If you have journal access, see the paper here: Journal of Fish Biology
Much of our work in fish ecology and conservation revolves around understanding the tradeoffs fish face by occupying alternative habitats. For migratory fishes we would like to know how a given environment influences key attributes like growth and survival. Ideally, we could compare growth and survival among habitats. However, measuring these attributes is laborious, often requiring extensive tagging and recapture to identify individuals through time. For numerous, highly mobile species, recapture rates of tagged individuals are often dismal. Therefore, a single capture method to measure growth would allow huge advances in our understanding and comparison of habitats. Until recently, a single capture growth measure remained elusive.
Recent advances in blood plasma hormone assays allows us to measure the hormone directly responsible to stimulating cell division and somatic growth, insulin-like growth factor 1 (IGF1). IGF1 has a number of roles, and is also involved in maturation, masking its growth effects in maturing fish. However, in immature fish numerous laboratory studies have demonstrated the efficacy of plasma IGF1 concentrations as a proxy for recent growth (primarily in fish length).
In facultatively anadromous species (e.g. cutthroat trout, bull trout, Dolly Varden, brown trout, etc.), that is, those that may vary widely in age at ocean migration, or may exhibit partial migration, we often wish to understand the growth potential of going to sea compared to staying in freshwater. In this study, we compared plasma IGF1 concentrations of fish captured in three estuarine sites with fish found in river and lake habitats. We also used otolith (earbone) chemistry to identify differences in residence time at different locations.
We were surprised to find different growth rates among estuarine locations. Dolly Varden are highly mobile fish capable of moving among these locations in a matter of hours (most are just a few km apart), yet otolith chemistry indicates these fish remain near their site of capture for at least several weeks. Moreover, these same sites become de-watered at low tide, so fish must repeatedly move into each site at high tide. Less surprising was that growth in estuarine environments was higher than lake sites at nearly all month-site combinations. However, there was no site effect in the lake, with equally low IGF1 at all sites.
Finally, we found that fish found in the river between the lake and estuary were of recent freshwater origin, with low growth, early in the season; gradually shifting to fish of likely ocean origin, with higher growth, later in the season.
In summary, this is one of the first studies to use IGF1 to measure growth in a wild fish, and demonstrates both the fine-scale residence and growth differences in an estuarine fish.