Atlantic salmon kelt physiology: endocrine regulation of growth, metabolism and appetite

Last week, we successfully sampled 27 Atlantic salmon kelts from the Nidelva River and 4 from the Storelva River, both in South-Western Norway. As an interesting comparison, we also sampled 12 brown trout kelts from Storelva. Great big thanks to the sampling team Linda Hasselberg Frank and Jari Parkkonen from Gothenburg and invaluable local collaboration and assistance from Tormod Haraldstad, NIVA.

The main focus of my lab, the Fish Endocrinology Laboratory is, of course, endocrinology.

In this project we'll look at ciculating GH and IGF-I levels, the pituitary GH expression and the hepatic IGF-I expression, the GH receptor expression in liver and muscle, and probably also pituitary GH content, utilizing a protocol established by Benedet et al 2010 in which both GH content and gene expression can be assessed in the same pituitary.

The main focus is, however, on leptin. We'll measure plasma levels, hepatic and adipose lep gene expression, as well as the expression of several leptin receptor isoforms in various tissues, including the hypothalamus, where we'll also assess expression of several appetite-regulating neuropeptides.

While not ignoring protein and carbohydrates, lipid deposits are the most important energy source available to the up-migrating salmon.

Therefore we need to obtain a fairly accurate estimation of the total lipid content of the sampled fish, but also in which tissues the lipids are stored as well as how and when they are utilized.

We use the same approach as we've used earlier for studies on rainbow trout (Johansson et al 2016), by classical lipid extraction from various tissues sampled.

In addition, we're now for the first time testing to use the Distell.com Fish Fatmeter Model FMM-692, using the SALMON‐1 calibration (see manual) to get an estimation of total muscle fat.

All in all, we hope to get a picture of how muscle, liver, and visceral fat reserves are mobilized for a) metabolism and b) gonadal development, how large the remaining fat reserves are at spawning, and what energy is left for the kelt migration.

One of the fundamental challenges of this project is to be able to catch wild salmon at different stages of its anadromous spawning migration and its subsequent post-spawning down-migration as kelts.

Blood samples for assessment of hormone levels and tissue samples for measuring expression of genes linked to metabolism, appetite and growth need, of course, to be obtained from live fish.

In Sweden, we aim to do this in the Ume-Vindelälven river system with strong support from local researchers and other stakeholders.

I would be very interested in hearing from any of you who may already have samples (plasma, tissues) from kelts, or have suggestions about other locations where samples could be obtained.

This work is a part of a 3-year (2016-2018) research project titled "LEPTIN REGULATION OF APPETITE AND ENERGY HOMEOSTASIS IN SALMONIDS: TARGETED RESEARCH FOR CONSERVATION BIOLOGY AND AQUACULTURE" and is funded by the Swedish research council Formas.

The rationale for including kelt work in this project is the likely involvement of leptin in regulation of appetite and metabolism, both being physiological mechanisms of key importance for kelt survival. As explained in the project application, "Atlantic salmon enter a voluntary anorexic state during their spawning migration (Kadri et al 1995, Johansen et al 2011) and post-spawned fish may have lost 60% their total energy reserves (Jons­son et al 1997). Despite this, a key life-history strategy is to overwinter at the spawning grounds as “kelts” and to return to the sea the following spring. Although survival of first-spawned fish to a sec­ond spawning can be as low as 10% (Fleming and Reyn­olds 2004), the importance of repeat-spawners is being increas­ingly recognized as these large, high fecundity individu­als can significantly add stability to vulnerable populations (Halttunen 2011). Their size is also of great value for commercial and recrea­tional fisheries. Early attempts to capture and recondi­tion wild Atlantic salmon kelts in order to increase their survival rates have found the fish to be highly anorexic and unwilling to feed (Crim et al 1992), but the physiological and endocrine mechanisms for the onset and offset of the anorexic state and the metabolic mechanisms of long-term energy conservation are currently un­known".