By Don Boyd
The fish populations on the Au Sable River system in fall of 1998 highlight some rather dramatic differences between river sections and also contain some hopeful results. The Mainstream and South Branch both show increasing fish counts and other positive trends while the North Branch generally shows little evidence of real improvement.
Readers may already know that the fish on the Mainstream are counted at two stations: WaWaSum and Stephan’s Bridge. Both brook trout and brown trout are more plentiful at Stephan’s Station, but despite this both stations showed a similar trend. Nearly all year classes in the Mainstream showed a slight improvement between 1996 and 1997 and an additional gain between ‘97 and ‘98. Two consecutive years of improvement in four to eight inch trout at WaWaSum may be a reversal of 15 years of decline. Brook trout between eight and 12 inches showed a particularly notable jump between the ‘97 and ‘98 population estimates. Brown trout estimates showed gains of a lower magnitude. Recall, however, that in 1997 the number of eight to 12 inch brook trout at the WaWaSum Index Station may have been the smallest in recorded history. At Stephan’s Bridge Station, brown trout made more surprising gains while brook trout were static or showed slight declines. The most significant gains were with eight to 12 inch brook trout at WaWaSum and the increase of 12 inch or larger brown trout at the Stephan’s Bridge Station. Again, when considering these changes, one must keep in mind that the magnitude of the change, while significant, needs to be contrasted with recent, very low numbers.
The South Branch, particularly at Chase Bridge, showed the most dramatic change of any location in the Au Sable system. In this area, all classes of brook trout showed remarkable increases. Normally increases in the four inch class results in some decline in the two to four inch class. The other South Branch Station, Smith Bridge, also shows substantial increases in brook trout.
Unlike the Mainstream and the South Branch, population trends at the North Branch stations continue to suggest either no change or decline. Eight to 12 inch brook trout at the Twin Bridge Station are either down slightly, or unchanged. Twelve inch or larger brown trout at the Twin Bridge’s Station are down and may be at an all time low. Dam Four Index Station shows a low and typically unchanged number of 12 inch or larger brown trout as well as very little change from 1997 in eight to 12 inch brook trout.
Data are also available regarding rainbow trout populations at the Stephan’s Bridge Station. Population for this group of fish show much broader fluctuations than brook or brown trout populations. 1994 may have been an all time low for eight to 12 inch rainbow trout. There is a slight increase in 1995, another increase in 1996, and again in 1998. Twelve inch or larger rainbow trout were uncountable in 1997 and were present in very small numbers in 1998.
Overall, the data continued to demonstrate two persistently occurring relationships. As brown trout numbers go up, brook trout numbers go down. (Competition?) Also, in nearly all instances as large fish increase, the number of small fish, particularly the two to four inch fish decrease (predation)? The decrease in small fish as an inverse proportion to the number of large fish was most notable for both brook and brown trout at the Stephan’s Bridge Station. The Smith Bridge Station on the South Branch recorded a second straight year of substantial brook trout increases. Again, this is contrary to declines in eight inch or larger brown trout at the Smith Bridge Station. Brook trout at Chase Bridge, in all size classes, showed dramatic increases and again this is contrasted with slight declines in the three largest class sizes of brown trout at that station.
Another trend was quite evident. Improvement in brook trout in the South Branch paralleled improvement in brook trout on the Mainstream. One could easily hypothesize that the very easy winter in 1997 enhanced these results. If that were true, however, one would not expect the continuing declines or static populations noted on the North Branch.
It is uncertain if this new data reflect a turnaround in the slope of the population decline for trout in the Au Sable system. But, three years of even modest improvement is hopeful. Both the Mainstream and Mason Tract of the South Branch continue to have catch and release regulations. It is possible that the cumulative effect of both catch and release and favorable weather conditions have reversed the trend, if ever so slightly.
Lastly, a look at the data shows high mortality between the two and four inch class and the four to eight inch class. Nearly 1,600 two to four inch class were present in the fall of 1997. In the fall of 1998, the number of four to eight inch fish at the same station (Stephan’s Bridge) was only about 300. None of these studies correct for the possible effect of migration. North Branch water temperatures, especially above Dam Four, are routinely high in mid summer. And the Mainstream above WaWaSum is usually warm during the summer. Fish leaving these sections, and then returning, may impact fish counts and skew population estimates. RWOL
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