Changes in branchial and intestinal osmoregulatory mechanisms and growth hormone levels during smolting in hatchery‐reared and wild brown trout

Abstract

Evidence of smolting was studied in Danish hatchery‐reared brown trout Salmo trutta L. Twenty‐four hour seawater (SW) challenge tests (28‰, 10°C) at regular intervals showed that maximal hypo‐osmoregulatory ability developed within a 3–4‐week period in March and April. The improved ability to regulate plasma osmolality, muscle water content and plasma total [Mg] developed asynchronously, indicating that developmental changes in the gill, the gastrointestinal system and the kidney may not necessarily concur during smolting. Gill Na+, K+‐ATPase activity peaked in April at the time of optimal hypo‐osmoregulatory ability. Na+, K+‐ATPase a ‐subunit mRNA level in gills was unchanged from January until April, but decreased in May in parallel with a decrease in the activity of the enzyme. In the middle region of the intestine, Na+, K+‐ATPase activity increased in February and remained high until April. In the posterior region of the intestine, the activity was stable from January until April after which it decreased. In vitro fluid transport capacitity, Jv, in the middle intestine fluctuated throughout the spring. In the posterior intestine, Jv was low until late March, when it increased fivefold until early May. Drinking rate in fish transferred to SW for 24 h surged during spring. Na+, K+‐ATPase activity in the pyloric caeca was elevated from March until May, and increased in response to SW transfer in June, suggesting a hypo‐osmoregulatory function of the pyloric caeca. Plasma GH levels surged in FW trout during spring, concurring with the increase in gill Na+, K+‐ATPase activity and SW tolerance, but peaked in May when gill Na+, K+‐ATPase activity and SW tolerance were regressing. GH levels were generally low in SW‐challenged fish, and there was no consistent effect of 24‐h SW exposure on GH levels. In wild anadromous trout, gill Na+, K+‐ATPase activity varied seasonally as in hatchery‐reared fish, but peaked at higher levels suggesting a more intense smolting in fish living in their natural environment.

Intestinal fluid absorption in anadromous salmonids: importance of tight junctions and aquaporins

“…As shown for stenohaline SW fish, salmonids in SW display elevated drinking rates (Usher et al, 1988 ; Fuentes et al, 1996 ) in association with elevated intestinal fluid absorption (Collie and Bern, 1982 ; Veillette et al, 1993 , 2005 ; Kerstetter and White, 1994 ; Nielsen et al, 1999 ; Genz et al, 2011 ; Madsen et al, 2011 ). The major driving force and hence the first step in intestinal fluid transport (Figure 1 ) in salmonids is considered to be basolateral located NKA (Loretz, 1995 ; Veillette et al, 2005 ; Madsen et al, 2011 ).…”

“…During smoltification, the intestine, as well as the other osmoregulatory tissues, will pre-adapt for a life in SW, while the fish are still in FW. A developmental increase in drinking rates (Nielsen et al, 1999 ) and increased fluid absorption have been observed in the distal intestine at the peak of smoltification in Atlantic salmon (Veillette et al, 1993 ) and brown trout ( Salmo trutta L.) (Nielsen et al, 1999 ), whereas the proximal intestine does not seem to show the same increase (Veillette et al, 1993 ; Nielsen et al, 1999 ). Increased fluid absorption at the peak of smoltification is accompanied by increased NKA activity, which in Atlantic salmon is apparent in both proximal and distal intestine (Sundell et al, 2003 ).…”

Association Mapping Based on a Common-Garden Migration Experiment Reveals Candidate Genes for Migration Tendency in Brown Trout

“…The function of these pumps is well documented in salmonids ( e.g. , Zaugg 1982; Nielsen et al 1999; Larsen et al 2008) as they play significant role for osmoregulation. Similarly, PLEC plays a role in coping with osmotic stress (Osmanagic-Myers et al 2006) and has been shown to be upregulated in seawater-exposed whitefish Coregonus lavaretus (Papakostas et al 2012) while ABCF3 is differentially expressed in gills of freshwater or seawater exposed rainbow trout, demonstrating its putative role in acclimation to seawater (Leguen et al 2015).…”