Background

The large units in salmon farming require effective storage and transport, and, to minimize pellet breakage, pellets with high physical quality is demanded. However, as shown previously in this project, pellet quality may affect feed intake and feed utilization in fish. In the latter experiment, soaking feed increased feed intake.

It was hypothesized that the variation in feed intake when feeding feeds of different physical quality may, at least in part, be due to differences in gastrointestinal passage rate. In the present trial therefore, fish was fed either a dry (as is) or a soaked diet, and fish was sampled at given times after feeding to investigate the passage rate. 

Methods

Atlantic salmon of 1kg bodyweight was fasted until the gut was empty, and then fed one meal of either a dry feed (10 mm) or the same feed soaked in sea water for 2 h prior to feeding. The dry feed and the soaking procedure was the same as used in a previous trial. The fish was euthanized at 2, 6, 12, 18, 24 and 48 h after feeding, and the gut removed. The content was collected and divided in content from 1) stomach 2) pylorus and mid intestine and 3) hind gut. Material from three fish was pooled, and three pooled samples were collected at each time for both treatments (triplicate). The samples were weighed and analyzed chemically.

Results

There was unexpectedly large individual variation in gastrointestinal passage rate among the salmon. Two hours after feeding, significantly more of the feed (% of ingested dry matter) had moved from stomach to pylorus and mid intestine in salmon fed soaked feed compared to those fed dry feed .This shows that the gastric evacuation rate was increased when the feed was soaked. Several of the measurements indicated (non-significantly) that the soaked feed had a higher passage rate through the gastrointestinal tract than the dry feed. This information is valuable for optimizing feeding, and will be investigated further in a new trial.