In recent years fish stocking has become an increasingly important practice. Factors such as
climate change, habitat degradation and overfishing have driven the decline of many fish populations.
Fish stocking serves to provide stocks for recreational fishing, and serves as the backbone of the fish
farming industry, but can also play a crucial role in stabilizing wild fish populations helping to avoid local
and regional extinction. The increasing economic, recreational, and ecological role of fish stocking has
led to an increase in research on the physiological effects of stocking. At this point much of the
discourse in this area focuses on how chronic stress affects growth, information relevant in maximizing
recreational and fish farming yields. Yet this research limits itself to studies in current generations and
neglects to assess the long term transgenerational effects of stressors on existing fish populations, a
more valuable focus for conservation purposes.
There is however a small subset of research looking into the transgenerational effects of
maternal and paternal chronic stress. These studies focus on the effect of stocking density on key
physiological indicators of chronic stress such as, Cortisol Levels, IgM Levels, and serum lysozyme with
Cortisol being the primary focus. (Lund et al, S. Hinch, D. Montero). As the focus of these studies are
limited to the expression levels of these specific hormones and proteins there is a lack of information
regarding genetic processes behind these stress induced changes in expression and how any changes are
transmitted to later generations.
Research into different fish species has shown the regulation of cortisol in response to chronic
stress varies greatly between species, indicating an array of potential regulatory mechanisms at play
that may be unique to different clades and even to individual species. Cortisol, often used as the
standard to measure the effects of chronic stress, is seen to increase with varying levels to a plateau in
some species, whereas in others it increases only temporarily after which point it is then downregulated
likely to prevent or reduce the potential detrimental effects of a sustained overexpression.
Unfortunately, due to the inherent variation in transgenerational stress management strategies, model
organism may not be the most suitable for studying the mechanisms at play here. As such it is important
to investigate multiple species to try to determine whether a pattern can be found in transgenerational
transmission of modified stress response genes.
We believe a good place to start is by building a model around the basic reproductive strategies
observed in teleost fishes, semelparity and iteroparity. Semelparity describes any fish species which only
has one reproduction cycle in its lifetime, fish are observed to spawn once and then die. Iteroparity on
the other hand, describes any fish which have multiple reproduction cycles (birth events) through their
lifetime. Then there are some fish species in which some populations express iteroparity and others
semelparity based on the conditions in which they live, even though they are identified as the same
species. It is possible that the ways in which fishes reproduces effects the ways in which they manage
epigenetic inheritance.
We propose a model in which iteroparous fishes, with many chances to produce viable offspring
have fewer mechanisms in place to remove methylated histone proteins and cytosine base groups in sex
cells. On the other hand semelparous fish, with only a single chance to produce viable offspring, should
have more mechanism in place to regulate how methylated products are passed onto offspring. In order
to test this model we will focus on salmonids, comparing the effects of parental stocking density induced
chronic stress on the resulting histone and cytosine methylation patterns in offspring of O. merka
(Sockeye Salmon), O. mykiss (Steelhead Trout) and S. salar (Atlantic Salmon). The reason for choosing
these species is that they are all closely related, being salmonids, have similar migration patterns for
spawning, but differ in the number of times in which the reproduce. O. Merka are completely
semelparous, O. mykiss have some semelparous and some iteroparous population, and lastly S. salar are
an iteroparous species.
The question this study aims to answer is whether the reproductive strategies of salmonid species is
correlated with different degrees of regulation of the transmission of methylation associated with
cortisol regulatory elements. We predict that S. salar, being entirely iteroparous will have the least
regulation and as such offspring methylation patterns will very closely resemble that observed in the
parents cortisol regulatory regions and elements. O. Mykiss, a species which can express both
iteroparity and semelparity will have slightly more regulation as producing viable offspring the first time
become an increasingly significant concern. As such there will be some notable differences between the
methylation patterns observed in O. Mykiss cortisol regulatory regions of parents and their offspring.
Lastly O. merka, being an entirely semelparous species will have the greatest difference between
methylation patterns observed in parents who experienced chronic stress and their offspring.