Each summer the MNDNR deploys fisheries crews from each of its 29 offices to find out exactly what’s swimming in Minnesota lakes and rivers. These surveys aren’t meant to give the DNR a solid population count for any species. Rather the studies provide biologists with the relative abundance and general size range for each species. This data is not only used in important management decision-making, it is made available to the public via the DNR website.


Scott Mackenthun, the Hutchinson Area Fisheries Supervisor, shared an inside look at one such survey this summer. Along with intern Garrett Ober, a senior in Bemidji State’s Aquatic Biology program, Mackenthun collected valuable data on Meeker County’s Spring Lake in late June and early July. Like with every other survey being conducted at the time all over the state, Mackenthun and Ober used both gillnets and trapnets to passively catch fish.


Trapnets are shaped somewhat like an arrow. A long stretch of netting runs perpendicular from where it is anchored near shore. As shallow fish find their path along the shore obstructed, they follow the netting to the first chamber of the trapnet in an attempt to circumnavigate. Each of the five chambers funnels fish towards the front, which the crew will open when the net is pulled.


Gillnets are 250 foot long, 6 foot tall rectangular nets that are placed above the oxygen drop-off (in Spring Lake during the survey, the drop-off was at about 12 feet). The netting begins at ¾” wide and gets bigger every 50 feet; the final frame’s netting is 2” wide. When fish swim into the netting, they become caught around their gills. Unfortunately the fish are killed in this process, but their contribution to the survey far outweighs their ecological importance. Gillnets catch fish that would be impossible to catch any other way.


According to Mackenthun, using both types of nets is crucial so that no species are missed. For example, pike and walleye are caught best in gillnets and sunfish are caught best in trapnets. Smallmouth and largemouth bass are not represented well in either, so when crews target black bass they use electrofishing.


Each net runs – or is left in the water – for 24 hours. Once the nets are pulled, the fish are quickly removed, sorted by species, counted, and measured. Fish caught in trapnets are placed into buckets of freshly aerated water to keep them healthy until they are returned to the lake.


After the crew removes and sorts its catch, the net is reset in a new location. These locations are predetermined and used year after year to maximize consistency. The number of nets in each lake is determined by acreage. Because of the overwhelming time and effort that goes into these surveys, lakes are usually only evaluated every five years.


While lowering a trapnet back into the water, Ober pointed out the bright orange flags that demarcated the submerged nets. He explained that the flags are more to warn away any boats that may get caught in the line; thankfully the vast majority of anglers know to leave the DNR nets alone. As he happily shared with a curious onlooker, the crews know when the nets have been tampered with, and luckily it happens only rarely.


As the biologists, technicians, and interns process the fish, they keep an eye out for lesions and deformities. These abnormalities are noted, but the DNR relies on separate surveys dedicated solely for this purpose to check on fish health. Crews also remove a few eater-size fish for contaminant testing to be checked for harmful components such as PCBs and mercury.


When asked if there was anything that he wanted Minnesota sportsmen to know, Mackenthun replied that the fishing here is unlike anywhere else – you just have to be willing to put in the time learning different bodies of water. “And we’re on your side!” added Ober with a smile.


Originally published in Outdoor News Minnesota