Friday, April 1, 2016

Turning predators into prey

Keith Greenwell had a question about the status of tiger trout in Meadow Creek Reservoir. Excellent question, Keith. Thanks for the idea. Let me back up and give you the recent history of Meadow Creek.
     I've got several waters that contain classic examples of stunted brook trout populations, and Meadow Creek is at the top of that list. How do I know if a population of brookies is stunted? Let me explain.
     I have surveyed the fish population at Meadow Creek three times in recent years: in 2007, 2011, and 2015. Up until 2007, we were stocking small (1.5" or so) cutthroats and catchable, put-and-take rainbows there, on top of what we knew to be a robust wild brookie population. One of the main questions I wanted to answer in the 2007 survey was whether or not the cutthroats were doing anything. As is almost always the case when you stock cutthroats on top of a dense brook trout fishery, in 2007 I found that the cutthroats weren't persisting at all. Because we don't stock cutthroats with the purpose of feeding brook trout, I immediately pulled them off of the stocking schedule for Meadow Creek. So, in addition to not finding any cutthroats, here is the size structure of the brook trout population that I observed in 2007:

     Very few fish larger than 9", and no fish over 12".  Below is the relative weight plot of the 2007 fish. If you've read any of my other posts hopefully you understand what relative weight is - a measure of "plumpness" on a scale of 100. It can be thought of as a grade of body condition.  What I saw in 2007 was modestly declining relative weights as the fish got larger, as indicated by the trend line on the graph below. 

     The X-axis in this one is size of fish in mm; I didn't convert to inches. Taken together, these two pieces of information - no large fish and declining body condition as the fish get larger - are strong evidence of a stunted population. That is, there are more fish than there is food available, and so their growth slows down as they get larger and they are having trouble maintaining or adding to their body weight. 
     Now fast forward to 2011. I don't get to lakes like Meadow Creek every year - it's not necessary. Every few years is fine unless I'm trying to accomplish something specific, as we'll see later. What was different in 2011 compared to 2007, is that now the lake had gone four years without the annual plant of 16,000 1.5" cutthroats that it had recieved through 2007.  Here is the size structure of the brook trout population in 2011:

     Now we've got lots of fish at the 7" mark and not many making it past that. Let's look at the relative weight plot for these fish:

     Average body condition for these brookies in 2011 was only 73, compared with 92 in 2007. For you statheads out there, this difference was highly significant (p< 0.01).  Also, the trend line of body condition as fish get larger now slopes downward more steeply (-0.14 in 2011  vs. -0.08 in 2007).  Stathead that I am, I like to test for significance of different slopes. These two slopes test with a p-value right at 0.10.  Being a management biologist rather than a researcher or academic, that's good enough for me. I actually tend to fixate on a p-value of 0.2 for decision-making purposes. Academics would be aghast, but I'll take 80% odds that I'm right any day, and we're not talking about life-and-death decisions here. Aside from the life and death of a few starving brookies, that is. And actually, if you look at that 2011 plot, there are three or four outliers right at the 230 mm mark that bucked the trend. Without those fish, all these parameters would have been tighter. There's a good chance that those were the cannibals in the sample. 
     Anyway, I digress. The bottom line is that the fish in 2011 were definitely worse off than they were four years earlier. And why was that? Remember that I had pulled the cutthroat plant. In all likelihood those cutthroats were providing a prey supplement that was propping up the larger brookies to a moderate extent. Also, you'll remember that 2011 was an extremely high water year. This probably played a role too, with colder water longer into the summer, and delayed insect activity making food scarce.
     So at this point I'm thinking that I need to try something else. I love tinkering with these small-to-mid-size lakes because you can really effect major change over a fairly short time frame, and it's never been very satisfying to me to simply put them on the catchable rainbow schedule and leave it at that. Put-and-take rainbows, in my mind, are the last and least-creative management option. 
     Enter the tiger trout. For those that aren't familiar with them, tiger trout are the crime-against-nature hybrid of a female brown trout and a male brook trout. Because the fish aren't very closely related genetically (not even in the same genus), you don't get huge success rates when you try to make these fish - something like 15-30% of the eggs get fertilized. So we never have them available in big numbers, but recently our guys in Denver responsible for egg trades and procurement were able to work out a deal with Wyoming, so in the future it looks like our supply of these fish will be more reliable. Apparently Wyoming has a better situation for making tigers - we don't have a captive brood stock of either brown trout or brook trout, which means that to make tigers you have to go out into the wild, collect milt from male brookies that are ripe, and get it to the eggs of female browns from somewhere else in the wild. Getting the timing and logistics of that proposition dialed in is not an easy task, and there are many opportunities for things to not work out. It's far better to have captive broodstocks available of both species, preferably in the same hatchery. 
     Anyway, the reason we want to use tiger trout in some waters is that they are apparently a good predator, and because of their hybrid genetics they're reproductively sterile. So we can have complete control over how many teeth are in a water body. They may be used to try to impact sucker populations (I'm not aware of an example of this actually working - suckers are extremely difficult to control), but more importantly, to thin stunted brook trout populations.
     So, with the help of Chris Crowder down at the Monte Vista hatchery, in 2014 I was able to get my hands on 2,000 tigers for Meadow Creek. They were stocked on July 9, 2014 and averaged 3.7". This was a relatively conservative number of fish (20/acre), for two reasons: 1. we don't have a ton of them available, and 2. you have to be very careful not to overdo it in this situation.
     We surveyed Meadow Creek again on July 7, 2015. So the tigers had been in there almost exactly one year. Below is the graph of the fish that we captured.

     Out of the three sampling occasions, the size structure of the brookies in 2015 was actually the best it's been. There were more fish in the 10" range, but still nothing making it to 12". Below is the relative weight plot. 

     The average relative weight for these brookies in 2015 was 84, so it split the difference between the 2007 and 2011 samples. However, the trend line is the steepest yet with a -0.17 slope. So even though there are more 10" fish which represents a slight improvement in the fishery, they're having the hardest time they've ever had maintaining their body condition when they get to that size - which could be a direct result of the higher number of 10" fish creating density-dependent competition and food scarcity.
     And we captured 20 tigers. This was the main purpose of the 2015 survey: to see what kind of survival and growth we had out of those 2,000 fish. To catch 1% of them with this level of sampling effort was pretty great as far as I was concerned. Their average length was 8.0", with the largest fish at 9.8".  Below are a couple photos. 

     Predatory trout will eat prey that is up to 2/3 of their own body length. So I think our survey came right at the time when these tigers were on the verge of being able to start tearing through some of the smaller brookies. I strongly suspect (maybe I should say "hope") that at least the ones pushing 10" started doing that by the end of the summer. 
     I plan to get back to Meadow Creek again in 2016. As you know I haven't been surveying it every year, but I'm very excited to track the progress of these fish. The goal will be to again get a feel for survival and growth of the tigers, get a sense of whether my stocking rate is right, and look for signs that they are in fact using the brook trout as prey. I don't expect to see improvement in the brook trout fishery quite yet; I think it will take at least a couple years of thinning by the tiger trout to see that. But when we do see that, I hope to see brookies pushing past that 12" growth barrier, and the slope of that body condition trend line should flatten out.
     I have a request in to stock another 2,000 in Meadow Creek this year. It's been approved, so apparently barring unforseen complications, we'll get that second year-class in there this year.
     So Keith, there's a long-winded answer to your question.