Observing, predicting, and controlling when it happens
At the November 5 TIC workshop, we discussed two of the biggest challenges of raising brook trout:
"Swim-up" is a stage that occurs when alevin become fully developed. Prior to that point, alevin have been living on the bottom of the breeder basket, but when they approach being 100% developed, they start making tentative movements towards the surface. Experienced fisheries biologists like Roxbury hatchery's Jeremy Whalen can recognize the fairly subtle cues that indicate the alevin have reached the swim-up stage (when they'll be called fry). But TIC teachers and volunteers, who observe only a comparatively small number of alevin and who raise trout just once a year, usually find it very difficult to spot the telltale signs of the swim-up stage. As a result, we need to use data to predict when fry will swim up.
And for this, we've created a data entry sheet and a special "swim-up calculator," which you can find here. Thanks to the efforts of Lorena Schwarz and with some help from Castleton University statistics professor Abbess Rajia, this new file is much improved over the one we demonstrated at the workshop. A quick note about the Excel file attached to the link above: it contains four worksheets. The first provides instructions, the second is used to PREDICT swim-up and to record water temperature data, the third will allow you to DETERMINE or CONTROL when the alevin swim up, and the fourth you won't need to use, it simply allows a formula to perform a "lookup" function. Below I've included a screenshot of the four tabs that give you access to the three worksheets you'll need to use.
(For those who want to refine their ability to spot signs of the swim-up moment, check out this Idaho TIC document.)
Predicting the swim-up stage
Development is a function of water temperature and time. With each day that hatchery or tank water is at a particular temperature, a constant is added to the previous day's cumulative Development Index (DI). This chart (below) shows the relationship between temperature (in tenths of a degree) and the constant that is to be added to DI.
So, for example, at 38.3 degrees, the value of 0.456 is added to whatever the cumulative DI was on the previous day.
When a school gets its eggs in January, enter the date you receive your eggs into cell B4 in the "mm/dd" format, e.g., "01/12" would represent January 12 (image below). Either hatchery staff or your egg deliverer will be able to tell you how developed the eggs are on that day, that is, what their Cumulative DI is. This number is very important and should be entered into cell C5 of sheet B.
From that point forward, all you need to do is enter the daily water temperature into the cell in Column B that corresponds to the correct date. The spreadsheet will automatically calculate the new Cumulative DI.
It is important to enter water temperature data for every day, including weekends and school breaks, even if you haven't checked the tank temperature. For those days when you don't visit the tank, estimate (or "interpolate") the water temperature. So if the temperature was 51 when you left on Friday afternoon and it was 49 when you returned to the classroom on Monday morning, enter a temperature of 50.5 for each of the weekend days.
As your Cumulative DI approaches 95, you should remove the front and top foam insulation (this will help stimulate the swim-up instinct), and you need to become very attentive. Look for evidence of alevin swimming to the surface. (You may want to revisit the above-mentioned "Idaho TIC document.") Alevin require careful monitoring at this stage and should be inspected several times every day, including weekends and school breaks. If some fry start swimming up, provide the tiniest pinch of food. Remove whatever is not eaten after ten minutes. When fry have begun eating reliably, you can feed them as often as five times a day, but always just the tiniest pinch.
And don't let the bottom of the net breeder get too dirty! (More on this later.)
Joe Mark, Lead Facilitator, Vermont Trout in the Classroom
In June 2012, I retired after 40 years in higher education, having spent the last 32 years of my career as dean at Castleton. One of the first things I volunteered to do in retirement was to work with Jim Mirenda to help the Dorset School, where his kids and my Vermont grandkids attend, start a TIC program. Gradually that commitment grew into my current role, which is both demanding and highly rewarding.