Thinking Outside The Stream

Watershed analysis holds the key to the stream and its proper management.

Smart anglers know to use their quarry’s habitat preferences to help locate and catch them. Fishery biologists use habitat management to improve the quality of sportfish populations.

While both have long been true, there is growing awareness that the quality of habitat includes land uses far beyond the water’s edge. This has been facilitated by advances in spatial-analysis technologies, like geographic information systems (GIS), that rapidly assess characteristics and land uses of watersheds from aerial photographs and satellite imagery.

Many recent studies have demonstrated that the watershed has significant effects on the composition of the fish community and its health. I found one by Brian Alford and Don Jackson at Mississippi State University especially interesting because fish abundance was assessed by hook-and-line. Thus, Alford and Jackson actually developed relationships between watershed variables and angler catch rate.

The study sites were wadeable streams—moderate size waters generally less than three feet deep at normal flows—throughout Mississippi. Fish were sampled during summer with ultra-light spinning gear.

Surprising Findings
Rigorous statistical analysis found angling success for black bass and sunfish was greater in stream reaches with more forest cover and higher stream and road density. The relationship with forest cover makes sense. Compared to urban and agricultural lands, forests are less disturbed and tend to contribute less sediment that destroys stream habitat and smothers invertebrates. Forested riparian zones stabilize banks, contribute woody materials that improve hab- itat and, via leaf fall, provide organic matter to fuel fish production.

The positive relationship between catch and road density is counterintuitive. More roads generally mean more development, more disturbance and more smothering sediment. But the roads in the study were rural ones, and even in areas with high road densities, there were fewer miles per acre compared to other parts of the United States.

It may also be important that most of the streams were located in relatively flat watersheds where erosion associated with road development and maintenance would be minimal.

Reasons for the positive relationship between catch rate and stream density are not obvious. I would expect, however, that sites with more streams nearby might have more stable flows and water depths because runoff would be distributed to more streams rather than all funneling into one or two.

There are important implications from this and similar studies. First, the sportfish potential of streams can be quickly and economically predicted from satellite imagery, and managers can use these predictions to effectively channel resources to waters with the greatest potential. Conversely, managers can identify stream reaches where fishing can best be improved by managing the watershed rather than adding in-stream cover, manipulating harvest regulations or stocking.

There are implications for anglers, too. You’ll find better fishing opportunities if you use satellite imagery to locate streams with more forest and less agricultural land and urban development in the watershed.

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