal to give the species an evolutionary advantage. Environments change and it is critical species have the ability to adapt. If there are only a few alleles for a certain gene in a population, and none of them are doing the trick, evolution has to wait for a mutation to occur, and that can take a long time. Instead, if a population has lots of alleles, it is more likely that an allele already exists that is best suited for the current environment. Inbreeding (the mating of close relatives) can reduce the genetic diversity in a population. If populations are small and isolated, then they may have little choice but to mate with a relative. Royal families in the past who condoned this practice in order to maintain “royal blood” felt the consequences through numerous afflictions. Inbreeding increases the chances that an individual is going to have two copies of the same allele for a particular gene (i.e., homozygosity). For many alleles that result in detrimental traits, two copies of the bad allele are necessary in order to exhibit the trait. A common example is sickle cell anemia. Those with the disease get a copy of the bad allele from both their mother and their father. Inbreeding can have devastating consequences in many animal and plant species . Fish are no exception. In a study of the threespine stickleback, both survival and reproductive success were significantly impacted by one generation of brother-sister matings (Frommen et al. 2008). Inbred families of Atlantic salmon also had decreased survival compared to non-inbred families (Ryman 1970). Reduced survival and growth have also been observed in hatchery-reared Pacific salmon (Kincaid 1983). Genetic diversity may exist in a species as a whole through the maintenance of different stocks. If a fish species consists of several stocks representing different lineages, then the genetic differences between stocks contribute to the diversity of the species as a whole. By assessing the genetic connectivity of stocks, managers can maintain linkages between connected stocks. Stock differentiation may be The Genetic Structure of Stocks n...