Talk to Lake Ontario steelhead fishermen or check up on the message boards and you'll see a lot of talk about thiamine and steelhead mortality. It's been reported that there have been numbers of dead steelhead as well as some sickly looking steelhead around on the Salmon River and other Lake Ontario tributaries over the past two months and anglers are justly concerned. The Thiamine issue is a complex one and one of the most challenging issues that fisheries managers face when trying to maintain our fisheries and develop self-sustaining populations of native and non-native salmonids.
There's a lot of good information online about this topic but reading fisheries studies is the best way to get informed and these are obtained online through the American Fisheries Society and can be read at College libraries or purchased online. For those of you who do not know, in the Finger Lakes we do not have any significant natural reproduction of Landlocked Atlantic Salmon. Region 8 found one naturally produced Landlocked Atlantic Salmon in Catherine Creek during the summer of 2013. The percentage of wild salmon in Cayuga and Seneca Lakes is probably in the low single digits - like 1% or slightly more. Out of around 15 to 20 salmon taken from Cayuga Inlet around 10 years ago, I believe one female had adequate thiamine levels. Trout and salmon don't need a lot of thiamine, but they get very little by eating alewives.
Lake trout spawn successfully in some lakes, like Keuka, Skaneateles, Canadice and usually Seneca. We are seeing wild lake trout in every Finger Lake these days and every year it seems like we get good news on this front. But in lakes with alewives as a forage base, reproductive success tends to be quite variable. Forage fish like alewives and to a lesser extent smelt have an enzyme in them called thiaminase that breaks down thiamine (a much needed vitamin) in the predators that consume them. Fish like rainbow trout, walleyes and other non-salmonids tend to eat a more varied diet, hence the effects of thiaminase are often negated somewhat.
I'm no expert on the topic, but I have spent a fair amount of time researching it and have worked with an expert in the field and was able to ask him quite a few questions over the years. One point not mentioned is that the levels of thiaminase in alewives vary a lot. There is some thought that stress levels in alewives cause higher levels of thiaminase to be produced. So a "healthy" alewife driven fishery with plenty of forage and a good balance may not show a lot of outward signs of thiamine deficiency in trout and salmon. However, when alewives are stressed - for example due to a heavy winter (starvation,) or their levels are down or zooplankton are scarce, it's possible that thiaminase levels increase. Less alewives do not necessarily translate to less thiamine deficiency! It's actually the opposite in many cases. That may be what we are seeing on the Salmon River and elsewhere.
The lack of Thiamine causes weakness in adult fish. They are more stressed and likely more susceptable to disease. The Thiamine issue likely results in some adult fish dying out in the lake - especially Atlantics. King Salmon haven't shown the signs of Thiamine deficiency thus far to the best of my knowledge. Cohos, Atlantics, Rainbows and walleyes have. It's even been theorized that smelt suffer from this problem, since smelt hatch before alewives and young smelt consume a lot of alewife fry. This is probably why smelt numbers dropped so precipitously on Seneca and Cayuga Lakes during the late 1980s and 1990s.
We continue to see the impacts of invasive species in our fisheries. But there are no simple answers, despite the palaver on the message boards. Given the poor King salmon fishing experienced by many anglers on Lake Ontario, combined with the poor salmon runs and now these issues with steelhead, it's clear to me that this fishery continues to change rapidly and present managers with ever greater challenges. I've said it before and it bears repeating - we are going to see some major changes in salmonid fishing as we know it on Lake Ontario over the next decade.