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Location: Green Bay, WI | In doing a little research this weekend, I ran across this excellent paper by Dr. Casselman:
http://www.muskiescanada.ca/attachments/Muskellunge-growth-rate-cha...
It's a very interesting read, but he basically talks about how the fish that succumbed VHSv soonest (in the St. Lawrence fishery) seemed to be the slower-growing adults. Those with higher growth rates seemed to be less susceptible to the disease, at least initially, but then many of the fish in their sample later succumbed to angling stress. I found these findings very interesting, and wonder if it bodes well for the fish in the Green Bay? This population seems to be capable of very significant growth potential, and since there has been VHSv verified in the fishery, is this actually protecting the population in some way?
TB
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Posts: 1291
Location: Hayward, Wisconsin | One other interesting thing Dr. Casselman has learned is that the remaining population of fish in the St. Lawrence are now growing FASTER! Some kind of compensation thing going on I guess. Ma Nature is an interesting creature! | |
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Location: Green Bay, WI | Right...sort of that "empty niche" phenomenon, I suppose. But given that muskellunge density is quite low anyway (as the apex predator), I am somewhat surprised to see it has increased so significantly. I wouldn't have thought it would have made that much of a difference, to be honest.
I spoke with our last biologist over in Green Bay, about this very concept. The DNR argued (early in the 54" size limit effort) that the fish were growing so quickly that their longevity might not be sufficient to achieve super-tanker status. Sort of the "fast-growers, but short-livers" idea. But we argued that the carrying capacity of the bay ecosystem was so far from being full, that these fish were simply doing what mother nature told them to--eat lots, and grow quickly. And then we argued further that, once the population was more stable, we might expect to see growth rates decline to some degree--possibly to the point of being more consistent with other populations considered to be stable, long-term, fisheries (like Georgian Bay and the St. Lawrence fish, for instance). Whether this happens or not though remains to be seen.
But having said that, I found it very interesting that Dr. Casselman found such an increase in growth rate in the remaining adult fish. Whether this was a compensatory response from the remaining fish, or simply an indication of a higher growth rate in fish that survived the VHSv infection, remains to be seen I suppose. But at the very least, I thought that the way he wrote the paper made it quite evident that he was excited about the possibility of having a bunch of BIGGER (compared to the pre-VHSv fish) muskies swimming around out there in a few years!
TB | |
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Posts: 2024
| tcbetka - 4/3/2012 8:53 AM
But having said that, I found it very interesting that Dr. Casselman found such an increase in growth rate in the remaining adult fish. Whether this was a compensatory response from the remaining fish, or simply an indication of a higher growth rate in fish that survived the VHSv infection, remains to be seen I suppose.
TB
John seemed to indicate it was not a compensatory response because they were able to back-calculate length-at-age and found they were fast-growing even before the VHS die-off.
"At first, one would assume that the surviving individuals grew faster after the VHS die-off, producing a larger ultimate size. However, reconstructed growth histories are used here, reflecting the overall growth history, and there is no evidence of growth stanzas in the growth trajectories. Furthermore, the survivors survived the die-off by only a few years at the most, and it would have been impossible for this to affect the overall growth history and trajectory. It is well known that density-dependent growth changes can occur if a population is thinned, resulting in more abundant prey for the survivors. However, muskellunge in the upper St. Lawrence River are a relatively rare species and would never be abundant enough to be prey-limited. They seem to prefer redhorse suckers; however, they readily take large-bodied fish such as smallmouth bass, which are plentiful, and also gorge on yellow perch and alewife, particularly in early spring." | |
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| I hope all of you keep posting as this is great reading...thanks. | |
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Location: Green Bay, WI | Good point Sean...I do remember reading that now. Thanks for clarifying it!
So then it's interesting that these faster-growing fish seemed to be less susceptible to VHSv infection, compared to the slower-growing fish in his data set (which, as I recall, were older). Certainly the fact that those fish were growing faster preceding the infection might imply that they were in fact genetically predisposed to do just that...grow faster. But it also might indicate that they were younger and at a different point on their growth curve than older fish, which therefore may have been growing more slowly at the time of infection. In humans, the very young and the very older are more susceptible to infection--both viral and bacterial. But this is for vastly different reasons: The young have more infections because their immune systems have not yet been challenged by those infective agents, and thus (assuming they no longer have passive immunity from their mother) they have no inherent immunity. However older individuals seem to have a less vigorous immune response in general, regardless of the fact that they may have seen that particular antigenic agent (organism) in the past. In fact, this is one of the reasons we now see older people get a second vaccination against pneumococcal pneumonia.
So while I in no way claim to be a Ichthyologic Immunologist, I can think of several plausible explanations for the apparent discrepancy Dr. Casselman's data uncovered. I will need to go back and re-read the paper again however, because I usually discover things that I missed the first time through...like what Sean posted, LOL.
Fascinating stuff though!
TB | |
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Casselman paper on VHS mortality risks